<P> Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation (SO). Though its reading of SpO (peripheral oxygen saturation) is not always identical to the more desirable reading of SaO (arterial oxygen saturation) from arterial blood gas analysis, the two are correlated well enough that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in clinical use . </P> <P> 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 . </P>

A noninvasive method to monitor oxygen saturation levels in arterial blood is