Brands of O₂ analysers

CIG - Ohmeda

1. Advantages

a) Can measure accuracy of blenders

b) Alarm state

c) Placed at last outlet to oxygenator

d) Detects major disconnection

e) May not determine minor disconnection

2. Disadvantage

a) Another part of gas line to be disconnected

Calibration method & alarm system for oxygen analyser

1. Exposure to room air; calibrate oxygen analyser to indicate 21% oxygen

2. Disconnect from blender & connect to direct oxygen outlet; calibrate oxygen analyser to indicate 100% oxygen

a) Never use blender to check/ calibrate analyser

b) As are assuming that blender is correct (which defeats the purpose of monitoring FiO2)

3. Set low oxygen alarm to no lower than 21%; ideally set to slightly lower than minimal FiO₂ to give early indication of failing oxygen supply

4. Analyser probe should be positioned distal in the gas supply line to monitor for any breaks in the integrity of the gas supply to the oxygenator

Methods of measuring oxygen concentration

1. Fuel cell [

a) Mechanism

i) At the gold cathode [similar reaction to oxygen electrode]:

a) O₂ + 4e + 2H₂O ® 4(OH)^-

ii) The current flow depends on the uptake of oxygen at the cathode and thus on the partial pressure of oxygen

iii) At the lead anode production of electrons by:

a) Pb + 2(OH)^- ® PbO + H₂O + 2e

iv) Thus a voltage is produced which is proportional to the PO₂

b) Operation considerations

i) Unlike the oxygen cell, no battery is needed because the cell itself acts as a battery producing a voltage

ii) Like other batteries, the cell has a limited life of several months [checked & changed every several months]

iii) The cell is often connected to a small indicator unit to provide a portable & reliable oxygen analyser for clinical & bedside use

iv) Response time of 30 secs

v) Inexpensive

vi) Slower response than Clark cell but requires less maintenance and has a guaranteed lifespan

2. Paramagnetic

a) Mechanism

i) Based on fact that oxygen is paramagnetic - is attracted into a magnetic field

ii) The pressure differential is measured between a stream of reference gas (air) and the sample gas as the streams are exposed to a changing magnetic field

iii) An electromagnet is rapidly switched on & off, creating a changing magnetic field between its poles

iv) The streams of sample & reference gas have different oxygen partial pressures

v) The pressure between the entrance and the exit of the respective gas streams differ slightly because of the magnetic force on the oxygen molecules

vi) A sensitive pressure transducer is used to convert this force into an electric signal

vii) This signal can be filtered and used to display the oxygen percentage

b) Operation considerations

i) As with all instruments, must be calibrated before use (eg 100% N2 & 100% O2)

ii) Like most gas analyses, water vapour affects readings - sample gases should be dried by passage through silica gel prior to analysis