Performed intermittently or continuously, pulse oximetry is a relatively simple procedure used to monitor arterial oxygen saturation noninvasively. Pulse oximeters usually denote arterial oxygen saturation values with the symbol SpO₂, whereas invasively measured arterial oxygen saturation values are denoted by the symbol SaO₂.

In this procedure, two diodes send red and infrared light through a pulsating arterial vascular bed, like the one in the fingertip. A photodetector slipped over the finger measures the transmitted light as it passes through the vascular bed, detects the relative amount of color absorbed by arterial blood, and calculates the arterial oxygen saturation without interference from surrounding venous blood, skin, connective tissue, or bone. Using the ear probe, oximetry works by monitoring the transmission of light waves through the vascular bed of a patient’s earlobe. Results will be inaccurate if the patient’s earlobe is poorly perfused, as from a low cardiac output. (See How oximetry works, page 618.)