Some science behind the scenes

Pulse oximeter

A finger mounted pulse oximeter taking measurement

through the fingernail

Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's haemoglobin.

It is a particularly convenient noninvasive measurement method. Typically, it uses a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with a wavelength of 660 nm, and the other is infrared, 905, 910, or 940 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form; therefore, the oxy/deoxyhemoglobin ratio can be calculated from the ratio of the absorption of the red and infrared light.

The monitored signal bounces in time with the heart beat because the arterial blood vessels expand and contract with each heartbeat. By examining only the varying part of the absorption spectrum (essentially, subtracting minimum absorption from peak absorption), a monitor can ignore other tissues or nail polish, (though black nail polish tends to distort readings) and discern only the absorption caused by arterial blood. Thus, detecting a pulse is essential to the operation of a pulse oximeter and it will not function if there is none.

A wrist mounted remote sensor pulse oximeter with plethysmogram

Clinically, a pulse oximeter is useful in any setting where a patient's oxygenation is unstable, including intensive care, operating, recovery and hospital ward settings, for assessment of any patient's oxygenation, and determining the effectiveness of or need for supplemental oxygen. Because of their simplicity and speed, pulse oximeters are of critical importance in emergency medicine and are also very useful for patients with respiratory or cardiac problems, especially COPD, or for diagnosis of some sleep disorders such as apnea and hypopnea.

Portable battery-operated pulse oximeters have also been used by pilots operating in a non-pressurized aircraft above 10,000 feet where supplemental oxygen is required. Portable pulse oximeters are also useful for mountain climbers and athletes whose oxygen levels may decrease at high altitudes or with exercise.

Pulse oximetry is not a complete measure of respiratory sufficiency. It gives no indication of base deficit, carbon dioxide levels, blood pH, or bicarbonate concentration. The metabolism of oxygen can be readily measured by monitoring expired CO2, but saturation figures give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin; in severe anemia, the blood will carry less total oxygen, despite the hemoglobin being 100% saturated.

Erroneously low readings may be caused by hypoperfusion of the extremity being used for monitoring (often due to a limb being cold, or from vasoconstriction); incorrect sensor application; highly calloused skin; or movement (such as shivering), especially during hypoperfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform.

It is also not a complete measure of circulatory sufficiency. If there is insufficient bloodflow, tissues can suffer hypoxia despite high oxygen saturation in the blood that does arrive.

In June, 2009, video game company Nintendo announced an upcoming peripheral for the Wii console, dubbed the "Vitality Sensor", which consists of a pulse oximeter.