Medisinsk-teknisk avdeling, Rikshospitalet
Fysisk institutt, UiO
Chapter 9
GAS
INSTRUMENTATION
av
Sverre Grimnes
2008
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INTRODUCTION
In each cell the complex mechanisms of life are based upon the simple use of two gases:
oxygen supplied - carbon dioxide produced. In the lungs these two gases are separated by
the gas/blood membrane but transported as gas on the ventilation side and as liquid
(dissolved blood gases) on the blood side. Most tissues of the body do not contain gas in
the gas phase; gas bubbles in the small blood vessels are dangerous because they act as
emboli hindering blood flow. The guts and the lungs are the only organs where gases are
to be found normally. Gas instrumentation in medicine serves first and foremost the
lungs, and both for diagnosis (gas analysers); therapy (aerosol nebulizers) and support
(ventilators, anaesthesia workstations). We can not be without lung ventilation for many
minutes; therefore support instrumentation is critical equipment with respect to technical
malfunction and wrong use.
.
Table 1 Content of air
Volume %, equal to kPa if the barometric pressure is 100 kPa.
nitrogen
oxygen
argon
carbon dioxide
water vapor
dry
saturated 37oC
78.1
73.4
20.9
19.6
0.9
0.8
0.04
0.04
0
6.3
Table 1 shows the content of air. Notice the influence of water vapour. Oxygen and
carbon dioxide are called blood gases, together with nitrogen these gases are dissolved in
the blood and therefore are also in the liquid phase. Nitrogen is not used by the body, so
there is no net nitrogen transport across the lung membrane. In blood most of the oxygen
transport is performed by oxygen chemically bound to haemoglobin (bluish) forming
oxyhaemoglobin (reddish).
If anaesthetic drugs in gas or vapour phase (e.g. N2O or sevoflurane) they are supplied
through the ventilation. Scavenging systems remove such gases before they reach the
operating room ambiance.
AIRWAY AND LUNG ANATOMY
The lower airways below the throat comprise the trachea and the bronchi. The trachea is
split into the two main bronchi, at the distal part they end in the alveoli (Fig.1). Here the
air and the blood meet but separated by the very thin membrane of the air sac (alveolus).
On the tissue side blood capillaries envelop an alveolus.
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Oxygen is transported as O2 gas molecules in the trachea down to the alveoli, as dissolved
gas and chemically bound to haemoglobin in the blood, and in the end diffuses the last
tenths of a millimetre from blood capillaries through the extracellular liquids up to the
living cells. Oxygen supply is from outside of the body, and it is therefore a concentration
gradient with falling values from the mouth to the cells. Carbon dioxide is produced in
the cells, diffuses to the blood capillaries and is then transported by blood to the lung
capillaries, diffuses through the lung membrane
and is expelled from the body as CO2-gas
through the airways. The CO2 gradient is
therefore with the highest values in the cells and
lowest in the mouth.
The gas exchange takes place in the alveoli.
Most textbooks present alveoli as a bunch of
grapes, but pulmonary alveoli are prismatic or
polygonal in shape, i.e. their walls are flat. There
are about 600 millions of them in our two lungs.
The membrane surface in an adult healthy
person is about 160 m2 and this assures a very
effective gas exchange between the air and the
blood. The exchange is as a gradient driven
diffusion process through the membranes, tissue
and the walls of the blood capillaries.
Figure 1 Airways with larynx,
trachea and bronchi
Lung volumes, lung capacitance
The total volume of both lungs of an adult
healthy person at maximum inspiration is about 6L, fig.2. The residual minimum volume
at maximum expiration is about 1L: it is impossible to empty the lungs completely all the
way to collapse. The difference (5L) is the vital capacity. The tidal volume is the normal
inspiration or expiration volume under quiet breathing, for instance 0.5L.
Lung compliance, pneumothorax
Each lung is enclosed in a gas-tight pleural volume by the double-walled lung sac
membrane. The outer membrane is fixed to the thorax cage, the inner to the lungs.
Because of the surface tension of the liquid films a lung tends to contract and reduce its
volume. Therefore the intermembrane volume has a negative pressure of about -4 cmH2O
with respect to atmospheric pressure. During inspiration the diaphragm pulls the lower
surfaces of the pleural volume down increasing the lung volume and thereby increasing
the negative pressure in the alveoli. A puncture of the lungs destroying the negative
pressure is critical for the patient. The lungs will collapse and the patient will not be able
to breath (pneumothorax). Normally a pressure change of as little as -1cmH2O (+1
cmH2O during expiration) in the alveoli is sufficient for a quiet respiration. When the
patient is breathing spontaneously the inhalation is caused by the work of the lung
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muscles resulting in the alveolar negative pressure. During expiration little muscle work
is done, it is the relaxation process of the stretched tissue which brings the air out. During
forceful ventilation also the rib rise increases the pleural volume and increases the
negative pressure. Then also special muscle groups actively compress the pleural volume
during expiration.
Figure 2 Lung volume parameters
The lungs may be soft and easy to fill, meaning that a relatively large inspiration volume
is obtained with only a small negative pleural pressure change. Compliance is a much
used parameter to describe the expansibility of the lungs, compliance C is defined as:
Equation 1
Compliance
C = ΔV / ΔP
[L/Pa, L/cmH2O]
The compliance of the normal lungs and thorax is about 0,13 [L/cmH2O]. Reduced
compliance makes the patient more difficult to ventilate. A therapy is the use of
surfactants, substances which lowers the surface tension at the inside alveoli surfaces. A
near ideal zero compliance closed volume is a gas supply bottle, a near ideal maximum
compliance volume is the closed volume spirometer.
Flow resistance, gas viscosity
The trachea is equipped with cartilage rings so that it will not collapse at negative
pressure. The basic model for flow resistance in tubes is based upon the law of
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Poiseuille1, describing the resistance R to flow through a tube of radius r and length L
under the influence of gas viscosity  [Pa s]:
Equation 2 Poiseuille
R
L
4 r 4
[Pa/m3/s = pressure / flow rate]
Validity
1) Laminar flow in a straight tube geometry
2) Gases and liquids (fluids), but better model for gases than for liquids
3) Flow rate in [m3/s], not [mol/s]
4) Gas viscosity is increasing with temperature (in contrast to liquids). It is pressure
independent, and R is therefore independent on the mean pressure level in the tube
Figure 3 Parabolic velocity profile in accordance with the Poiseuille model.
Thus, the resistance is not dependent on friction between the fluid and the walls, only on
the internal friction in the fluid. At the walls the velocity is zero, increasing to maximum
at the centre of the tube. Fig.3 illustrates the Poiseuille ideal flow model in a tube, the
flow profile is parabolic. The frictional forces between layers of the fluid are forces
parallel to the flow, they are shear forces. Ohms law for electrical parameters is V=RI
where V is voltage difference [V] and I current flow [A] through R. As a parallel to
Ohms law P=RQ where P is pressure difference [Pa] at the wall and Q is mean flow
[m3/s]. In spite of the variable velocity illustrated in Fig.3, R is therefore related to the
mean velocity. By measuring P we have a gas mean velocity sensor, see subchapter on
gas sensors.
The extreme dependence on the tube radius shown in Eq.2 has very important
consequences e.g. with catheters and syringes for the injection or aspiration of fluids. It is
also important in obstructed airways (airway resistance work, asthma). It is a very
1
Jean-Louis Poiseuille (1799-1869), French physicist
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effective regulating mechanism in the body when the arterial blood vessel walls are
equipped with muscles to contract and reduce the radius of the vessel.
Turbulence
When the velocity of a fluid is increased beyond a threshold value, the flow modus
changes from laminar to turbulent.
The resistance to flow is increased
and Poiseuilles law no longer
describes the process correctly. The
flow resistance is not determined so
much by the fluid viscosity as by
fluid density.
Turbulence is important in many
Figure 4 Flow lines with local hindrance and
parts of the body, both in the airways
a back eddy (non-laminar zone)
and in the blood stream, in particular at
bifurcations and around heart valves. The laminar model is useful, necessary and
important, but its validity range must always be kept in mind.
.
GAS PHYSICS
Water has a very low compressibility because of the strong polar bonds between the
molecules. The molecular bonds in oil are somewhat weaker, and oil is therefore slightly
compressible. Air at room temperature and 1 bar has a density of about 1,25 kg/m3, about
thousand times lower than that of water, and the distance between the molecules is
accordingly roughly 10 times larger than in water. A gas is very compressible, but if a gas
is at a temperature higher than its critical temperature, it is impossible to press the
molecules together into a liquid phase.
The amount of gas substance in a closed compartment can be characterised according
to two traditions: either by volume and pressure or by the number of mol. Flow rate can
accordingly be given by mass: kg/s; or volume: m3/s; or substance: mol/s. The
metabolism of the body is based upon the chemical reactions between molecules, so the
number of molecules (mol) is perhaps the most basic unit for medical gases used by the
body.
Dynamic gas model and the universal gas law
Gas molecules (or atoms or small particles) at room temperature are not at rest. It was the
British botanist Robert Brown who in 1827 discovered in his microscope that small
grains of pollen in water moved and collided, he thought it was a life process. The
average effect of gas molecule collisions with the walls constitutes the pressure of the
gas. It was Avogadros2 great discovery that the pressure is proportional to the number,
not the mass, of the particles. With reference to Avogadro the number of particles
2
Avogadro (1776-1856), italian autodidact chemist and physicist
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therefore has got its own numbering system: the mol which is ≈6·1023 particles and
corresponds to the molecular weight [gram] of a gas, e.g. 1 mol of oxygen is 32 g
oxygen. The type of particles should be specified, but often we mean both atoms (e.g. Ar)
and molecules (e.g. O2). The reason for the number dependence is that smaller particles
move faster so that their contribution to pressure statistically is the same. Accordingly,
usually for a certain amount of substance the pressure will be dominated by the smallest
particles because they are usually more numerous (the mass of spheres is proportional to
the radius r3, so with equal mass it is 106 as many 0,1 μm spheres as 10 μm spheres).
