iThoughts

Equipment 6

gas measurement

Blood gas

Polarographic O2 Clarke electrode

Platinum cathode

O2 + 2H20 + 4e- —> 4OH-

Ag/AgCl anode

Ag + Cl- —> AgCl + e-

KCl/phosphate electrolyte
Blood separated by plastic membrane permeable to O2
0.7V applied by battery
CURRENT measured by galvanometer
can be used for gases AND liquids
Uses up O2
may be affected by halothane

pH electrode

Hg/HgCl (calomel) reference elecrode
Ag/AgCl sensing electrode surrounded by KCl buffer encased in pH sensitive glass
H+ in blood able to pass through H+ sensitive glass bulb, equilibrates with one side in KCl but not the other due to the presence of buffer which creates a potential difference proportional to the H concentration
No battery, measures VOLTAGE

Severinghaus CO2 electrode

modified pH electrode
KOH and NaHCO3 in solution
rubber/teflon membrane only allows CO2 to pass into sodium bicarbonate solution, some H+ ions are formed by the Henderson-Hasselbach equation and CO2 can be measured

ALL

require regular 2 point calibration
need to be kept at 37 degrees

solubility of gases increase w. decreased temp. so hypothermia reduces pO2 and pCO2

membranes can be damaged

innaccuracies

heparin

acidic, so will cause a spuriously low HCO3- and pH with high K (will actually decrease CO2 due to dilution with the saline)

delay

continued metabolism, so low pO2 and pH with high pCO2

air bubbles

increase in pO2 and pH, decrease in pCO2
will cause decrease in pO2 if it is >21kPa

temperature

increased solubility of gases with lower temperature so will give falsely high pO2 and pCO2 with low pH if hypothermic patients blood is measured at 37 C
pH drops with increasing temperature as more H+ ions dissociate

oxygen

electrochemical

polarographic Clarke electrode
galvanic fuel cell

gold (Au) cathode

O2 + 4e- + 2H2O —> 4OH-

lead (Pb) anode

Pb + 2OH- —> PbO + H20 + 2e-

KOH electrolyte
No battery, measures VOLTAGE
20 second response time
affected by nitrous oxide (reacts with anode to produce nitrogen)

physical

paramagnetic

water vapour reduces accuracy, theoretical interference from nitric oxide (paramagnetic) and CO2, NO2 and volatiles (diamagnetic)
due to 2 unpaired electrons in outer shell of oxygen
types

new

test gas and reference gas flow through parallel tubes with a pressure transducer between them, 100Hz pulsing magnetic field applied - the higher the oxygen content the more attracted to the magnetic field and the higher the pressure transduced
breath by breath, no moving parts, no need for calibration

old

2 nitrogen filled spheres, joined by a bar, suspended by a wire in a magnetic field. Oxygen is attracted to the field and rotates the bar in proportion to its concentration, measured by observing the deflection of a beam of light

non-deflection

measures current required to prevent rotation

may be affected by nitrous oxide and water vapour

spectroscopy

oximetry

CO-oximetry

measures levels of different types of Hb
on blood gas machines
red cells disrupted
path length fixed
sapphire glass

pulse oximetry

2 wavelength pulse plethysmograph
660 and 940nm (infrared) LEDs
pulsatile component is approx. 2% of total absorption
PRO’s

accurate > 75%
no issue w. HbF

8 CON's

carboxyHb gives falsely high SpO2 (96%)

think carbs have high RQ

metHb gives falsely low SpO2 (85%)

with mets you have a LOW chance of living

also underread with indocyanine green, methyline blue and nail varnish

dyes and varnish are pretty low end

interference from other devices
does not inform about oxygen content or delivery
inaccurate below 75%
inaccurate in arrythmias
Average measurement, so some delay
can cause burns very rarely!

laws

Lambert’s

the absorption of radiation through a substance increases exponentially as the length of the substance increases

Beer’s

the absorption of radiation through a substance increases exponentially as the concentration of the substances increases

transcutaneous oxygen electrode

heats skin to 44C
must be changed every 4 hours
slow, can cause burns

9 oxygen safety features

Schradar valves
pin index system

air

1-5
(ONE word)

oxygen

2-5
(O2)

nitrous

3-5
(3 molecules)

carbon dioxide

1-6

helium

no pin
(so light)

heliox

2-4

entonox

7
‘people in bristol love balloons’ (Severn)

Cyclopropane

3-6

NIST
Omeda Link 25 system
O2 last on the back-bar
Oxygen knob ridged
Ritchie whistle

7 seconds
60 decibels at 1 metre
sounds when pressure < 200kPa

galvanic fuel cell with alarm

should be checked weekly by disconnecting oxygen hose

paramagnetic analysis with alarm
pulse oximetry

vapourisers

variable bypass

in circuit

draw over

rely on negative pressure
Goldman

no wick

Epstein and Macintosh of Oxford (EMO)

heavy, ether only, bellow compensation, water heat sink

Oxford miniature vaporiser (OMV)

different volatiles (need to change scale), can be used in series, glycol heat sink, used with tri-service apparatues

out of circuit

plenum

gas pushed through by positive pressure
Ohmeda TEC (temperature compensated) MK 2, 3, 4, 5
Aladin Cassette
maximise surface area for vapourisation with wicks and baffles
dense metal heat sink

Boyle’s bottle

glass: low heat capacity so cooled rapidly and reduced SVP
improved with water bath

temperature

stabilisation

high heat capacity
high thermal conductivity

compensation

bimetallic strips
anaeroid bellows

measured flow

TEC 6

desflurane
injection

Copper kettle

bubble through

Gas measurement contd..

others

historical

Engstrom Emma Piezoelectric

only measure vapours that are soluble in oil (volatiles - cannot distinguish and cannot measure other gases)

Drager Narcotest
UV absorption for halothane

200nM
mercury lamp

current

IF absorption spectrophotometry

IF = 1 - 100 uM
molecules w. dissimilar atoms absorb IF
volatiles 3.3uM and 8-9 (better)
CO2 4.3uM
N20 4.5uM
limitations

collision broadening

N20 increases CO2, but can be compensated for

sensitive to water vapour
sevo, water vapour and nitrous all overlap with CO2

photoacoustic spectrophotometry
gas chromatography

different gases are slowed down different amounts in a carrier gas
flame ionisation

better for organic gases

thermal conductivity

beter for inorganic gases

electron capture

better for halogenated gases

‘(Ciara) O’FITHE’
very accurate, but expensive, need prior knowledge of likely gases and can’t measure continuously (only for research)

Raman scattering

most photons do not change energy state when argon laser is passed through (Raleigh)
very small number do and specific ones can be detected by a photodetector (Raman)
very expensive and currently unavailable in UK

refractrometry

measures velocity of light in a gas compared to a vacuum
GOLD STANDARD - Rayleigh (>1m long)
smaller - Riken
not used clinically
can only use for volatiles
do need calibrating
influenced by water vapour

mass spectroscopy

mass spectrometry
quadrapole spectrometry

very accurate
bulky and expensive

N20 and CO2 both have MW of 44 so are difficult to measure and one may be over-estimated