Pulse Oximetry, Part 5

What does a pulse oximeter measure?

• The oxygen saturation of haemoglobin in arterial blood - which is a measure of the average amount of oxygen bound to each haemoglobin molecule. The percentage saturation is given as a digital readout together with an audible signal varying in pitch depending on the oxygen saturation.
• The pulse rate - in beats per minute, averaged over 5 to 20 seconds.

A pulse oximeter gives no information on any of these other variables:

• The oxygen content of the blood
• The amount of oxygen dissolved in the blood
• The respiratory rate or tidal volume i.e. ventilation
• The cardiac output or blood pressure

Systolic blood pressure can be estimated by noting the pressure at which the plethysmograph trace reappears during deflation of a proximal non-invasive blood pressure cuff.

Principles of modern pulse oximetry
Oxygen is carried in the bloodstream mainly bound to haemoglobin. One molecule of haemoglobin can carry up to four molecules of oxygen, which is then 100% saturated with oxygen. The average percentage saturation of a population of haemoglobin molecules in a blood sample is the oxygen saturation of the blood. In addition, a very small quantity of oxygen is carried dissolved in the blood, which can become important if the haemoglobin levels are extremely low. The latter, however, is not measured by pulse oximetry.

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