Oxygenation and Ventilation Measurements
The monitoring of oxygenation and ventilation is important in the assessment of sleep-releated breathing disorders. However, because oxygen values can change often during the course of sleep, repeated measurements must be taken to ensure accuracy. The direct measurements of arterial oxygen tension only offer a static glimpse, and repeated measurements from invasive procedures such as sampling arterial blood for oxygen will disturb the patient’s sleep; therefore, noninvasive methods are preferred such as pulse oximetry, transcutaneous oxygen monitoring, transcutaneous carbon dioxide, and pulse transit time.
Pulse oximetry measures the oxygenation in peripheral capillaries (such as the fingers); however, an article written by Bohning states that pulse oximetry may not be that useful, such as diagnosing obstructive sleep-apnea, due to the differences in signal processing in the devices.
Transcutaneous oxygen and carbon dioxide monitoring measures the oxygen and carbon dioxide tension on the skin surface respectively, and the pulse transit time measures the transmission time of an arterial pulse transit wave. For the lattermost, pulse transit time increases when one is aroused from sleep, making it useful in determining sleep apnea.