The thermal movement of gas molecules has an important consequence: in a mixture
of particles such as air, the particles do not necessarily sediment in layers according to
their weight. It is the mass of a particle at a certain temperature which determines
whether the molecule settle under gravitational forces or thermal movements are forceful
enough to overcome gravity. At room temperature light particles such as N2 (molecular
weight 28), O2 (32), CO2 (44), Ar (40) and H2O (18) will not settle with the heaviest
molecules at the lowest levels. Larger molecules like sevoflurane (200) however do settle
to the floor to a noticeable degree. According to the gas law a certain gas pressure is
obtained by fewer molecules the higher the temperature. Accordingly, warm gas is lighter
and ascends into the air (warm air balloons).
The universal gas law is based upon Avogadros discoveries. It is given in a variety of
forms; here by the Boyle&Mariotte / Gay-Lussac3 version using the amount of substance
n [mol] as a parameter:
Equation 3
Universal gas law
PV = nRT
n is the number of all particles in the enclosed gas volume, that was Avogadro's great
discovery, The particles may be atoms (noble gas), molecules (e.g. O2 or CO2) or any
small particle from electrons to droplets. The contribution to pressure is from each
particle, it is the number that contributes and not the size. P is the pressure [Pascal], V is
the volume [m3], R the universal gas constant (8,3 [Joule/ºKmol]) and T the temperature
[ºK].
Validity
1) Closed volume, all n contained in V
2) Ideal gas (= far from the condensation point)
3) Static conditions
Other versions
 P1V1 = P2V2 (constant number of particles and constant temperature)
 P = (n/V) RT (n/V is number density)
3
Boyle (1627-1691) Irish physicist; Mariotte (≈1620-1684) French physicist; Gay-Lussac (1778-1850)
French physicist.
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
Closed volume but soft walls so that V is a function of P, i.e. compliance C > 0, see
chapter on lung compliance
There are two different pressure scales in common use: absolute pressure (AP) or relative
to atmospheric pressure (RP). Often it is not defined but determined by convention. Most
pressures are relative pressures. Bottle filling is RP, an oxygen bottle is “empty” when it
has atmospheric pressure, and negative pressure is not good as ambient gas may be
aspired into the bottle. Absolute negative pressures do not exist, so if negative pressure is
given it must be RP. Also with suction devices relative (negative) pressures are often
used. Higher negative pressure then means higher vacuum and lower absolute pressure.
PV diagrams, non-ideal gases, condensation
Fig.5 shows a pressure-volume (PV) diagram of a gas in a closed volume. At high
temperatures the gas is more ideal following the Boyle-Mariottes law, Eq.3. In this region
the gas can not be compressed into a liquid, irrespectively of how high the pressure is. At
lower temperatures the curves loose their hyperbolic form, and at the critical temperature
Tc a point is reached where it is possible to compress the gas into a liquid. At
temperatures below Tc there is a constant pressure range where the substance is more or
less liquefied. A liquid is not very compressible and when completely liquefied the
pressure rise is very rapid at a further lowering of volume. Tissue (except lungs and guts)
does not normally contain gas, so tissue is like a liquid, highly incompressible, but may
easily change its form.
Figure 5 PV-diagram for a closed volume
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Above the critical temperature it is not possible to compress a gas into the liquid state,
the vibrational thermal energy is high enough to break the tight bonds between the
molecules. Below the critical temperature the gas may be compressed to a liquid, the gas
in this range is called a vapor. In daily talk we do not make the precise division, for room
temperatures we should say water vapor, nitrous oxide vapor and oxygen gas. This would
clarify the important fact that a bottle of carbon dioxide at room temperature may contain
gas in the liquid phase. Then the bottle filling must be determined by weighing and not by
pressure measurement. The oxygen bottle can not contain liquefied oxygen at room
temperature, and the degree of filling can therefore directly be determined from reading
the manometer pressure.
Examples: a) Calculate how many liter of gas you have left in a 10L oxygen bottle at 120
bar. Solution: Tc for oxygen is -119 oC, so the oxygen must be in the gas phase. We use
the Boyle-Mariotte gas law in the form P1V1=P2V2 so that 120.10=1.V2 and V2=1200 [L].
b) Calculate how many liter of gas you have left in a bottle of N2O having weighed it and
subtracted the empty bottle weight (tared) and found the N2O content to be 2.2 kg.
Solution: Tc for N2O is 36.5 oC and the N2O may be in the liquid phase. We need not
know the ratio of liquid to gas, our weighing takes care of all the N2O molecules. The
molecular weight of N2O is 44 and 1 mol is therefore 44 gram. The amount of substance
in the bottle is 2200/44=50 [mol]. We use Eq.3: PV=nRT and put P=100kPa and T=300
K
, then V=1245.3 [L].
Table 2 Critical temperature Tc, pressure Pc ; boiling point (1 bar) Tb.
Tc [oC]
-268
-147
-122
-119
31
36,5
374
Pc [bar]
2,4
33,6
49
50,3
73
72
218
Tb [oC]
-269
-196
-186
-183
*
*
100
Transport and storage of
liquefied gas is practical
because of the reduced
volume, a large hospital is a
big consumer of e.g. oxygen
and nitrogen. According to
Table 2 the storage of liquid
oxygen must be done in large
thermos
bottles
at
*can not exist in liquid phase at 1 bar (fig.34).
temperatures lower than the
Tc. Oxygen can e.g. stored at -119 oC and 50 bar, or -160 oC and 6 bar. To reduce the
storage pressure to atmospheric pressure the temperature must be lowered, for oxygen in
an open bottle (1 bar) down to the boiling point -183 oC. Nitrogen is used for cooling in
the laboratories, for tissue long term storage and for cryospray. It is often practical to
transport small volumes of liquefied gases in open thermos bottles at 1 bar and boiling
temperature.
Water is an important substance in medicine. Air can absorb water in the gas phase,
such water vapour is invisible to the human eye, just as oxygen and nitrogen. The warmer
the air, the more water molecules can be absorbed. Air saturated with water vapour
corresponds to a relative humidity (RH) of 100%. At 37oC the partial pressure of the H2O
molecules is then 6kPa. If the gas is saturated with water vapour and the temperature is
Helium (He)
Nitrogen (N2)
Argon (Ar)
Oxygen (O2)
Carbon dioxide (CO2)
Nitrous oxide (N2O)
Water (H2O)
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lowered, water condensates in small droplets. This is mist or fog and is visible to the
human eye, it is particles (droplets) and not single molecules.
Laplace law
In a blood vessel the blood pressure exercises a force against the walls which is
counteracted by three different force components in the wall: 1) elastic tissue tension, 2)
surface tension, and 3) active muscle tension (tonus). Tension T is measured as force pr
meter length perpendicular to the force [N/m], Fig.6. Laplace4 found the following
formula for the pressure P [Pa] in a cylinder of radius r [m] and the total wall tension T
[N/m]:
Equation 4
Laplace
P = T/r
(cylinder)
Some peculiarities of this equation are linked with the fact that it does not contain pi, that
pressure increases beyond limits as r  0, and that the pressure and the tension
components are orthogonal, ref. Problem 10. T may itself be a function of r, so that the
increase in pressure with small values of r may be modified.
A tube has one dimension of curvature, and since the sphere has two such dimensions,
the pressure for a sphere is doubled: P = 2T/r .
Figure 6 Laplace cylinder model
These equations are applicable in many medical situations also with active muscles in the
walls, e.g. blood vessel, spherical pathological enlargement of blood vessels (aneurisms),
pressure in the ventricles of the heart during systole, pressure in the urine bladder,
pressure in the airways of the lungs. In the lungs the alveoli are prismatic or polygonal in
shape, i.e., their walls are flat, and the Laplace law applies only to curved regions. Alveoli
do not readily collapse into one another because they are suspended in a matrix of
connective tissue "cables" and share common, often perforated walls, so there can be no
pressure difference across them. Surfactants have important functions along planar
surfaces of the alveolar wall and in mitigating the forces that tend to close the small
airways. Laplace’s law as it applies to cylinders is an important feature of the mechanics
of airway collapse, but the law as it applies to spheres is not relevant to the individual
alveoli.
4
Pierre Simon Laplace (1749-1827), French astronomer, mathematician and physicist
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Solubility, partial pressure
Gases do dissolve in liquids like oil, water and blood. This process is called physical
solubility if there are no chemical reactions between the gas and the liquid. The gas
molecules find their positions between the liquid molecules, and if the gas molecules fit
well in the space between them the solubility is high. The gas becomes a part of the liquid
phase, and is not to be regarded as small gas bubbles. There is a transport of gas across a
gas/liquid interphase as long as there is a concentration difference. The concept of partial
pressure is essential in this respect as a practical measure of gas concentration in a liquid.
Henry’s law states that the amount of gas dissolved in a liquid is proportional to the
partial pressure of the gas in equilibrium with the liquid. Partial pressure in a liquid may
be measured with a sensor covered by a membrane permeable to the gas but not to the
liquid.
Figure 7 Left: dry gas mixture, right: after insertion of a water filled dish
On Fig.7 a closed chamber at 37 oC is filled with dry nitrogen gas up to a pressure of 80
kPa and then with dry oxygen gas so that the total pressure is 100 kPa (1 bar). At 37 oC
no chemical reactions between the two gases occur, we have a mechanical mixture.
According to Daltons law the partial pressure of each gas contributes to the total
pressure as if it were alone. The partial pressure of oxygen (Fig.7) is therefore 20 kPa.
On Fig. 7 (right) we then introduce a small dish of water into the chamber. The water has
already been in prolonged contact with room air so that the water is in equilibrium with
the oxygen and nitrogen of the air. With the dish inside no net transport of oxygen and
nitrogen occurs across the gas/water interphase, but as the gas was dry a transport of
water molecules into the gas starts (evaporation). This goes on until the chamber gas is
saturated with water vapour. The relative humidity (RH) is then 100%, at 37oC
corresponding to a water vapour partial pressure of about 6 kPa. The total pressure in the
chamber has increased to 106 kPa. The slightest temperature fall somewhere in the
chamber will then start water vapour condensation.
