Article: Pulse oximetry

Pulse oximetry is a non-invasive method which allows health care providers to monitor the oxygenation of a patient's blood.

A sensor is placed on a relatively thin part of the patient's anatomy, usually a fingertip or earlobe, and red and infrared light is passed from one side to the other. Based upon the ratio of absorption of the red and infrared light caused by the difference in color between oxygen-bound (red) and unbound (blue) hemoglobin in the capillary bed, an approximation of oxygenation can be made.

This is especially useful in an intensive care setting, for assessment of emergency patients, determining the effectiveness of or need for supplemental oxygen, as well as monitoring during anesthesia and during post-anesthetic care. Assessing a pateint's need for supplemental oxygen is useful in the detection and prevention of hypoxemia. Although pulse oximetry is used to monitor oxygenation, it is possible that it can also be used to detect abnormalities in ventilation. However, the use of pulse oximetry to detect hypoventilation is impaired with the use of supplemental oxygen, as it is only when patients breathe room air that abnormalities in respiratory function can be detected reliably with its use. 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.

This is a measure solely of oxygenation, not of ventilation, and is not a substitute for blood gases checked in a laboratory as it gives no indication of carbon dioxide levels, blood pH, or sodium bicarbonate levels. While oximetry does indicate the patient's pulse rate, it does not measure pulse intensity and therefore should not substitute for more accurate methods of pulse reading.

Falsely low readings may be caused by hypoperfusion of the extremity being used for monitoring (often due to the part being cold or from vasoconstriction secondary to the use of vasopressor agents); incorrect sensor application; highly caloused skin; and movement (such as shivering), especially during hypoperfusion. To ensure accuracy, the sensor should return a steady pulse and/or pulse waveform. Falsely high or falsely low readings will occur when hemoglobin is bound to something other than oxygen. In cases of carbon monoxide poisoning, the falsely high reading may delay the recognition of hypoxemia (low blood oxygen level). Cyanide poisoning can also give a false high reading.

See also

• Oxygen sensor
• Oxygen saturation
• Pulse oximeter