Pulse oximetry does not provide information on (1) the oxygen content of the blood (e.g. in anaemia); (2) delivery of oxygen to the tissues (e.g. in shock); or (3) on ventilation (normal SaO2 does not exclude CO2 retention).
Oximeters work by the principles of spectrophotometry: the relative absorption of red (absorbed by oxygenated blood) and infrared (absorbed by deoxygenated blood) light of the systolic component of the absorption waveform correlates to arterial blood oxygen saturations. Emitters and detectors oppose each other in the probe. Probes (wrap or clip style) are usually positioned on the fingertip or earlobes
Central cyanosis, the clinical sign of hypoxaemia, is an insensitive marker occurring only at 75-80% saturation.
Individual pulse oximetry readings - can be invaluable in clinical situations where hypoxaemia may be a factor – for example, acute asthma, bronchiolitis or croup. If oxygen saturation is less than 92% in air, consider the attack potentially life-threatening.
In pneumonia pulse oximetry is helpful to assess the need for oxygen therapy.
Sources of error
- Excessive movement (check wave-form).
- High level of ambient light, including infrared heat lamps, and nail polish.
- Cold hands - warm extremity if local poor perfusion.
- Pulse oximetry cannot differentiate between different forms of haemoglobin. Overestimation of true saturation levels if carboxyheamoglobin is present.