Overnight Pulse Oximetry

Pulse oximetry is a procedure that determines if oxygen levels in the blood are low. This is called hypoxia. Normal levels during the night would generally rule out sleep apnea. With this procedure, a device called a pulse oximeter is attached to the patient's finger. The oximeter transmits red and infrared light through the capillaries in the finger. Part of the light waves is absorbed by hemoglobin, a molecule in the blood that carries oxygen. The ratio of the two light beams provides the measurement of oxygen. The test is not always accurate, however. A combination with polysomnography, especially heart rate measurements, may be best for diagnosing sleep apnea.

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 patient’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 calloused 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.

Home oximetry monitors are available to rule out sleep apnea but their accuracy is unclear. A 2003 study indicated that home oximetry alone was not very helpful in discriminating between patients with or without sleep apnea. Home oximetry however, may be helpful in identifying patients with unsuspected and seriously low oxygen levels