Table 3 shows the solubility coefficient of different gases in blood and oil. It is practical
to give the amount of a dissolved gas as shown: litre gas per litre liquid [L/L=1], the
Ostwald solubility coefficient which is temperature, but not pressure dependent. If the gas
pressure is doubled the amount of dissolved gas is also doubled according to Henry’s
law. But that is as amount of substance [mol], not as volume [L], because the doubled
pressure has also halved the volume. The number of dissolved molecules pr litre liquid
[mol/L] may therefore be more physiological relevant, but less practical because it is
pressure dependent.
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Table 3
Solubility of gases in blood and oil at 37 oC
Nitrous oxide
Halotane
Enflurane
Isoflurane
Desflurane
Sevoflurane
Ether
Oxygen
Carbon dioxide
Nitrogen
gas/blood gas/oil
[L/L]
[L/L]
0.5
1.4
2.3
224
1.8
96
1.4
91
0.4
19
0.6
53
12
65
0.02
0.8
0.015
Blood gases are transported in the
circulation, and Table 3 shows that the
solubility of oxygen is low and the transport of
dissolved oxygen is therefore not sufficient to
supply the metabolism of the body. To increase
the transport capacity blood is therefore
equipped with haemoglobin also binding
oxygen chemically. The carbon dioxide
solubility is much higher so that the transport
is not so dependent on chemical binding for the
exhalation of CO2.
If nitrogen is to be replaced by nitrous oxide in
an anaesthesia, the transport of nitrous oxide in
the blood stream is more rapid than the wash out of nitrogen (why?), therefore gas
volumes filled with nitrogen e.g. in the guts can increase dramatically but transiently
when large amounts of nitrous oxide suddenly arrives.
Oil may seem to be a curious choice of liquid, but it equals fat sufficiently and body
fat may store large amounts of gas and so influence gas kinetics strongly. After a
prolonged anaesthesia it may take a long time to wash out the anaesthetic gases from the
fat during wakening. Oil data is also used because oil is more stable and give more
reproducible data than fat.
BREATHING SYSTEMS (SUPPORT)
In the operating room or intensive care unit the breathing
system connects the patient to the ventilator or anaesthesia
machine. For resuscitation smaller and simpler portable
units are used with a patient mask. For longer periods of
time the patient is intubated. A tube is positioned in the
patients airways with the help of a laryngoscope, see Fig.8.
The patient must be unconscious. The laryngoscope
contains battery and light source to ease correct positioning.
With the tube correctly positioned in the trachea a cuff is
inflated so that a tight coupling with the lungs is obtained.
Figure 8 Laryngoscope
and tube insertion.
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One-way breathing system
In such systems all the expired gas leaves the system and nothing is recirculated back to
the patient. One-way systems are used with ventilators when anaesthetic gases / vapours
are not used. Fig.9 shows an example, a simple resuscitation model used for supplying
oxygen-rich gas to the patient via a mask, more effectively than by the mouth-to-mouth
method. Fresh gas from an oxygen bottle continuously flows into the system; the flow
control is outside the illustration as a part of the bottle with manometer, flowmeter and
pressure reducing valve. Flow direction valves are an essential part of such a system.
Squeezing the bag starts the inspiration cycle, the gas inlet valve flap A is pushed to the
left, and the fresh gas is flowing into the patient. The operator feels the patient lung
pressure in her hand and the resistance to gas flow, and perhaps also the rise and fall of
the thorax surface. When the operator considers the lungs to be adequately filled the bag
squeezing is stopped. The pressure drops and the expiration cycle starts driven by the
lung pressure. The valve flap A is pressed to the right and the fresh gas starts to fill the
bag. The expiration airway is free out to ambient air. When the operator considers the
lung empty, the bag is again squeezed and a new inspiration cycle starts.
The pressure in the bag will not raise proportional to its filling. According to Laplace
law the bag pressure P = 2T/r, cf. the subchapter on Gas Physics. However, as r increases
also T increases and usually roughly proportional to the circumference of the bag and
therefore r. The result is that the pressure in the bag is not very dependent on bag filling.
The tubes are often of a spiralled type so that they do not collapse at low pressures
(vacuum).
Figure 9 One-way small portable resuscitation system
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For short term use a special humidifier is not necessary even if the fresh gas is dry.
Usually the parts are single-use low price items, so disinfection or sterilization by the
users is not necessary.
Circle system, rebreathing
In these systems a part of the expired gas is returned to the patient. The advantages are:
water is returned to the patient airways, heat is returned, use of costly inhalation drugs is
reduced. An example of such a system is shown in Fig. 10. The expired gas is partly
returned to the circle, partly leaving the system via the pop-off valve. The carbon dioxide
is absorbed in the CO2 canister, this also develops heat. Fresh gas flows continuously into
the system from an anaesthesia machine, and the breathing rhythm is determined by the
bag squeezing. The patient is intubated with the tube coupled to the Y-piece. The unidirectional valves secures the correct flow direction.
The inspiration starts when the bag is squeezed. The one-way valve 2 is closed, and
the content of the bag, humidity included, flows through the absorber and valve 1 and is
mixed with the dry fresh gas. The gas mixture enters the lung via the Y-piece. The gas
will not continue in the circle because valve 2 is closed. With one hand on the bag the
operator palpitates lung filling and lung pressure. When the lungs are adequately filled
the operator stops squeezing, valve 2 opens and valve 1 closes. The spring pressure on
the pop-off (also called the automatic pressure limiter (ALP) valve plate determines at
which pressure the valve opens, and thereby how large part of the gas is expelled. The
opening phase of the valve corresponds to the high pressure phase at the end of
inspiration.
The surplus gas enters a reservoir open to the ambient air, from the same reservoir a
suction system aspirates at a flow rate high enough to secure that no gas escapes into the
room. The open reservoir is an important safety measure so that the suction tube can not
bring negative pressures to the circle and the patient lungs.
A Y-piece is used as near to the patient as possible in order to separate inspired and
expired gas. Dead space is the problem; the first gas inhaled to the lungs is the newly
expired gas content of the patient airways. The dead space is the part of a breathing
system common to inspiration and expiration. The volume of the trachea is a natural dead
space. One-directional valves must be included in such systems to clearly define the
inspiration and expiration tubes.
Fig.10 shows the main components of a circle system, however there are many ways of
putting the components together. The one-directional valves may for instance be put in
the inspiration and expiration tubes, or the bag tube may be connected to the right of the
lower one-directional valve. The function in normal mode is perhaps not so different, but
if something goes wrong the difference and consequences for the patient may be very
dependent on the exact configuration.
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Figure 10 Rebreathing circle with uni-directional
valves 1 and 2
The bag may be
tight bottle, and
volume may be
pressurized by a
put into a
the outer
cyclically
ventilator.
Such a bag-in-bottle system is a part of the ventilator.
What happens if the bag is not squeezed? The patient is not ventilated, the fresh gas flows
continuously directly to the pop-off valve.
Risk considerations Breathing systems:
The uni-directional valves have important safety functions. If they are not functioning
correctly (e.g. open all the time), tube connections swapped, pop-off (scavenging) valve
closed, manometer or pressure relief valve at the Y-piece, water condensation in the
expiration tubes, too high fresh gas flow. The wheel of a trolley pressing tube to closing.
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Humidifiers and nebulizers (therapy)
Passive humidifiers
The humidifier adds water vapour to the breathing system. A simple way is to insert a
HME filter (Heat and Moisture Exchanger) at the Y-piece, cf. Fig.12. Expired gas
saturated with water is cooled when leaving the patient and the condensed water is
absorbed in a sponge with a hygroscopic material. At the same time the latent heat of the
condensation process contributes to reduce the patient temperature loss at the next
inhalation.
Active humidifiers
Active humidifiers are usually positioned on the inspiratory side of the breathing system.
Often the condensation in some part of the system is so strong that water traps must be
installed. The problem is that a breathing system is not isothermal. If the gas is water
saturated in the warmer parts, the colder parts will cause water condensation. This can be
avoided by using an electric heating wire inside the tube.
Hot water humidifier
The inspiration gas is lead over an electrically heated water bath with a sufficiently large
contact area between water and gas. Added advantage of heat supply to the patient.
Ultrasonic vibrator humidifier
Ultrasonic vibrating plate near the water surface or water drops falling on the plate
creates a water mist.
Gasdriven jet Bernoulli humidifier
is a suction device (see last chapter) aspiring water into the jet stream and thus generating
droplets. They may too large to be able to penetrate down to the bronchia.
Nebulizer (aerosols)
The nebulizer is a therapeutic device for the inhalation of pharmaceuticals in aerosol
form. Aerosols are particles (powder or droplets) suspended in gases, the therapeutically
useful size spectrum is the diameter range 0,5 – 10 μm. The largest particles carry the
main amount of substance (volume of a sphere is proportional to r3). For the larger
particles sedimentation is an important deposition process. For the smaller particles
diffusion is the most important deposition process (collisions with the walls). The smaller
the particle, the deeper it penetrates into the lungs towards the alveoli. However, in the
upper airways the smaller particles tend to evaporate in the air, the larger to agglomerate.
Two important nebulizer types are based upon jet generation and ultrasonic generation.
Risk considerations Nebulizers:
Condensed water forms traps impeding intended gas flow. Jet humidifier may introduce
high pressure in the breathing system. Growth of micro organisms in humid atmospheres.
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GAS MEASUREMENTS
Important variables characterising a gas mixture are pressure, flow (volume) and
concentration. Each desired variable has a separate transducer being able to selectively
measure the variable. Probe is perhaps the broadest concept; sensor is a little more
specific comprising the transducer and its protective housing perhaps with a sampling
part bringing the transducer in correct position with respect to the gas to be measured.
Sensor considerations include biocompatibility/disinfection/sterility. Transducer is the
part which converts the energy correlated to the variable into (usually) electric form.
Sensor response time
A response time better than 0.1 – 0.2 s is needed in order to obtain in-vivo undistorted
real time curves during patient respiration.
Sensor selectivity
A measuring instrument is constructed to be maximum sensitive to the intended (desired)
variable(s). By selectivity we mean the degree of reduced sensitivity to other variables.
