Other gas/blood gas analysis

CO2 electrode - Severinghaus

• Essentially a pH glass electrode held in a NaHCO3 solution by a porous cellophane membrane
• CO2 diffuse through semi-permeable membrane
• Glass electrode then detects changes in pH
• Temperature control also very important
• Slower than H+ electrode because CO2 needs time to diffuse
• Accuracy +/- 1mmHg

pH electrode

Temperature control is very important because dissociation of acid and base increases with temperature

IR photoacoustic spectrometer

1. A single pulsatile infrared beam into the measuring chamber through a filter
2. Each gas absorbs IR radiation at a particular wavelength
3. Gas expands and contracts according to the frequency of the pulsation
4. Sound wave generated
5. Amplitude is proportional to partial pressure of the gas

Cannot distinguish between volatile agents.

Ultraviolet

Similar to infrared analysis

Halothane decomposes into toxic products and must not be returned to circuit

Used only for volatile agents

Lloyd-Haldane apparatus

Measure O2 volume in gas.

Passage through KOH to absorb CO2, then pyrogdiol to absorb oxygen.

Slow and difficult

Van Slyke apparatus

Measures oxygen content directly in blood

1. Add a saponin and potassium ferricyanide solution to blood sample
2. Causes haemolysis and converts Hb to MetHb
3. Oxygen released
4. Amount of oxygen released measured by change in pressure in a constant gas volume

Also slow and difficult to do

Lex-O2-Con

Blood haemolyzed and oxygen free carrier gas flushed through the sample (to remove O2)

Oxygen measured using fuel cell