Physics – insights
Dr. S. Parthasarathy
MD., DA., DNB, MD (Acu),
Dip. Diab.DCA, Dip. Software statistics
PhD (physio)
Mahatma gandhi medical college and research institute,
puducherry, India
Definitions
• What is a gas?
A gas is a substance that is in its gaseous phase, but is
above its critical temperature.
Critical temperature is the temperature above which a
gas cannot be liquefied no matter how high the
pressure.
A vapour is a substance in the gaseous phase but is
below its critical temperature.
For example
• 20 degrees – gas – apply 50 PSI – liquifies
• 30 degrees gas – apply 500 PSI – liquifies
• 36.1 degrees – apply any PSI does not
• 36 – critical temperature
Pressure, volume and temperature
• Pressure = f/a
• Units = 1 bar = 1 atm = 100 kpa = 760 mmHg=
14.7 PSI
• 138 bar = 2000 psi
• Temperature – units –
00 C = 320F = 2730k
• Volume == litres or Cubic cm ( cc = ml)
There are some gas laws which inter
relate the three variables
• Boyle’s law
• “For a fixed mass of gas at constant
temperature, the pressure is inversely
proportional to the volume”
P is proportional to 1 / V or PV = K
Volume decrease – pressure increase
Application in anaesthetic practice
Oxygen cylinder of volume 10 L, molybdenum steel –
138 bars.
So how much oxygen is stored ?
P1V1
=
P2V2
138*10 =
1*V2
So,
V2
=
1380 L
Transfer
• 100 bar – 15 litre cylinder
• P1 V1 = P2V2 one
• 1500 litres of oxygen
• 5 litres / minute --- 300 minutes approx
Charles law
• “For a fixed mass of gas at a constant
pressure, the volume is directly proportional
to the temperature”
V is proportional to T or V / T = K
Charles’ Law (Temperature-Volume Law)
Gas volume varies directly with
temperature at a constant pressure
V1/T1=V2/T2
Application of charles law
• Respiratory gas measurements of tidal volume
& vital capacity etc are done at ambient
temperature while these exchanges actually
take place in the body at 37 OC.
Pressure law- Gay lussac law
• “For a fixed mass of gas at a constant volume,
the pressure is directly proportional to the
temperature”
P is proportional to T or P / T = K
Gay lussac law
• Cylinders are kept in high temperature??
• Volume same
• Pressure raises to explode
• Molybdenum steel can withstand 210 bars , if
there is any damage , temperature rise ??
Universal gas law
• PV = nRT
• Look at the gauge of the oxygen cylinder
• Pressure
• Volume, R, temperature constant
• P∝n
• Look at the gauge and tell
• But for nitrous ??
Is there a pneumonic ??
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Pakistan Tele Vision -- Can Be Good
P constant – C
T constant – B
V constant – G
Avogadro’s hypothesis
• Equal volumes of gases, under the same
conditions of temperature and pressure,
contain equal numbers of molecules.
• One mole of a gas
• Oxygen (O2) = 32
• 32 grams of oxygen will have 6.022 * 1023
atoms
If we describe in volume
• One mole of a gas will occupy 22.4 litres
• Gram molecular weight –( Oxygen – 16 ) =O2- 32
• i.e 44 gm of CO2, 2 gram of hydrogen, 32 gm. O2,
• Will occupy ??
• 22.4 litres
Take sevoflurane as example
• MW = 200
• i.e. 200 gm of sevo will occupy 22.4 litres
• 20 gm = 2.24 litres
• flow of oxygen through vaporizer of 224 litres
this 2.24 will be 1 %
Nitrous oxide ??
N2O is stored in cylinder as liquid.
Exists partly as liquid and partly as gas.
So customary to weigh the cylinder along
with its contents.
From known cylinder wt. and measured
wt. amount of N2O and usage is found out
using Avogadro’s hypothesis
E.g.
Wt of cylinder with N2O - 5.6kg
Tare wt.
- 4.5kg
So, wt of N2O
- 1.1kg
44 g of N2O = 22.4 l
Therefore 1.1 kg of N2O =
22.4∗1100
44
= 560 l
So, if we give 2 liters of N2O / min, this
cylinder will come for 280 min or 4.6 hrs
Dalton’s law
• total pressure of a gas mixture was the sum of
the pressures of each of the gases if they were to
exist on their own.
• P = p1+p2+p3…..
• If a cylinder of air = 100 kPa
• Nitrogen is 79 kPa and
oxygen 21 kPa
Entonox 100 kPa == ??
Dalton’s law
• What is the partial pressures of O2 and N2O if you are
administering a ratio of 70/30?
N20 70% X 760 mmHg = 532 mmHg
O2 30% X 760 mmHg = 228 mmHg
760 mmHg
• Would this differ if you were administering anesthesia at
Ooty General Hospital?