Important unintended variables interfering with the measurement may be temperature,
ambient pressure, water vapour, alcohol etc. Medical gas measurements are usually done
in multigas systems, and interfering variables may then be all other possible gases than
the intended. Example: The sensitivity of a paramagnetic oxygen analyzer to nitrous
oxide (NO) (unintended) in an oxygen (intended) gas mixture. Selectivity is dependent on
the measuring principle and whether the sensor is directly sensitive to the intended
variable or the measured variable is recalculated to the desired variable. Example:
Oxygen sensor sensitive to partial pressure [kPa], result to be given as oxygen saturation
[%].
A special case is water, as vapor or condensed water droplets. There are two problems:
the sensor may be sensible to water vapor in an unintended way interfering with he
results. Or the sensor function is disturbed by being covered by liquid water. Some
instruments dry the sampled gas before it is measured. The concentration of the intended
gas may then be too high relative to what it is in the patient airway. In many cases the
sensor is heated to cancel water condensation.
Sensor calibration
Single point calibration, for instance a zero point calibration with an oxygen sensor
placed in pure nitrogen. Two-point calibration with the oxygen sensor placed in pure
nitrogen and then in pure oxygen; three-point calibration (checking linearity) adding
measurement in air. NB! The measurements may be disturbed by interfering variables
such as ambient pressure, electromagnetic radiation, relative humidity, mechanical
position etc.
Calibration intervals are dependent on sensor stability and needed accuracy.
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Gas sampling
Usually we are interested in the variables (flow, pressure, concentration) as near
(proximal) to the patient as possible. The choice of sampling position is of interest, from
proximal somewhere near the Y-piece to distal inside the ventilator. In the ventilator a
sensor is well protected, at the Y-piece it must stand rough handling and the cables are a
source of annoyance. These factors are well illustrated in the sidestream and mainstream
sampling systems.
Sidestream sampling
Figure 11 Sidestream sampling to a multigas analyzer
A constant gas flow is aspired through a thin tube from the breathing system as shown in
Fig.11. The choice of internal diameter and sampling (aspiration) flow rate must be
carefully considered and be based on a compromise (See problem 12). The sampling flow
rate [mL/min] should of course be small in comparison with the respiration flow rate.
Even with a small sampling flow rate the gas velocity should be high so that the delay
between the sampling and display instants is small. The gas concentration is a function of
time at the sampling position on the respiration tube, but a function of position along the
length of the sampling tube. The sampling flow is continuous, and along the tube length
there will be gradients according to the concentration variation. A longitudinal diffusion
process will occur, smearing the peaks out, but leaving the area under the curve
unchanged. The curve will be softened, and the high frequency components reduced (low
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pass (LP) filtering).To reduce this LP filter effect the gas velocity in the sampling tube
should be as high as possible.
A HME (Heat & Moisture Exchange) filter is often used to reduce patient water and
heat loss. Such a filter also reduces mucus (slime) in the distal part of the breathing
system, so the sampling position should be at the distal part of the HME filter. The
minimum internal sampling tube diameter is related to the trouble with tube obstruction.
Sampling directly from the inspiration tube will be much easier, but expiration data is not
obtained.
At the entrance of the instrument the sampled gas must pass a trap to take away
mucus and water. The measuring chambers are accordingly spared for contamination. If
the sampling point is chosen to be on the right (patient) side of the HME filter the risk of
sampling tube and instrument contamination is larger.
Due to the suction pump there will be an increasing negative pressure in the sampling and
measuring system from the Y-tube along the sampling tube, the trap, and the measuring
chambers to the pump. (see problem 12.). The aspired gas can be brought back to the
breathing system, or sent to a scavenging system. Paramagnetic oxygen instruments
mixes the unknown gas with a reference gas (room air), and the returned gas to the
breathing tube is therefore not the same as the aspired gas. By sampling just a small gas
volume pr min the unknown gas is not very disturbed.
Some characteristic properties of sidestream sampling:
Different measuring chambers may be mounted in series to form a multigas analyzer.
Sampling gas is aspirated from the breathing system, this poses problems in paediatric
anaesthesia particularly.
Sample gas flow rate must be small and the tube thin to obtain sufficient high gas
velocity so that concentration gradients along the sampling tubes are not smeared out.
Measuring results not in real time but delayed e.g. 0,2s.
Thin sampling tube can easily be obstructed. Humidity and mucus must be filtered out in
a special trap before the gas can be allowed into the measuring chambers.
The transducers are well protected inside the instrument.
Mainstream sampling
The sensor is situated in the mainstream as shown on Fig.12. Even if there are no thin
sampling tube there may also here be problems with humidity, poor transparency and
mucus build up. The sensor must be heated to avoid water condensation. Characteristic
properties of a mainstream sampling system are:






Measurement in real time.
Difficult to realize multigas analysis in one sensor head.
No thin tube which can be obstructed.
Sensor head optics must be cleaned frequently.
Sensor head increases dead space.
Sensor head heavy, fragile and warm (anti condensation precaution).
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Figure 12 Mainstream sampling
Sampling inside the ventilator
Sidestream and mainstream sampling are parts of the breathing system proximal to the
patient. However, the sampling position may be moved to inside the ventilator/
anaesthesia machine avoiding extra cables/tubing outside the box. But then the results do
not necessarily reflect true patient data. In a test procedure before use on the patient the
Y-tube may be connected to a special connector on the machine so that the machine can
apply gas a short moment and measure volumes and compliances of the breathing system.
In this way the machine to a certain extent can calculate the true patient data continuously
during use. In such a system the tubing must not be changed during use.
Gas concentration measurements
Three different measuring principles in widespread use are shown in Table 4.
Table 4 Three measuring principles
Measuring principle
1a Spectrophotometric
1b Spectrophotometric
Chapter 9
medi
um
gas
variables
time comments
const
CO2, H2O, 0.1s capnography included
agent vapors
blood O2
1-10s also
in-vitro
cuvette-
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pulsoximetry
2a Paramagnetic, contin.
2b Paramagnetisk, pulsed
3a El.chem.
fuel
cell,
membrane covered
gas
O2
gas
O2
gas or O2
liquid
10s
0.2s
30s
3b El.chem. polarographic
membrane
covered
(Clark)
3c El.chem.
membrane
covered (Severinghaus)
gas or O2
liquid
0.120s
gas or CO2
liquid
30s
oximetry and in blood gas
analyzers
sample gas unchanged
sample gas changed
limited lifetime, drifts and
frequent calibration, single
use
membrane & el.lyte change
and reuse, used in blood gas
machine
used in blood gas machine
Table 4 shows different in-vivo and in-vitro principles for blood gases. This is of interest
for quality control, but also raises questions with respect to which result is the most
correct one. There are e.g. often discrepancies for the same patient between the oxygen
results obtained with pulsoximeter (in-vivo), the sidestream paramagnetic analyzer (invivo) and blood samples analyzed on a stationary blood gas analyzer (in vitro). There are
many reasons for these differences: the handling of the in vitro samples from the patient
to the measuring instrument perhaps in a remote laboratory, different calibration,
recalculation of data obtained with different measuring principles. In order to assess such
problems it is important to know the different measuring principles and their
characteristic properties. In this chapter a survey is therefore given of the different
measuring principles and a more detailed description of gas analyzers. The non-gas
instrumentation is more detailed described under clinical chemistry and intensive care.
Gas spectrophotometry
It is well known that he colour of oxygen-rich blood is reddish, of oxygen-poor blood
bluish. When the photon absorption is within the visible spectrum such colour changes
illustrates the spectrophotometric principle based upon the selective absorbance of light.
Spectrophotometry is measurement of colour (colorimetry) and it may be used both in
liquids and gases. Many gases are transparent and colourless in the visible spectrum (e.g.
nitrogen, oxygen, water, argon) meaning that there is no photon absorption in that range.
In the infrared (IR) spectrum however many of the gases of interest do absorb. Each gas
absorbes in a characteristic way (Fig. 13), so that selective measurements are possible.
The following gases show selective absorption in the IR spectrum: CO2, N2O, water,
anesthetic agent vapours. IR gas monitors measure the absorption at several wavelengths
in the 3.3 or 8–12 µm areas and then solve a series of simultaneous equations to calculate
the concentration. Multiple wavelengths are required in order to identify the anesthetic
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Figure 13 IR absorption spectra for some anaesthetic agent vapours.
Datex Ohmeda Division, Instrumentarium Corporation
gases used, and the 8–12 µm range is preferred as this represents the area of the infrared
spectrum where anesthetic gases show maximum absorbance (Fig.13). Automated
anesthetic agent identification is then possible. But often the instrument must be told
which gas is the intended; it is only the concentration which is unknown. The absorption
may follow the Lambert-Beers law:
Equation 5
I = I0 e-Lμ
or
ln(I0/I)=Lμ
if the molecules are much smaller than the wavelength. Referring to Eq.5, I is the
measured photon flux, I0 the input flux to the sampled gas, L absorption length in the gas
and μ the linear attenuation coefficient (the product Lμ is the absorbance and must be
dimensionless). If there are larger particles in the sample other attenuation mechanisms
related to scattering will take place, not necessarily obeying Eq.5.
The linear attenuation coefficient μ is dependent on the gas, wavelength and
concentration [mol/L]. Concentration is the measured variable, but the displayed variable
may be percentage [%] of the total volume. If the total pressure in the measuring chamber
changes because of the suction sampling system or the barometric pressure, the
concentration or partial pressure is proportional to the total pressure, but the percentage
is independent.
Oxygen and nitrogen gas can not be measured spectrophotometrically because these
gases do not have characteristic absorption bands in the optical spectrum.
Fig.14 shows how multiple wavelength measurements are possible. The sampled gas is
aspired into the measuring chamber, where some photons from a filtered IR source are
absorbed by the gas and others reach the IR detector on the other side. A rotating filter
wheel inserts 6 different filters corresponding to different gases in rapid succession. The
IR detector must be fast enough to discriminate between each filter. Each rotation also
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involves a reference filter and a stop filter. Partly overlaying spectra can be separated by
using multivariat analysis on the different wavelengths measured data.
