NOTES ON MEASUREMENT
HUMIDITY :
Humidity
P C A Kam
risk static sparks drying of mucosa heat loss (vapourisation)
Standard for O.T. > 55%
Absolute : H2O per vol gas (mg/L)
Relative : H2O content
Saturated content at heat To
Hygrometer 1) cool a metal tube and measure To at which dew forms = DEW POINT
Absolute humidity
2) Hair
length α humidity
Wet & dry thermometer : Tables given relative humidity
Mass Spectometer : accurate, rapid response very expensive.
HUMIDITY MEASUREMENT
(i)
Hair Hygrometer
Humidity
hair
Hair length
(ii)
Wet & Dry Hygrometer
T = T2– T1
T1
T2
α RH
used in meteorology
H2 O
(iii)
wet
dry
Regnault’s Hygrometer
Dew Point : Temp at which ambient
when air is fully saturated
RH =
Air
SVP at dew pt.
SVP at ambient temp
Ether
(iv)
Transducer : R or C
(v)
Mass Spectrometer
(vi)
UV Light Absorption :
s with temperature
absorption with
H2O vapour.
MEASUREMENT OF GAS FLOWS
(a)
Constant Pressure (or variable orifice) flowmeter
eg. Flowmeter on Boyle’s machine
Low flow = flow between wall and bobbin is Laminar
High flow = flow between wall and bobbin is turbulent
R
L
Accuracy + 5%
Errors : Vertical + Static
Electricity
Leaks
Back pressure effect
Coxeter
(b)
Heidbrink
VARIABLE PRESSURE (Constant Orifice)
MEASUREMENT OF GAS FLOW
Pneumotachograph (Fleish) (rapid response constant orifice flowmeter)
heating coil, prevents water condensation
x
x
x
Longitudinal fine bore tubes to ensure laminar flow
F α P2 - P1
x
x
P1
x
P2
Mesh Type
P
α V2
P
WRIGHT’S SPIROMETER
Accuracy + 2%
Affected CO2 water vapour
OXIMETRY
•
•
•
Measures infrared & red absorbance
Determines Hb saturation
Transmitted light +I e-kcl
ORDINARY OXIMETRY
a)
Absorbance oximetry
Haemolysed sample placed in light path 805 mm absorbance gives total Hb 640 mm
absorbance composed to above and SaO2 calculated
b)
Reflectance oximeter
Now developed with can be left indwelling in Pulm. Art. Catheter to give constant
SVO2
Absorbance
Coefficient
isobestic Point
Hb O2
Hb
650
805
wave length (um)
Pulse Oximetry
Now readily available due to ; 1) Easily available monochromatic LEDs
2) Microprocessor technology.
Light Source
LEDs - Red 940 um
- Infra Red 660 um
Background
• Probe cycles Red, I.R. OFF during “OFF detector gives ambient light which is
subtracted moment to moment.
• Cycles at multiple of mains AC to effects of flickering [at multiple of O R mains
supply]
Sources of error
1.
Abnormal Hb: Hb CO : Also measured as HBO2 !
Met – Hb : weights reading towards 85
Met Hb
HbO2
HbCO
660
800
Hb
940
2)
BO2.
Shifted O2 dissociation curv: will cause any given SaO2 to be associated with a different
3)
Dyes and Pigments
Methylene Blue : Absorbes at 660 um
Limitations
1)
Need adequate Pulse
To
BP
V/C
alters
SaO2
Ineffective
Cardiopulmonary bypass
2)
Synthetic fingernails
3)
movement artifact, eg. shivering
4)
dirt
5)
Abnormal Hbs
6)
I.V. Dyes
7)
Diathermy
8)
Ambient light
9)
Uncalibratable
10)
Cost
Probe reliability
Acuracy Depends on instrument - Usually > 2% for 70-100 sat%
Up to 20% for < 70%
Ear probe more accurate.
PULSE OXIMETER
1.
2 LED (a) Red
660mm
(b) Infra red
940mm
LED Switch on + off @ several hundred times per sec.
2.
Photocell SENSOR detects transmitted light
3.
PAUSE with both diodes off allows microprocessor & photocell to compensate for effects
of ambient light.
ac
Arterial
Absorption of light by tissues has 2 components
4.
ac component = arterial component
dc
Non pulsatile arterial
dc component = all non pulsatile component
Venous and capillary blood
5.
