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
- A single pulsatile infrared beam into the measuring chamber through a filter
- Each gas absorbs IR radiation at a particular wavelength
- Gas expands and contracts according to the frequency of the pulsation
- Sound wave generated
- 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
- Add a saponin and potassium ferricyanide solution to blood sample
- Causes haemolysis and converts Hb to MetHb
- Oxygen released
- 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