Figure 14 Multigas spectrophotometric gas analyzer with rotating filter wheel
The better the optical systems with filters and lenses, the better the selectivity. Filters in
the 10 μm range do not look very transparent to the human eye!
Paramagnetic oxygen gas analyzer
Oxygen is one of the few gases which are paramagnetic. Paramagnetism and
diamagnetism are the weak magnetic forces in contrast to ferromagnetism. Most
substances are diamagnetic, meaning that the substance is repelled by the magnetic poles.
Oxygen however is paramagnetic and will be attracted to the magnet poles. Around the
poles of a permanent magnet the oxygen concentration is therefore higher than elsewhere
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in the room. Unpaired electrons in the outer
shell give the atom magnetic properties; this The force F [newton] on a magnetic
is the case for oxygen. Table 5 gives the moment m [Am2] in a magnetic field of
magnetic susceptibility (intensity of flux density B [tesla=weber/m2]:
magnetization) of some respiratory gases
and shows that the selectivity for oxygen gas
Equation 6 F = grad (m  B)
is due not to the gas being paramagnetic (+
sign), but that it has a more than 100 times Accordingly, if the magnetic field B is
higher magnetic susceptibility than many strong but constant there is no force on
(not all!) of the other gases. As Eq.6 shows, m.
the force will be proportional to the
magnetic moment, which will be proportional to the oxygen concentration [mol/L] or
partial pressure [kPa], and therefore also to the chamber total pressure.
Table 5 Magnetic molar susceptibility m of respiratory gases. SI unit: [m3/mol], but
according to customary practice, cgs units are used and given here as m/10-6cm3mol-1
(CRC Handbook of Chemistry and Physics).
gas
oxygen O2
nitrogen N2
nitric oxide NO
nitrous oxide N2O
nitrogen dioxide NO2
water vapour H2O
carbon dioxide CO2
argon
m
+3449
-12
+1461
-18.9
+150
-13.1
-21
-19.3
m
relative
+100
-0.35
+42
-0.55
+4.3
-0.38
-0,61
-0.56
The measuring principle is shown in Fig.15, it was invented by Nobel laureate Linius
Pauling in 1946; The Beckman Oxygen Analyser. A diamagnetic gas (e.g. nitrogen) is
enclosed in two spheres fixed to the end of an arm which is fixed to a suspended metal
wire so that the arm can rotate. The rotation is read by a light beam reflected from a
mirror fixed to the arm. The magnetic field is from permanent magnets.
Increased oxygen concentration disturbs the magnetic balance and the spheres are
driven out of the magnetic field. The time response is slow, e.g. 5-15s, because of the
large chamber volume and the mass of the dumbbell. A somewhat quicker version is
made by fixing a magnetic coil to the arm. The coil is supplied with an electric current
from a servo system so that the bell positions are virtually unchanged under different
oxygen concentrations. The measurement result is read from the coil current.
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Figure 15 Paramagnetic oxygen analyzer. The construction is enclosed in a tight box with inlet
and outlet for the gas to be examined, the reference gas is enclosed in the two spheres.
Fig.16 shows a more rapid system using a pulsed magnetic field. It is a differential
measuring principle, using a differential pressure transducer (microphone) to detect the
difference in magnetic action on the unknown gas and a known reference gas, usually
room air. Increased concentration of oxygen leads to increased suction of the oxygen
molecules into the magnetic gradient field zone, and therefore reduced pressure outside
the zone. In order to have a response time < 0.1s, the magnetic field is switched at a
frequency of a few hundred hertz. The differential measuring principle implies that the
gas output is not the same as the aspired unknown gas.
An important advantage with the paramagnetic measuring principle is long term
stability and minimal need of maintenance (if the measuring chamber is kept clean). The
differential principle of the pulsed type may also be advantageous. However, the
measuring chamber is heavy (the magnet) and must therefore be mounted in a sidestream
sampling system.
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Figure 16 Paramagnetic oxygen analyzer using pulsed magnetic field. Gray lines are
tubes.
Multigas analyzers
Fig.11 showed a multigas analyzer in a sidestream sampling system. The gas first arrives
to the spesctrophotometer where CO2, N2O, water and anesthetic agent vapors are
measured. Then the gas sample is mixed with the reference gas and oxygen concentration
is measured.
Problems with such instrumentation are the risk of contamination of the measuring
chambers. Water condensation in the chambers must be avoided, by warming the
chamber and/or filtering the aspired gas before it enters the chambers. This filtering
system is an important part of the construction and determines to a large extent the
robustness of the system. If mucus and other contaminations still reach the measuring
chambers, special rinsing liquids plus days of dry gas flushing may bring the instrument
alive again.
Other gas measuring principles
Membrane covered electrochemical electrodes for oxygen and carbon dioxide are
described in chapter 10.2. They can be used for measuring partial pressure both in liquids
and gases.
Mass spectrometers (MS) are large and expensive instruments which therefore must use
sidestream sampling. A MS consists of a vacuum chamber where the gas is ionized. The
ions are accelerated and focused in an electric field by suitable electrodes and then enters
a magnetic field zone where they are deflected according to their mass. The selectivity is
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very good, but molecules with identical mass numbers will interfere with each other, for
instance N2O and CO2 which both have mass number 44. It can measure several different
gases fast enough to measure respiration in real time. The instrumentation maintenance is
expensive, and the instrument is best adapted to research or in a central installation
serving several patients simultaneously.
More on measured and calculated variables
If the total pressure of a gasmixture e.g. air is increased (Fig.17), the partial pressure of
oxygen (pO2) increases, but the oxygen volume % is constant. Sometimes we are
interested in the volume %, sometimes in the pO2. Let us take the example that oxygen is
measured by a partial pressure (pO2) sensitive sensor, but the result is recalculated to
volume %. The volume % will display false values if the total pressure varies. This
illustrates that the transducer working principle should always be known.
Figure 17 Closed variable volume
Another example is the one shown in Fig.7, where a water dish is inserted into a dry gas
chamber. Then the % oxygen will fall, but not the pO2. In medicine the inspired gas will
always have a lower water content than the expired gas.
Gas pressure sensors
Pressure is force per area: P = F/A. The classical measuring device is a liquid filled tube
measuring level difference. Often it is formed as an U, if closed in one end it measures
absolute pressure, if open it measures relative (gauge) pressure. If it is filled with water
cmH2O may be the preferred unit, if filled with mercury mmHg may be preferred. A
mechanical pressure measuring instrument is called a manometer. A pressure sensor
alone is not a manometer.
Fig.18 shows two sensors both based upon a precision moulded thin membrane as a
part of the sensor house, cf. Fig.2.4 in chapter 2 Webster. The membrane thickness and
material properties determine the deflection pr change in pressure level. The thinner the
membrane the more sensitive the sensor, but also the more fragile the membrane. The
deflection is in principle not a linear function of pressure. The deflection also determines
the compliance C of the sensor: C = ΔV /ΔP [mm3/kPa]. ΔV is a volume which implies a
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transport of inert substance (gas molecules in this case) to and from the membrane
according to the variable pressure. Compliance is therefore important for the dynamic
response, not for static or slowly varying pressures. A high quality sensor shall have a
stiff membrane and low compliance. The membrane deflection may be measured by a
piezoelectric beam (left) or an optical reflection system (right). The sensor principles can
be operated with the interior volume closed. Then absolute pressure is measured, but if
the closed chamber is gas filled this introduces a temperature dependence according to
Eq.3. With the interior open to the surrounding air relative (gauge) pressures are
measured. With a tube connected to the interior we have a differential transducer which
e.g. may be used for flow measurements, see under gas flow sensors. A dome closing the
volume above the membrane may be connected to a second tube to adapt it better for
differential measurements. The dome can be made of plastic with a soft membrane
interfacing the sensor membrane. The complete dome may be sterilized so that it can by
used in invasive applications. The whole sensor house may also be sterilized for use as a
tissue implant.
Figure 18 Pressure sensors. Left: piezoelectric transducer with an optional dome to be
positioned so as to form a closed volume above the membrane. Right: optical
transducer
Important specifications for a pressure sensor are: sensitivity, compliance, linearity
(membrane property), hysteresis, membrane absolute maximum pressure, size,
temperature zero drift, temperature sensitivity drift, temperature range, long term
stability, negative pressure properties, sterilizability, biocompatibility.
Gas flow sensors
Rotameter and turbine flow meters, mainstream
A rotameter consists of a slightly conic vertical glass tube with a bobbin at
the bottom. With gas flow through the tube the bobbin lifts and starts to
rotate. The scale is engraved on the tube external surface so the flow rate
can be read. The rotameter is the classical instrument for measuring flow,
however it is non-linear, gas viscosity dependent, critically dependent on the
conic boring and the bobbin size, and it is a unidirectional device. The
calibration is valid only for one gas, and it must be used in a vertical
position.
The free bobbin may be replaced by a propeller or turbine with a fixed
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axis. Such a device is bidirectional, and the rotations pr minute may easily be read by an
optical system so that an electronic flow rate signal is available.
Hot wire flow sensor, mainstream (also Fig.8.13 in chapter 8)
Figure 19 Hot wire flow meter with two termistors, cross section shown to the right
Two temperature dependent resistors (thermistors) are used in a tube, one of them is used
for measuring gas inlet temperature (Fig.19). The other is heated by an electric current at
the same time as its resistance can be measured (how?). The flowing gas with a lower and
known temperature cools the heated thermistor according to the gas velocity and the
temperature is followed by monitoring its electrical resistance. It is a spot sensor, and the
thermistor transducers can be made very small with thin wires, with just a small
disturbance of the flow profile. The construction is robust and lightweight, but the
position of the thermistors is critical. In its simplest form as shown it is a unidirectional
device. The sensitivity is dependent on the thermal properties, heat capacity in particular,
of the gas. It is sensitive to gas pressure.
