Pulse oximetry measures the ratio of haemoglobin saturated with oxygen compared to the total functional haemoglobin, and is expressed as a percentage. Functional haemoglobin is the amount of haemoglobin capable of binding to oxygen. This oxygen saturation percentage, SpO2, can be translated to an approximate PaO2 using the oxygen dissociation curve. However this is an approximation and can be widely inaccurate as other factors can influence the relationship between SpO2 and PaO2.
It’s also important to note that pulse oximetry cannot identify whether the concentration of haemoglobin is normal or not, nor can it identify non-functional haemoglobin. In an anaemic patient, for example, whilst the haemoglobin may be fully saturated (giving a normal SpO2 reading), the oxygen carrying capacity of the blood may be significantly reduced.
Similarly, haemoglobin may be fully saturated (showing a normal SpO2) but with dysfunctional strands, such as carboxyhemoglobin or methemoglobin. Carboxyhemoglobin levels are elevated in heavy smokers. Methemoglobinaemia may occur in patients undergoing nitrate or lidocaine therapy.
Haemoglobin also binds much more readily to carbon monoxide than to oxygen, so the body’s haemoglobin may be fully saturated (giving an SpO2 of 98%) but with carbon monoxide not oxygen!
Oximeters use the principle of light absorption by haemoglobin. Oxyhaemoglobin (HbO2) and reduced haemoglobin (Hb) absorb light at different frequencies.
Within the pulse oximeter device, there are two lights at different frequencies; 660nm (red) and 940nm (near infrared).
A typical pulse oximeter uses a pair of small LEDs facing a light sensor through a translucent part of the subject’s body, for example a fingertip. Absorption of light at these wavelengths differs significantly between blood loaded with oxyhaemoglobin and blood loaded with reduced haemoglobin. The light sensor measures the amount of light that is transmitted, and therefore not absorbed; these signals fluctuate with the patients’ pulse. The ratio of measured red light compared to infrared light represents the ratio of oxygenated haemoglobin to deoxygenated haemoglobin, and this ratio is converted to SpO2.
Pulse oximetry can be used in any situation where non-invasive measurement of oxygen status is sufficient. Examples include monitoring effects of therapy such as oxygen therapy and diagnostic tests such as overnight pulse oximetry for obstructive sleep apnoea.
There are lots of factors, which can interfere with the validity of the measurements, such as poor peripheral perfusion, dark skin pigmentation and nail polish.