Oxygen Measurement

Pulse oximetry

Beer-Lambert
Absorption of radiation by the compound of interest in a sample increases as the concentration increases.

Measurement
Absorbance of light by Hb depends on its O2 saturation.
HbO absorbs less red light at 660nm than Hb.
HbO absorbs more IR light at 940nm than Hb.
Absorb the same at 805nm (isobestic point).
Comparisons of the absorbances at the 2 wavelengths means the saturation can be calculated.
Red and IR LEDs pass light radiation through tissue.
Detected by a photocell.
Light emited every 5-10microseconds (several hundred times a second).
On:off / off:on / off:off (later compensates for ambient light).
Non pulsatile component from tissues and venous blood ignored.
Therefore measures only the pulsatile (plethysmographic) blood – SpO2.

Errors
Calibrated against volunteers so accurate only down to 70%.
Ambient light – see above.
Loss of normal pulse – hypoperfusion, hypothermia, vasoconstriction, narrow pulse pressure, arrhythmias.
Interference – movement, electrical.
Rejection of movement artefact improved by averaging results over a number of waveforms → slower response time.
Absorption by other substances:

  • COHb – reads at 96%
  • Biliruben – abnormally low readings
  • MetHb – reads at 84%
  • Dyes – methylene blue – false low reading
  • Other – nail varnish
Interpretation
  • Does not detect CO2 (resp failure masked if supplemental O2)
  • Anaemic patients will have normal sats but ↓ DO2

O2/Clark/Polarographic electrode

Anode – silver/silver chloride.
Cathode – platinum.
Solution – potassium chloride.
0.6v applied → O2 diffuses to cathode combining with electrons and water → hydroxide ions + current flow proportional to O2 conc.
Temp maintained at 37.

Fuel cell

Similar to O2 electrode but generates own potential.
Anode – lead.
Cathode – gold.
Solution – potassium hydroxide.
Hydroxide ions produced at cathode combine with lead anode → lead oxide and electrons.
Temp maintained at 37.

Paramagnetic analyser

O2 is paramagnetic – attracted to magnetic field.
Pulsed magnetic field applied to sample and reference gas.
Reduced pressure on O2 side measured by diaphragm of pressure transducer.
Nitric oxide also paramagnetic (other gases diamagnetic – repelled).

Mass spectrometry

Gas molecules charged by passing through an electron beam.
The ionised particles deflected by a magnetic field proportional to their mass.