Vane deflection flow sensor, mainstream
Figure 20 Vane flow sensor in a tube, cross section shown to the right
Vane deflection is dependent on gas velocity and density as well as gas pressure. Nonlinearity dependent on the back eddy behind the vane, less pronounced if the vane is soft
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and deflects. Direction sensitive. Vane disturbs the parabolic flow profile, and it is more a
cross sectional area sensor than a spot sensor. Transducer may be of a piezoelectric or
straingauge type.
Pitot flow sensor (also with remote transducer)
Figure 21 Pitot flow sensor in a tube, cross section shown to the right
The Pitot tube flow sensor is based upon Eq.8 (Bernoulli) and the so called kinetic part of
it: ½ v2. The sensitivity is accordingly dependent on gas density  (and therefore gas
pressure) and the calibration factor is dependent on gas type and the gas mixture. The
output is proportional to the square of gas velocity, and Fig.21 shows two Pitot tubes with
the tube opening with and against the velocity direction. The sensitivity is doubled with a
kinetic factor v2. Actually it is a gas velocity spot sensor, and a velocity profile has to be
assumed to obtain flow. The disturbance of the velocity profile, turbulence included is
considerable as small diameter Pitot tubes destroy the dynamic properties of the sensor. If
the tubes and transducer are low compliance components mainstream sampling with a
remote differential transducer is possible. The sensor is direction sensitive. The pressure
differential transducer may be based upon light reflection from the sensing membrane,
discuss other possible technologies.
Poiseuille flow sensor (also with remote transducer)
This sensor measures differential pressure ΔP across a well defined flow resistance, it is
also called a pneumotachometer. Because it is based upon the law of Poiseuille (Eq.2) the
sensitivity is gas viscosity dependent. The dimension of R in Eq.2 is [Pa/m3/s]. If the
mean pressure in the tube doubles the molar flow [mol/s] doubles, but R is constant
because gas viscosity has a surprisingly low pressure dependence. Volume flow [m3/s] is
therefore the intended variable of this sensor, not molar flow. The viscosity dependence
is a problem if it is a mixture of different gases; each gas will have its own calibration
factor.
The pressure difference is proportional to gas flow if the flow pattern is laminar. The
pressure difference is measured at the wall where the gas velocity is zero, and the
pressure difference represents the whole cross sectional area. Flow direction can be
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determined. If the sampling tubes and transducer have low compliance the transducer
may be remote as a part of a main stream sampling system, it may be then be realised as a
very robust system.
Figure 22 Poiseuille gas flow sensor (pneumotachometer)
The flow resistance can be obtained with or without a narrow passage zone as shown on
Fig.22. A narrow zone increases sensitivity ΔP/Q (Poiseuille). A so called Fleisch tube is
a special resistance with many small gas channels (capillaries) in parallel. Thin capillaries
will be vulnerable to accumulation of secretions or other contaminants and from the
condensation of water vapor. Therefore the tube must be heated if used on expired gas.
Doppler flow meter, mainstream
A usual ultrasound Doppler flowmeter for blood is not useful for ordinary gas
measurements. Blood flow can be measured because the blood cells are sufficiently large
(about 5-10 μm), small molecules do not give sufficient reflected signal strength.
However, in a gas the flow can be measured in transmission instead of reflective mode.
The velocity of sound will be higher if sound direction is in the gas flow direction. The
problem is that such transmission mode requires separate transmitting and receiving
probes on each side of the organ.
VENTILATORS (SUPPORT AND THERAPY)
When the patients are unable to breathe themselves they must have artificial (assisted)
respiration. As this can go on for many hours and days (intensive care units) it must be
taken care of by a machine, the ventilator5. We are talking about lightening the bagging
burden for medical personnel, giving them a “third hand”. In addition to this support
function the ventilator is used therapeutically by controlling pressure and flow to the
optimum condition for lung healing.
5
Respirator and ventilator are usually considered to be synonyms, perhaps with a certain tradition that
respirators are simpler devices, e.g. not electrically driven.
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As we have seen earlier in this chapter the natural lungs initiate inspiration by lowering
the diaphragm thus increasing the lung sack volume and creating a negative pressure in
the lung alveoli so that external gas is inhaled. The ventilator functions the opposite way,
during inspiration the ventilator creates a higher pressure pushing fresh gas into the lungs.
It is only the “iron lung”6 with the whole patient (except head) enclosed in a chamber that
reproduce natural physiological conditions: chamber reduced pressure initiates
inspiration.
An advanced ventilator must, like a pacemaker, have a demand modus. If there are no
efforts from the patient, the ventilator should be in control. But as the patient recovers or
wakens, the patient starts to breathe spontaneously, and the ventilator can let the patient
gradually take over. The respiration cycle time parameters are important: inspiration
time, pause, expiration time. An advanced respirator may be set either to volume or
pressure controlled mode. In the volume controlled mode the inspired volume is
measured continuously, and when it reaches a preset level the inspiration phase is
terminated. Pressure is also measured continuously, and the user has chosen values that
shall not be exceeded or shall result in an alarm. For instance a selected PEEP (Peak End
Expiratory Pressure) modus decides when to leave the expiration phase so that the lung
pressure does not drop below a critical point where the lungs might collapse. In the
pressure controlled mode it is the pressure reaching a preset value which triggers the
ventilator to end the inspiration phase. The choice is made from what is considered to be
best for the patient. And remember that the flow and volumes which we just have referred
to are different in different parts of the airways inside and outside the patient, the
sampling position is important as already stated in the subchapter on gas sampling.
Technology: piston, bag-in-bottle, servo
Small ventilators for use outside hospitals may be purely gas driven, but all advanced
ventilators are electrically driven. The main components are:
1. A pressure generating and controlling device (gas under pressure or electric
pump)
2. A cycling device with timer, changing the modus between inspiration, pause and
expiration
3. Sensing elements and displays
The most direct version is to put the bag of a manually operated breathing system
(Fig.10) system into a bottle, and control the pressure in the bottle from an external
ventilator supply. Another way is to let a motordriven piston supply the pressure cycle.
Fig.23 illustrates a servocontrolled system, shown during the inspiration cycle.
6
Used for patients with poliomyelitis in the large epidemic outbreak in the 1950ies.
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Figure 23 Servocontrolled ventilator shown in the inspiration cycle
The fresh gas enters a pressure-controlled static reservoir. The pressure setting there
determines the maximum pressure which can be supplied to the patient, and is therefore
also a safety feature. The gas then enters the inspiration servo-controlled part. The flow in
this example is measured by the deflection of a vane and compared with the set-point
coming from the electronic control box. The minute volume is a basic parameter set by
the operator and used in the control system. Deviations result in a servo correction signal
which is sent to the control valve. The set point can represent different wave forms, not
just a square wave on-off function. The inspiration pressure is measured and the
information sent to the control box. Dependent on the mode chosen by physician the
pressure can control the servo e.g. during CPAP operation (Continuous Positive Airway
Pressure). During long-term patient treatment in an intensive care unit the gas must be
humidified. In series with the humidifier a vaporizer for anaesthetic volatile drugs may be
inserted for use in the operating room. When the control box so determines the
inspiration phase is ended, the inspiration valve closes and the expiration valve opens.
Also here the expiration flow is measured continuously and the control box can set up
any flow curve preferred. The expiration pressure can also control the valve if the
ventilator is set in such a modus (e.g. PEEP Positive End-Expiratory Pressure). Both
pressure measuring systems may be coupled to alarm circuits if a preset pressure level is
exceeded.
The ventilator shown on Fig.23 has no rebreathing system.
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Compression losses
Figure 24 Compression loss model
The pressure P in the patient system increases during the inspiration cycle when a piston
is pushed in as shown on Fig.24. The lungs are gradually filled until the machine ends the
inspiration cycle. Let us simplify and only consider the inspiration and expiration tubings
and with zero compliance. With a certain ventilation volume ΔVresp, the pressure build up
is dependent on the volume of the two tubes V1 = Vi + Vex. According to the universal
gas law P2=P1V1/V2 in a closed system. If V1= 1L, P1=100kPa and ΔVresp is 0.2L, then
P2=1·100/0.8 = 125 kPa. If the lung compliance CL = ΔVL/ΔP is low and the airway
resistance R is high, the rise in pressure ΔP = 25 kPa may result in a very low lung
ventilation ΔVL. The ventilation of the lungs may be much smaller than the ventilator
setting. Therefore: the larger the tube volume with respect to the ventilation volume, the
less part of the ventilator gas enters the lungs of the patient. This is called ventilation
loss, and must be taken into consideration especially with small and stiff lungs (children)
or long tubes.
Risk considerations Ventilators
Difference between inspiratory system and expiratory system with respect to humidity,
mucus (slime) and need of disinfection and sterility, especially in one-way breathing
systems.
Transducer breakdown. Gas leakage, wrong tube connection. System control before a
new patient is connected to the ventilator. Stops, electric power failure, gas delivery
failure. Accidental change of settings.
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ANAESTHESIA MACHINES (SUPPORT AND THERAPY)
Anaesthesia may be done by the injection of an agent into the blood and/or by inhaling a
substance like nitrous oxide or a volatile vapour like sevoflurane. In addition to gas
anaesthesia, anaesthetics are also given directly into the blood stream. Here we will
describe the anaesthesia gas / vapour delivery system as a separate machine, although it
may be an integral part of a ventilator and/or a patient monitoring system. Its main
purpose is to deliver fresh gas to the patient via a breathing system and perhaps a
ventilator. It may comprise also the gas scavenging system and the suction device. The
suction described later in this
chapter is an important additional
device, and the breathing system
may be directly coupled to the
machine or also to a ventilator
supplied by the machine. The
multigas analyzer may also be on
the same trolley.
Fig.25 shows the main components:
the supply of medical grade air,
N2O and O2 gases, the gas mixer
with flow monitors, the vaporizer
and some safety devices.
The gas supply may either be
from local gas bottles filled to high
pressure e.g. 60 bar (N2O) or 150
bar (O2), and equipped with
manometers, pressure reduction
valves and simple gas flowmeters.
Or it may be taken from the hospital
installed gas pipeline system at
medium high pressure (3-7 bar).
