Pulse Oximetry

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

A sensor 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. Light at red (660nm) and infrared (940nm) wavelengths is passed sequentially through the patient to a photodetector. The changing absorbance at each of the two wavelengths is measured, allowing determination of the absorbances due to the pulsing arterial blood alone, excluding venous blood, skin, bone, muscle, fat, and (in most cases) fingernail polish. Based upon the ratio of changing absorbance of the red and infrared light caused by the difference in color between oxygen-bound (bright red) and oxygen-unbound (dark red or blue, in severe cases) blood hemoglobin, a measure of oxygenation (the percentage of hemoglobin molecules bound with oxygen molecules) can be made.

Pulse oximetry data is necessary whenever a patient's oxygenation may be unstable, as in intensive care, critical care, surgery and emergency department areas of a hospital. Data can also be obtained from pilots in unpressurized aircraft, and for assessment of any patient's oxygenation in primary care. It is also very commonly used in the evaluation of the need for and the efficiency of the treatment of sleep apnea using positive airway pressure. A patient's need for oxygen is essential to life; no human life thrives in the absence of oxygen (cellular or gross). Although pulse oximetry is used to monitor oxygenation, it cannot determine the metabolism of oxygen, or the amount of oxygen being used by a patient. For this purpose, it is necessary to also measure carbon dioxide (CO2) levels. It is possible that pulse oximetry can also be used to detect abnormalities in ventilation. However, detection of hypoventilation is impaired by the use of supplemental oxygen, as it is only when patients breathe room air that abnormalities in respiratory function can be detected reliably. Therefore, the routine administration of supplemental oxygen may be unwarranted if the patient is able to maintain adequate oxygenation in room air, since it can result in hypoventilation going undetected.

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