N20 70% X 630 mmHg = 441 mmHg
O2 30% X 630 mmHg = 189 mmHg
630 mmHg
Dalton’s law
Graham’s law of diffusion
• rate of diffusion was inversely proportional to the square root of
the molecular mass of the gas
Applications:
1.
Flow meters: each gas with its own phy property must pass
through its own calibrated flow meter.
2.
Rate of diffusion is slower in liquids and thus local anaesthetics,
if not injected in close proximity to the nerve fibre will not be
effective.
3.
Helium, a lighter gas is used in airway obstruction
to improve diffusion and gas exchange
Gas B more mass than A
• A will effuse out of the alveolus quicker than B,
leaving behind more of B and so raising its
concentration.
• for example, halothane is more massive than
nitrous oxide,
• Graham’s law will indicate that the nitrous will
diffuse quicker and so raise the concentration
of the halothane in the alveolus.
Fick law
• Diffusion – concentration gradient
pressure gradient
1. DLCO
2. vapour into circuits
3. Nitrous into cuffs and air filled cavities
Carbondioxide is 20 times more soluble than
oxygen – membrane defects affect oxygenation
Henry’s Law
• Henry’s law states that for a gas-liquid interface the
amount of the gas that dissolves in the liquid is
proportional to its partial pressure.
• So Henry’s law helps to predict how much gas will be
dissolved in the liquid.
• The actual amount also depends on the solubility of
the gas.
Adiabatic expansion
• Adiabatic, when applied to expansion or
compression of a gas, means that energy is
not added or removed when the changes
occur.
– Compression of gas – temperature rises
– Expansion of gas – temperature falls
– Think there are no pressure regulators in Boyle s
machine
Reynolds number
• whether flow is laminar or turbulent
𝜌𝑣𝑑
𝑅𝑒 =
𝜂
Where
𝑅𝑒 – Reynold’s number
𝜌 − density of fluid
v – velocity of fluid
d – diameter of tube and
𝜂 – viscosity of fluid
Reynold s number
• Reynold s number of 2000 – borderline
• When
– Re < 2000 – laminar
– Re > 2000 – turbulent
 Viscosity is the important property of laminar flow
 Density is the important property of turbulent flow
 Reynold’s number of 2000 delineates laminar from
turbulent flow
Applications of reynold number
• 1- undersized ETT may cause a tremendous
decrease in the flow of gases
• 2- Every piece of anaesthetic equipment; because
of diameters & shape of connectors,FGF affected
• 3- In respiratory tract obstruction, oxygen – helium
mixtures are given to reduce density and improve
the flow.
• 4- Laminar flow during quiet breathing is changed
to turbulent during speaking & coughing- dyspnea.
• Flow meter – low flows – laminar flow – density
• High flows – turbulent flow – viscosity
Roughly speaking
• Numerical value for critical value in l/min for
O2 + N2O is same as ID of ETT in mm.
• Flow changes to turbulent from laminar.
• Flow – 5 litres , ok for 5 mm ID ET tube
Hagen poiseille equation
• Flow – Q is determined by a few factors
Pressure bags and height
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Viscosity of common infusions:
1.0 centipoise - Lactated Ringers
4.0 cP- Hetastarch
40.0 cP- 5% albumin
• Warming decrease viscosity
Joule-Thompson Effect
• When a compressed gas is allowed to escape
freely into an open space, cooling occurs.
• Condensation
of
water
or
frost
may
accumulate on the cylinder valve.
• A cryoprobe operates on the Joule-Thompson
effect.
(Joule-Thompson Effect)
• As gas escapes from a N2O
cylinder, the liquid N2O in the
cylinder vaporizes.
• Heat is lost as the liquid vaporizes
(latent heat of vaporization)
• the temperature in cylinder
falls
Hot summer – what is necessary ??
Bernoulli principle
• An increase in the flow velocity
of an ideal fluid will be accompanied by a
simultaneous reduction in its pressure.
Kinetic + potential = same
Venturi
• The effect by which the introduction of a
constriction to fluid flow within a tube causes
the velocity of the fluid to increase, therefore,
the pressure of the fluid to fall.
Venturi -- clinical uses
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Suction
Scavenging
Venturi mask
Nebulizers
Checking Bains circuit
Coanda effect
• This effect was named after a Romanian
aircraft designer Henri Coanda, after an
aircraft he designed went up in flames as a
consequence of this effect.
Coanda Effect
If a constriction occurs at bifurcation
because of increase in velocity and reduction in
the pressure, fluid (air, blood) tends to stick to
one side of the branch causing maldistribution.
Coanda effect
1. Mucus plug at the branching of tracheobronchial tree may cause maldistribution of
respiratory gases.
2. Unequal flow may result because of
atherosclerotic plaques in the vascular tree
3. Fluid logic used in ventilators employs this
principle to replace valves or mobile parts.
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Boyle law, charles law , Gay lussac, universal
Graham s law
Fick law
Bernoulli and venturi
Reynold number
Poiseulli
Joule thomson effect
Coanda effect
Thank you all