Figure 25. Anaesthesia machine
The gas mixer has individual gas
flow sensors [L/min] for each gas,
measured before mixing. Flow setting is adjusted with individual spindle valves. The gas
mixer is connected to the vaporizer, where anaesthetic volatile vapors may be added such
as: Halothane, Enflurane, Isoflurane, Sevoflurane, Desflurane and ether.
Often the machine comprises a gas scavenging system and suction for clearing
airways. The surgeon may have their own suction for use in the wound.
Risk considerations Anaesthesia machine
Loss of oxygen is of course critical, and a special oxygen flush can supply large direct
oxygen flow (NB! lung pressure). Functioning of the suction may be critical. Anaesthetic
dose is important, and concentration measurements in the breathing system near the
patient are very useful.
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SPIROMETERS (DIAGNOSIS)
A spirometer is an instrument for measuring lung volumes. There are basically two
different types: the water sealed and the pneumotachometer models. This is an
instrumentation which usually need not be sterile, disinfection procedures are sufficient.
Water sealed models
Fig.26 is an outline of the system. The patient is connected to the respiration tube via a
tight mask. With a few breaths the patient respires into a closed volume dominated by the
gas drum. The drum rises and sinks with minimal friction following the respiration. The
drum is balanced by a counterweight, and the vertical movement is registered by a pen
Figure 26 Spirometer, watersealed
fixed to the connecting wire. The scale is graduated in litre. The respirogram is then
drawn on the paper passing under the pen. Because of the inertia of the system the
instrument is best adapted to static or slowly changing volumes. Precautions must be
taken to avoid problems with temperature changes and humidity with condensation. An
important advantage of the instrument is stability and ease of calibration; it is well suited
to be a standard reference instrument in a laboratory. The spirometer represents a closed
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volume of maximum compliance. In the form shown on Fig.26 the spirometer can not
determine the residual lung volume. By using a dilution technique with a tracer gas 7
absolute volumes can be determined.
Risk considerations
The breathing system in its simplest form as shown is closed and without CO2 absorber,
the measuring time is accordingly very limited. It may have a bothersome cleaning and
disinfection procedure between each patient.
Electronic models
The electronic models may be very small,
wireless and convenient, and they are also well
adapted to dynamic measurements e.g. of FEV
(Forced Expiratory Volume, often during the
first second, FEV1). The model may be just a
mouthpiece with a differential pressure
transducer coupled to Pitot tubes or a flow
resistance in the form of a narrowing tube or
Fleisch flow resistor, se earlier in this chapter. In
this form they are also known as
pneumotachometers. The signal from the
transducer may be transferred wirelessly to a
computer system, where signal processing may
produce e.g. volumes by integration of flow,
peak flow values etc.
Risk considerations Spirometers
The tubes in contact with the mouth need not necessarily be sterile, but disinfected or
acquired for single patient use.
WHOLE BODY PLETHYSMOGRAPHS (DIAGNOSIS)
Determination of absolute lung volumes and airway resistance can be performed in a
whole body plethysmograph, Fig.27. The patient is closed into a chamber and breaths
into a mouthpiece with a pressure sensor and a flow outlet where the flow can be stopped
by a closing actuator. Chamber volume Vc is known, and the chamber pressure Pc and
mouthpiece pressure Plu are measured. At the end of a normal expiration the gas flow is
7
tracer means a substance not harmful to the patient and the concentration of which can be conveniently
determined.
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closed, and the patient performs forced ventilation against the closed mouthpiece. The
variations in chamber pressure ΔPc and airway pressure ΔPlu are measured as static
values (sufficient time at inspiration and expiration effort). In this way it is no flow at the
sampling moment so that the mouthpiece pressure is equal to the lung pressure. By
considering the whole body but the lungs incompressible the forced respiration pressure
diminishes the lung volume and increases the chamber volume by the same amount, and
reduces the chamber pressure Pc. By using Boyle-Mariottes law it is possible to show that
the lung volume (residual volume included) Vlu is given by:
Equation 7
Vlu = - Vc ΔPc /ΔPlu
Risk considerations
Some patients have a problem with being in a closed narrow chamber (claustrophobia).
Mouthpiece and airway must be disinfected or be of single-use types.
Figure 27 Whole body plethysmograph
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HYPERBARIC OXYGEN THERAPY
Figure 28 Hyperbar chambers
Hyperbaric oxygen therapy is performed with the patient in a closed chamber and by
slowly supplying pure oxygen washing out the nitrogen of the air in the chamber and in
the patient. Slowly the oxygen pressure is increased up to e.g. 2.5 bar absolute pressure.
At the end of treatment the pressure is slowly taken down to atmospheric pressure and the
pure oxygen replaced by air.
The chamber may be a complete room where patients can walk around (like the models
used for divers). In hospital smaller units (Fig.28) are used for each patient and several
patients can be treated simultaneously in one department.
Oxygen itself does not burn or explode, but it increases the burning rate of
combustible materials. A paper strip burns 30% faster at 25% than 20% oxygen. NASA
allows 25.9% in its space shuttles. Oxygen rich environment is defined as >25% or 27.5
kPa partial pressure by IEC (IEC 2005). 100% oxygen implies explosive combustibility.
In hospitals oxygen rich environment is used in infant incubators (chapter 13.7). It is well
known from such use that too much oxygen is dangerous for the infant, in particular the
eyes. The toxic property of oxygen is used in hyperbar therapy e.g. for enhanced healing
of selected problem wounds.
Risk considerations Hyperbaric chamber
Hyperbar oxygen therapy can only be used if all the necessary precautions have been
taken. These precautions must have been taken before the closing of the chamber for
oxygen pressure build-up.
 Pressure chamber certified for the necessary pressure, safety margin included.
 Most materials change from normal inflammable to explosive in pure oxygen.
 Prohibited to use inflammable liquids/vapours e.g. for disinfection just before or
under therapy.
 Use of open flame absolutely forbidden.
 Antistatic precautions inside the chamber so that no spark can ignite an explosion.
 Special precautions for patient monitoring equipment.
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VENTURI SUCTION SYSTEM (SUPPORT)
Suction may be very important during surgery and intensive care. The operating field
must be kept free from blood and liquids hindering visual control. During intensive care
it is critical to keep the airways free from mucus and other obstacles. The suction is a
vital part for respiratory systems.
Figure 29 Venturi suction system
Fig 29 shows the main components of a Venturi suction system, it is gas (air) driven.
Other models may use an electric pump to create the vacuum. The operator holds the
suction tube handle in the airways or as a sterile instrument in the wound. The aspired
debris is assembled in the bottle. The bottle is kept at low pressure via a second tube
connected to the Venturi. For the operator it is important that suction is available when
needed, and that the degree of vacuum and flow can be chosen. The suction must not be
too strong so that tissue is destroyed, but strong enough to ensure safe removal of debris.
The tubing must be reinforced so that it does not collapse at high vacuum. It must be
sterile for many of the procedures.
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The suction device can be characterised by static and transient parameters. The static
parameters are max. vacuum (min. absolute pressure) and flow capacity as a function of
drive gas pressure. These parameters are determined by the Venturi construction and the
driving gas flow rate. The dynamic parameter is the transient flow for instance in the
starting phase if the operator has closed the suction inlet for vacuum build up. Flow and
volume is then also dependent on the bottle volume which again is dependent on the
degree of bottle filling.
Venturi / Bernoulli principle
The Venturi principle is interesting and somewhat contra intuitive: Is it possible to create
vacuum pressure from high
pressure? The clue is to
make the gas molecules
pass a nozzle (cone) where
they
are
accelerated.
8
Venturi developed this
practical
method,
9
Bernoulli
had already
explained it from his
discoveries in kinetic gas
theory and mathematical
modelling, see Eq.8.
As Fig.29 shows, a high
pressure gas supply is
coupled to the Venturi,
where the gas must pass the
cone. The increased kinetic
energy during molecule
acceleration is taken from
the gas pressure. The local
Figure 30 Vacuum pressure as a function of static
pressure
is
reduced
suction flow
(Bernoulli text box) and
picked up by the perpendicular tube there. Notice that the Venturi outlet contains both the
driving gas and the aspirated gas from the bottle.
8
Giovanni Baptista Venturi, 1746-1822, Italian physicist, studied hydraulics, sound and colours.
Daniel Bernoulli, 1700-1782, part of an important Swiss scientist family studying astronomy,
mathematics, medicine, hydrodynamics.
9
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Equation 8 Bernoulli
Ps + ½ v2 + gh = constant
Ps = static (v=0) pressure [Pa].  = density [kg/m3]. v = velocity [m/s]. g =
acceleration due to gravity [m/s2]. h = height difference [m].
Validity range: Laminar flow, any geometry, valid at any point along a line of flow,
gases or liquids, no frictional (viscous) losses.
Actually the Bernoulli equation is about the conservation of energy along a flow line,
but it is usually given as here in terms not of energy, but pressure.
Fig.30 shows the pressure/flow curve for a Venturi driven by a gas pressure of 5 bar at 38
L/min. The bottle between the Venturi and the patient will act as a capacitor, so the flow
at the suction handle may be different from the Venturi suction flow. If flow is stopped at
the suction handle the Venturi will start emptying the
suction system. The vacuum pressure will increase Suction and aspiration
gradually until the Venturi no longer can draw any more are synonyms.
gas molecules from the system. The system can be
characterised with a time constant, the larger the bottle volume and the less the Venturi
flow, the longer the time constant. At zero suction flow a static (maximum vacuum)
pressure level is reached. With open flow suction handle maximum flow will occur, and
the vacuum pressure in the bottle will be small. Taking the time constant into
consideration the operator can chose at which vacuum level suction shall start. The time
constant will be dependent on the degree of liquid filling of the bottle.
The static vacuum pressure may be too high for certain procedures; the sudden start from
closed to open suction head may be too violent for the tissue concerned. A false air
leakage device between the pump outlet and the room air can be inserted so that max.
vacuum is reduced, without influencing the suction capacity at lower pressures.
Dynamic performance analysis
The suction system is a simple medical device which lends itself well for the use of
simple models to understand its function better. A usual practise is to use electronic
equivalent circuit models, Fig.31, as knowledge of electric network theory is widespread.
