What is pulse oximetry?

Pulse oximetry is a procedure used to measure the oxygen level (or oxygen saturation) in the blood. It is considered to be a noninvasive, painless, general indicator of oxygen delivery to the peripheral tissues (such as the finger, earlobe, or nose).

How does pulse oximetry work?

Oxygen in the air is breathed into the lungs. The oxygen then passes into the blood where the majority of the oxygen attaches to hemoglobin (a protein located inside the red blood cell) for transport in the bloodstream. The oxygenated blood circulates to the tissues.

Pulse oximetry technology utilizes the light absorptive characteristics of hemoglobin and the pulsating nature of blood flow in the arteries to aid in determining the oxygenation status in the body. First, there is a color difference between arterial hemoglobin saturated with oxygen, which is bright red, and venous hemoglobin without oxygen, which is darker.

Second, with each pulsation or heartbeat there is a slight increase in the volume of blood flowing through the arteries. Because of the increase of blood volume, albeit small, there is an associated increase in oxygen-rich hemoglobin. This represents the maximum amount of oxygen-rich hemoglobin pulsating through the blood vessels.

What is the process?

A clip-like device called a probe is placed on a body part, such as a finger or ear lobe, to measure the blood that is still carrying or is saturated with oxygen. The probe houses a light source, a light detector, and a microprocessor, which compares and calculates the differences in the oxygen-rich versus oxygen-poor hemoglobin. One side of the probe has a light source with two different types of light, infrared and red, which are transmitted through the finger to the light detector side of the probe. The oxygen-rich hemoglobin absorbs more of the infrared light and the hemoglobin without oxygen absorbs more of the red light. The microprocessor calculates the differences and converts the information to a digital readout. This information helps the doctor assess the amount of oxygen being carried in the blood and evaluate the need for supplemental oxygen.

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