Detection of hypoxia by clinical indicators alone is unreliable (Sinex, 1999). With a few exceptions (such as excessive patient movement (e.g. Parkinson's), anaemia or some types of nail polish - discussed further below), pulse oximeters can detect oxygen saturation to a high degree of accuracy (+/- 2%).
The first pulse oximeter was developed in 1975, and its use became widespread by the 1980s. Pulse oximeters make use of the fact that deoxygenated and oxygenated red blood cells absorb red and near infrared light to different degrees.
What is a pulse oximeter?
Pulse oximetry relies upon the pracical application of the Beer-Lambert law, which states that when the intensity and wavelength of the incident light, its path length, and the extinction coefficient (how it absorbs a specific wavelength) of the intervening substance are known, then the concentration of the intervening solution can be calculated.
Luckily, red and near infrared light penetrate tissues readily; if you've ever shone a torch through your hand as a child, you'll have realised this intuitatively, because your hand does not glow yellow, blue or green (these wavelengths are absorbed by the tissue in your hand), it glows red, as red is the wavelength which most readily penetrates. You'll also have noticed that when you cut yourself, well-oxygenated fresh blood is a brighter red than low-oxygenated blood, precisely because higher oxygen concetrations scatter more red light.
Pulse oximeters can take advantage of this by emitting red (660 nm) and near infrared (940 nm) light from light-emitting diodes through one side (e.g. of the finger), and measuring how much penetrates through to the other side (to the receptor). By registering how much light is absorbed, the oximeter can calculate oxygen saturation of the blood. An accuracy +/- 2% is the current standard (Ingram & Munro, 2005).
Contraindications and confounding variables
The below list of confounding variables is adapted from Chan et al., 2013:
• Poor perfusion due to a number of causes, e.g., hypovolemia, vasoconstriction, etc
Causes of falsely normal or elevated SpO2:
• Carbon monoxide poisoning
• Sickle cell anemia vasoocclusive crises (overestimation of FO2Hb and underestimation of SaO2)
Causes of falsely low SpO2:
• Venous pulsations
• Excessive movement
• Intravenous pigmented dyes
• Inherited forms of abnormal haemoglobin
• Fingernail polish
• Severe anemia (with concomitant hypoxemia)
Causes of falsely low or high SpO2
• Poor probe positioning
• Sepsis and septic shock
References and further reading:
Chan, E., Chan, M., Chan, M. (2013). Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations. Respiratory Medicine, 107(6).
Ingram, G. & Munro, N. (2005). The use (or otherwise) of pulse oximetry in geeral practice. The British Journal of General Practice, 55(516).
Sinex, J. (1999). Pulse Oximetry: Principles and Limitations. American Journal of Emergency Medicine 17(1).