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Figure 31 Equivalent electrical circuit for a dynamic suction system
Ohms law is ΔV=RI , and the equivalent pressure formula is ΔP=RQ. A voltage source is
therefore the equivalent of a suction pressure pump. The source voltage is designated as
V, but V is not in [volt], but in pressure [Pa]. The output is negative, so that direction of
flow Q (current I) is suction. With a series internal resistor Ri the voltage source becomes
non-ideal. The pump vacuum pressure will then be dependent on flow, in agreement with
Fig 30. The resistance of the tubings is not in [ohm], but [Pa s/m3] and can be calculated
with the Poiseuilles formula. The bottle is modelled as a capacitor, and its capacitance is
not [farad], but molecules/pressure, that is [mol/Pa] or [V/RT] where R is the universal
gas constant (≈8 [J/molK]). The output is
suction flow [m3/s or L/min].
Adiabatic gas law
Electric formulas such as Ohm’s law or the
relaxation time constant  =RC can now be The thermal effect of a gas volume
applied. The formulas are electric, but the expansion is described by the
quantities mechanical. From Fig.31 it can for adiabatic gas equation:
instance be seen that the time constant  for
Equation 9
T1 ·V1k = T2 ·V2k
attaining vacumm in the bottle is smaller with
closed handle ( = Rp||Rs  C) than with open
where
handle ( =Rp  C).
T = temperature [o Kelvin]
Example: Find the tube-bottle time constant .
The tube is 1m long with inner diameter 20 mm
and coupled to a closed bottle of volume 10L. R
is found from Poiseuille (viscosity 10-3) to be
0.3 106 [Pas/m3]. C is V/RT equal to 4 10-6
[mol/Pa]. =RC is equal to 1.2 seconds.
For consideration: The maximum suction flow
in [L/min], is it greatest in air or in water?
Chapter 9
V = volume [m3]
K = constant dependent on
gas, e.g. 0,4 for O2
Validity range
Closed volume thermally isolated
(adiabatic condition)
Ideal gas (far from condensation)
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Risk considerations Venturi
Gas dependent, not electricity dependent
Obstacles in the Venturi exit filter reversing vacuum suction pressure to positive blowing
pressure.
Using a tube of a diameter so that all debris may pass.
Room air pollution, central vacuum installation ?
Sterility, disinfections
CRYOTHERAPY
Cryo- is a word of Greek origin meaning cold and is the antonym to thermo- (thermostat
– cryostat). The cryo technique is used in general surgery and by ophthalmologists and
dermatologists to destroy tissue. It is often an ablation method, meaning that the dead
(necrotic) tissue is not cut out and taken away, but left in-situ. But it may also be a
technique where frozen tissue can be taken out as an alternative to scalpel surgery.
Once the cells are
destroyed, components of
the immune system primarily the white blood
cells - clear out the dead tissue. A killing mechanism is ice formation only outside a cell
that causes the cell to shrink as it gives up water by osmosis to replace the water that has
Figure 32 Cryo principle according to a Joule-Thomson
capillary model.
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turned into ice. As the area thaws, water rushes into the shrunken cell and causes it to
burst. For this reason, cry therapy often consists of a series of steps in which the tissue is
repeatedly frozen and thawed. At lower temperatures intracellular ice formation is
important, below approximately -40°C intracellular ice crystals begin to form that
destroys the cells completely. Tumour cells also die when their blood supply is choked
off by ice formation within small tumour vessels.
Figure 33. Principle
components of cryo
equipment according to
Joule-Thomson
The cryo source is a gas under pressure at room temperature. The effect is based upon the
temperature drop in an expanding gas, the Joule-Thomson effect. An expanding gas
performs work, and the energy is taken from the internal heat energy of the gas, and the
gas temperature drops. The basic model is described by the adiabatic gas equation 9 (see
text box). In the original Joule-Thomson experiment gas was flowing through an
insulated pipe with an obstacle in the middle, in the form of a porous disk or a silk
handkerchief. The temperature and pressure were measured on each side of the disc. The
usual practical construction is to replace the disk by a capillary, Fig.32. The high pressure
room temperature gas is brought into the capillary so that one end is at e.g. 67 bar and the
outlet at 1 bar. The pressure drop along the capillary is due to the viscous losses as
illustrated by the law of Poiseuille. These losses generate heat reducing the cryo effect.
Capillary length is determined to give the correct resistance and therefore a suitable flow
rate.
Due to the gradual fall in pressure the gas expands, and because the same number of
molecules (mol) must pass a cross-sectional area per second, the gas velocity increases
gradually. Due to the gas expansion the temperature drops if the adiabatic cryo effect is
larger than the heat effect from the viscous forces. Coming out of the capillary the gas is
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at its minimum temperature, and is sent right to a wall constituting the internal part of the
active cryo probe surface to be brought in contact with the tissue.
The basic components of a cryo instrument is shown on Fig.33. The freezing cycle is
started by pressing the footswitch. In approx. 5 seconds a freezing temperature of approx.
-80°C (-176° F) is reached. After release of the footswitch the freezing cycle stops and
the defrost cycle starts automatically as the pressure in the probe increases and the gas is
compressed and heat is liberated. In this way the cryoprobe is defrosted within 5 seconds
without use of electric heating. The tip is equipped with a cryometer (cf. thermometer).
The same equipment may handle either carbon dioxide or nitrous dioxide without any
modification. Special more performing systems use both argon and helium gas.
It is important that the supply gas is very dry, since water will freeze and tighten the
capillary nozzle. The gas passes the pressure regulator and then through a flexible but
highly armoured tube out to the capillary. In the defrost cycle the high pressure will
propagate also to the outlet tube, which accordingly also must be able to withstand
maximum pressure. Argon, CO2 and nitrogen are not polluting, while N2O should be
taken care of by a scavenging system.
Figure 34 CO2 phase diagram
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Risk considerations capillary cryo
1. Sudden loss of cryo effect (very thin capillary, tiny gas obstructions)
2. Room gas pollution
Cryo spray
The phase diagram for CO2 on Fig.34 shows that at room temperature the pressure in a
filled10 CO2-bottle is about 67 bar, and a part of the substance is in the liquid phase. If the
bottle is in the vertical position and the gas (vapour) is abruptly let out in the room the
pressure suddenly drops to 1 bar. Because of the gas expansion the temperature quickly
drops, for a short time down to -78 oC or lower. The CO2 gas is transformed to the solid
phase (“dry ice”) without passing the liquid phase. Because of the non-adiabatic
conditions the snow soon reverts to the gas phase. The liquid phase is an impossible state
for CO2 if the pressure is below 5.1 bar. The point in the phase diagram where all three
phases meet is called the triple point (5.1 bar, -56 oC).
A cryo source may also be cold, a liquid gas kept in a thermos bottle. Liquid nitrogen
(LN2) may be kept in a thermos bottle at -196 oC and 1 bar, or at higher pressures and
temperatures. Liquid nitrogen applied with a spray or probe (temperature of -196º C) is
much colder than liquid nitrogen applied with a swab (-20º C), than cryogens that come
in disposable spray cans (-55º C and -70º C), and than nitrous oxide (-75º C). Table 2
shows the boiling point of some usual gases.
In its most simple form for patients a cryo liquid or vapour is applied directly on the
tissue, e.g. the skin (cryospray). In dermatology or surgery liquid nitrogen (LN2) is often
used because of its powerful cryogenic effect. Cryosurgical freeze times vary according
to lesion type, size, depth, and location.
The handling of liquid nitrogen is of course cumbersome, and therefore Joule-Thomson
based cryo equipment is much used.
10
CO2 in liquid form is incompressible. Therefore a CO2 bottle completely filled with liquid is dangerous.
The supplier is not cheating when a “filled” bottle has a vapour pocket at delivery to allow for only a
moderate pressure rise at rising ambient temperatures.
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PROBLEMS
1. Find the compliance of the lungs when the ventilator has been set to a ventilation
volume of 3L and the pressure varies during inspiration from 10 to 15 cmH2O.
2. In a breathing system water droplets condensate on the sensor causing false
readings. How can the problem be solved?
3. How would you calculate how much [L] gas you have left on a bottle of N2O?
And a bottle of compressed air? Both bottles are equipped with a manometer.
4. What is sensor sensitivity? And selectivity?
5. Referring to Fig.9: Discuss fresh gas flow rate adjustment and patient lung
pressure safety (hint: bag as a pressure stabilising device).
6. What happens on Fig.9 if the bag is not squeezed? And at Fig.10?
7. An oxygen sensor has partial pressure as primary variable pO2 , and the results are
given in [kPa]. Does a varying barometric pressure influence on the
measurements if all other factors are constant? Explain.
8. What is the difference between a spirometer and a pneumotachometer?
9. Is the paramagnetic oxygen analyser sensitive also to diamagnetic gases? Discuss.
10. Derive the law of Laplace for a tube of radius r and wall tension T. (Hint:
consider half of the tube and put the tension contribution equal to the pressure
contribution. The pressure contribution has a radial direction and only the vertical
component is selected and integrated around the half tube circumference.)
11. Find the resistance for a 1 meter long suction tube with internal diameter 2 and 10
mm. Air viscosity 18.6 10-6 [Pa s]. Calculate the pressure drop at a flow rate of 30
L/min.
12. The aspiration flow to a multigas analyzer is 0.2 L/min through a sampling tube
of length 2 m and internal radius 0.4 mm. Calculate the signal time delay between
patient gas flow and analyzer. Calculate the necessary pump negative pressure to
assure the sampling gas flow under the assumption that pressure drop in the
analyzer itself is negligible. Gas viscosity 20 10-6 [Pa s].
13. Referring to the Venturi suction system equivalent circuit on Fig.31: Calculate the
time constant with 5L air in the bottle, tube internal diameter 8mm, suction tube
length 1 [m], patient tube length 2 [m]. Is the suction flow dependent on whether
suction is performed in air or in water. Discuss the effect of a parallel leakage
resistance from the suction pump outlet to the surrounding air.
14. For the paramagnetic oxygen analyzer, discuss the linearity problem on the basis
of eq.6.
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