- Pulse oximetry
- Used to show arterial oxygen saturation, it utilises 2x high intensity monochromatic LEDs and a photosensor linked to an electrical analyser which produces a pulsatile waveform.
- Utilises two laws, often combined as "Beer-Lambert's law"
- Beer's law
- Absorption decreases exponentially as concentration of a solution increases
- Lambert's law
- Light intensity decreases exponentially as distance travelled through the substance increases
- How they work
- LEDs emit red light (λ 660ηm) and IR light (λ 940ηm) in sequence then pause with both lights off (to compensate for ambient light). This sequence is repeated 100s of times per second. This allows changes in signal due to arterial pulsations to be detected. Light is absorbed by arterial and venous blood as well as tissues. The amount of light absorbed depends on the proportion of oxyhemoglobin (HbO₂), deoxyhemoglobin (deO₂Hb) and the wavelength of light. At 660ηm the absorbance of HbO₂ is lower than deO₂Hb, the reverse is true at 940ηm. The pulse oximeter detects absorbance at these λ to determine saturation. The pulsatile component is analysed and the non-pulsatile (venous & tissue components) component is eliminated.
- Isobestic points, wavelengths where the absorbance of HbO₂ and deO₂Hb are equal (805ηm & 590ηm), are used as reference points in older pulse oximeters
- Complications of use
- Inaccuracies
- Movement
- Diathermy
- Poor peripheral perfusion, vasoconstriction
- Excess ambient light
- Nail varnish
- Venous congestion (eg tricuspid regurgitation)
- Blood disorders
- MetHb
- COHb
- Methylene blue
- Note: ok with Fetal Hb, anaemia, dark skin, hyperbilirubinaemia and polycythaemia
- Limitations
- Less accurate below 70%
- Inaccurate below 50%
- Averages readings over 10-20s
- Doesn't measure actual O₂ delivery to tissues
- No information about CO₂
- Advantages
- Non-invasive
- Safe, reliable
- Simple to apply
- Early detection of hypoxia