Tissue
The pulse oximeter determines the ac component of each wavelength and divides it by its
dc component.
ac660
dc660
ac940
dc 940
R =
at λ 660 = reduced Hb absorption > HbO2 absorption
λ 940 HbO2 absorption > Hb absorption reduced.
NOTE : pulse oximeter does not rely on absorption at isobestic pt. (805nm)
[ absorption HHb = HbO2]
Measurement of Arterial Blood Gases
pH
- Principle : pH sensitive Glass
- Glass is a lattice of O + silicon with metallic cations within the lattice.
- A pH – gradient across the glass causes a voltage gradient.
? H+ displaces metallic cations
change
- The glass-electrode is made by exposing one side of glass to the sample and the other
side to standard : 0.1M HCL
Glass must be
- Thin enough
- Sufficient conductivity
- Clean (no protein poisoning )
- Kept at constant temp.
- The glass electrode is placed in series with 2 standard half cells which generate a
constant potential gradient.
- Difference in sample pH
measured voltage vs ~ 60 mV per pH unit.
Schematic Diagram :
Ag:AgCl
HgCl
0.1M HCl
Glass
Sample
Calomel
Electrode
In Practice : Electrode is a glass capillary
Surrounded by HCl with Ag : AgCl electrode
- whole apparatus in Water Jacket.
Problems : Temp. Sensitive
Electrode degrades with time
pH range 0 - 9
Calibration:
With 2 standard solutions
Phosphate Buffers
CO2 Content (Total CO2)
= dissolved CO2 + combined CO2 (Carbonate, bicarbonate, carbomino)
Measured by - adding acid to blood
- Drives out CO2
- Measure CO2 with CO2 electrode before + after adding acid.
- PcO2
is α total CO2
2.
PO2 can measure
-
PO2
SO2
CO2
related by O2 dissociation
curve (and O2 solubility)
PO2 Oxygen Electrode = CLARK electrode
- Blood sample (or gas) separated from electrode by
gas-permeable plastic mem.
- Constant voltage applied : 0.6V
- 2H2O + O2 + 4 e40M- Current α PO2 ( [O2] gas )
Schematic
Practice
Platinum
Wire
Reference
Anote
Ag :AgCl
Gas
platinum wire
Glass Rpd
electrolyte
Electrolyte s
Solution
Sample
Constant
Voltage
Plastic membrane
SAMPLE
Gas Permeable
2HO + O2 + 4OH
Plastic membrane
Accuracy 2mmHg
Response : 30-60 secs.
Probs. Strict temp control aging of electrodes, freq. Calibration.
SEVERINGHAUS CO2 ELECTRODE
Based on reaction of CO2 + H2O
CO2 + H2O
H2 O
H+ +HCO3-
Hence CO2 tension is related to H+ ion concentration
.
Electrode
HCO3
H+
Glass sensitive
Mesh
Membrane
BLOOD
CO2
MEASURING SYSTEMS
RECORDER
TRANSDUCER
PROCESSOR
AMPLIFIER
DISPLAY
ALARM
Converts one form
of energy into another
Modifies signal
Eg. Filters
Also Integrate = /differential=
Rate counting
The whole system must have :
1.
Frequ. Response
5-10 times natural frequency of signal
2.
Amp. Linearity
Across range of frequency of interest i.e. output and input.
3.
Phase Linearity
i.e. no phase distortion
4.
Gain stability
i.e. no drift over time
5.
No Hysteresis
Some systems have a different output for a rising vs. falling
signal (eg. thermistors)
6.
Zero stability
7.
Signal : Noise ratio : Pref. > 10,000
8.
High input Impedance : should be >> biological impedance of measured signals or
loos signal amplitude.
9.
Calibratable
minimisation
Input impedance maximal facilitations
Signal
signal
input
Output signal;
*
V signal
WHEATSTONE BRIDGE
x
y
- If all resistors equal then no potential difference
x & y.
- If one resistor varies with signal then can
measure :
a) Voltage
x –y
b) Current Flow x-y
c) Measure variation in the paired resistor needed
to rebalance the “bridge”.
Used for : Thermistors
- Pressure Transducer
(strain gauge)
Incorporating all and 4 elements into pressure
Sensor such that 2 elements resistance while
Other 2 elements them achieve sensitivity.
