Pulse oximetry

Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation (SO₂). Its reading of SpO₂ (peripheral oxygen saturation) is not always identical to the reading of SaO₂ (arterial oxygen saturation) from arterial blood gas analysis, but the two are correlated well enough that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in clinical use.

In its most common (transmissive) application mode, a sensor device is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot. The device passes two wavelengths of light through the body part to a photodetector. It measures the changing absorbance at each of the wavelengths, allowing it to determine the absorbances due to the pulsing arterial blood alone, excluding venous blood, skin, bone, muscle, fat, and (in most cases) nail polish.

Less commonly, reflectance pulse oximetry is used as an alternative to transmissive pulse oximetery described above. This method does not require a thin section of the person's body and is therefore well suited to a universal application such as the feet, forehead, and chest, but it also has some limitations. Vasodilation and pooling of venous blood in the head due to compromised venous return to the heart can cause a combination of arterial and venous pulsations in the forehead region and lead to spurious SpO₂ results. Such conditions occur with congenital cyanotic heart disease patients, or in patients in the Trendelenburg position,

In 1935, Karl Matthes (German physician 1905–1962) developed the first 2-wavelength ear O₂ saturation meter with red and green filters (later switched to red and infrared filters). His meter was the first device to measure O₂ saturation.

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