TEMPERATURE
Definition : Measure of the Thermal Energy of a substance. Heat flow; from a substance of
higher temp to one of a lower temp.
Scales : Kelvin : The triple point of water is defined as 273.16 Kelvin (unit “degrees Kelvin”)
Zero Kelvin is absolute zero, no thermal energy
Celcius : Temp (K) = Temp © - 273.15
Heat : Measurement of kinetic energy of molecules.
Importance of To in the body
Oral = 36.7oC, SD 0.2
Core To vs Shell To
Core 36 – 37.5oC
Shell much more variable eg. 32-35oC
Average Body To : 0.66 x Core To + 0.34 x Average skin To eg. = 35oC
Balance between production + loss of Heat.
Production ~ 50 w/m2 ~ 100W in 70kg man
With physical activity
Assimilation of food
BMR
Fever
Shivering
x 2 Heat Production
Loss
Radiation 40%
Convection 30%
Skin evaporation 20%
8% latent heat of vaporisation
Respiration 10%
2% heating of inspired gas
Body fluids 1%
Not conduction
Radiation Mostly infrared
If surrounding objects are cold
By surrounding sources of radiant heat
By heat reflective blankets
Connection
by trapping layer of
clothing, blankets
Depends on
Surface Area
Insulation
fat
clothing
Surface Evap. Perspiration rate
Heat less if air humidity (p press gradient)
ELECTRICAL THERMOMETERS
(A)
Resistance Thermometers
Platinum Wire
T α
Disadv.
R
R (linear)
Not sensitive
αRαT
wire
Battery
T
(B)
Thermistor
oxide of
metal
R
To
R exponentially with Temp.
Adv : small - rapid change
- accessible to remote location
Disadv : Drift in calibration
(C)
Thermocouple (Seebeck Effect)
Reference Junction
Copper
Constantan
Junction
Potential
mV
Measuring junction
Temp
TEMPERATURE MEASUREMENT
SITE :
Rectum
Oesophagus
=
=
Tympanic Memb.
Nasopharynx
Skin
Axilla
Core ( ~ 0.5oC > oral)
Upper 1-2o < Core (resp. gasses)
Mid x heart 0.5oC < core
usu ~ 25cm pas larynx
~ brain temp.
Easily displaced ~ trauma
As above ( risk trauma)
Affected by blood flow to skin
Muscle temp (eg. M H) best measured in axilla.
Thermometer :
a)
Liquid expansion
Hg or Alcohol
i)
Bulb of liquid
ii)
Fine capillary
iii)
Scale
Advantage : Cheap
Accurate
Reliable
Disadvantage : Fragile
Slow response
No remark reading
Toxic Hg (with recurrent
Exposure)
Or Bourdon Gauge Type
Scale
Criteel tube
Alcohol filled
Sensing element
b)
Bimetalic Strip
Robust
cheap
bonded metals with differing
coefficient of expansion
Inaccurate
c) Thermocouple
Junction of 2 different metals
P. Diff (α To)
Seebeck effect
Metal 1 eg Cu
V
Metal 2 eg. Constantin
Ref
Temp
(eg.Ice)
Measured
Temp
Adv. Large linear range
can be v small
Disadv. Small output 40μv
Need v sensitive measurement
o
(Reverse process can be used for creating a T
difference by applying current
protector
destroying tissue lesions).
Infra-Red (eg. tympanic membrane sensors)
GAS & VAPOUR ANALYSIS
A.
Chemical Methods
CO2 : Haldane device
- CO2 absorbed by KOK solution
- Vol (or P) gives amount of CO2 in sample
- Accurate O.1%
•
N2O also absorbed by KOH
O2 :
B.
Solutions which absorb O2 also absorb CO2
2 chamber device : removes CO2 first.
Physical Methods
Physical Properties of Gases :
Density
Viscosity
Thermal Conductivity
Refractive Index
Velosity of Sound
Magnetic susceptibility.
Density : Wallers chloroform balance
Chloroform x 5 density
Chloroform %
density
Buoyancy of bulb.
1000ml gas filled bulb
Thermal Conductivity
Cooling of a heated thermometer is compared to a reference gas eg. with wheatstone
Bridge.
Sample
Heating current
v
Used for He
CO2
Reference
2.
a)
SPECIFIC Physical Properties of gasses
Electromagnetic Radiation Emission (EMR)
All gasses emit EMR when excited
Used for measuring N2 eg. N2 washout for VD anatomic
Gas passed through charge of 1500-2000V emitted light measured (filtered for N2
wavelength)
Advantages : Rapid Response
Disadvantage : expensive > $20,000
b)
Absorption of EMR
i)
Infra-red : Any gas with > 2 atoms absorbs infrared ( gas will heat as absorbs
and this can be used as the sensor i.e. To)
Absorbance at specific wavelengths concentration of gas
Problems. N2O and CO have absorbance peaks close to CO2
N2O : Peak 3.9 μm
CO2 : Peak 4.3 μm.
N2O in circuit
6%
measured CO2
Some machines (a) analyse at N2O peak as well to give
- correction to PcO2
- absolute PN2O (as an added bon vs_
(b) Calibrate continuously using reference cell
USE : PcO2
Accurate 0.1%
Response time .1 sec (xpt long. If sample drawn up long tubing
ii)
Need last response to give a waveform.
Halothane , Etc. absorb in U.V. spectrum 0.2% accuracy for halothane 0-5%
CAPNOGRAPH
Infra-red CO2 analyser based on Beer-Lambert Law.
REFERENCE
CHAMBER
Known CO2
detector
Light
Source
Filter
CO2 ζ peak = 4.3μm
Sample
Chamber
O2
Sapphire glass
windows
Rotating Wheel
“CHOPPER”
alternately allows
light from each beam
to reach detector
Detector : heat Asthenometer etc.
2 Types : Side line
Main line
Errors : Other gases that may interfere
Asymetric molecules : O2, N2O, H2O.
May be corrected by:(a) Use additional wavelength to measure N2O independently
(b) “Collison broadening”
ETCO2 monitoring
- Expensive fragile detector
1.
Main Stream : - Bulky, heavy sensor at end of ETT
- dead space
- Optimal CO2 Wave form
- No gas withdrawn from circuit.
Infra-red light across the gas stream through sapphire window
Light interrupted by wheel
Empty not used
Chamber with N2 only (reference)
Chamber with CO2 of Sat C3
2. Side Stream : CO2 / N2O
-
Multiple gas analysis
Min. Equi. At ETT
Gas sip
off
containment.
Infra-red absorbption
Some units require setting of N2O separably and then attempts to allow
to overlap
.
O2
Pressure gradient derived from diamagnetic prop of O2
Sample
Reference
Pressure T’ducer
Magnetic Field
Volatile Anaesth
Infra-red : all measured at 3.3 μm
Different gain set to allow for different agent
USES OF GAS ANALYSIS TECHNIQUES
Method
O2
CO2
N2 O
/
/
?
V/A
Chemical
Halothane
Physical
Density (Waller’s Chloroform balance)
/
Thermal conductivity
/
Refractive Index
/
Solubility
Silicon Rubber
D
crystal
/
/
Magnetic Susceptibility
/
EMR Emission
EMR Absorption
I.R
U.V
/
/
/
/
Mass Spectrometry
/
/
?
/
/
/
Raman Scatter Spectrometry
/
O&H
O2 MEASUREMENT
(PARAMAGNETIC ANALYSER)
O2 - electrons in outer orbit unpaired
Paramagnetic i.e. attracted to magnetic field.
N2O - diamagnetic and repelled from a magnetic field
N
O2 attracted to magnetic field
Reflector
mirror
Dumb-bell
N2
N2
Suspensory
Filament
Light
Source
S
O2 MEASUREMENT
(FUEL CELL) only used for gas samples.
Imp. Feature : No battery in system because cell produces a voltage
Electrolyte
KOH
GOLD
CATHODE
(mesh)
LEAD
ANODE
Membrane
Pb + 20H-
PbO + H2O + 2e
O2 + 4e- + 2 H2O
4oH
Disadvantage : N2O reacts with P6 to produce N2
Hence sorter life-span.
damage &
O2 MEASUREMENT – CLARK ELECTRODE
(POLAROGRAPHIC ELECTRODE)
May be used in gases or in blood (if electrode is protected by a plastic membrane)
Current α O2
0.6mV
applied
e-
(electrons)
Ag/AgCl
Anode
platinum
_ CATHODE
Electrolyte
Eg. KCl
Plastic membrane
If used for blood
4ẽ + O2 + 2H2O
4OH
NB Battery attached to circuit to apply the O.6V potential difference.