Pulse oximetry is a technique that uses a special device to measure the oxygen-carrying capacity of a component in your red blood cells called hemoglobin. Normal pulse oximetry readings typically indicate oxygen levels near maximum carrying capacity, while abnormal readings may indicate oxygen levels low enough to cause life-threatening, health complications.
Understanding Blood Oxygen
Hemoglobin is a blood compound that contains a combination of iron and amino acid chains called polypeptides, according to the University of Texas Medical Branch (UTMB). When you breathe, the iron in hemoglobin gathers oxygen and transports it throughout your bloodstream and body tissues. Roughly 97 percent of the oxygen in your body is transported by hemoglobin, UTMB reports.
Understanding Pulse Oximetry
A pulse oximeter, formally known as a transcutaneous pulse oximeter, uses light-emitting diodes to pass infrared and red light through your body tissue at any point where a pulse can be found, explains UTMB. Sensors on the oximeter then measure any light not absorbed by your tissue, and the device uses these measurements to calculate how much of your hemoglobin carries oxygen. Typically, an oximetry reading takes place over a series of pulses, or heartbeats.
If 97 percent of your hemoglobin contains oxygen, your blood oxygen saturation is at maximum capacity, UTMB reports. Normal results on a pulse oximetry reading range between 96 and 100 percent of this maximum value. If your pulse oximetry readings fall below 90 percent of maximum, you enter a clinical state called respiratory failure, characterized by inadequate oxygen supplies to your vital organs and body tissue.
A number of factors can decrease the accuracy of your pulse oximetry results, states Merck Manuals. These include wearing nail polish during testing, having dark skin and having medical conditions, such as hypotension, highly constricted blood vessels and an irregular heartbeat. In some cases, a pulse oximetry reading may inadvertently include specific types of hemoglobin, such as methemoglobin, which do not carry oxygen, and, thereby, falsely elevate the estimates of your blood oxygen levels.
UTMB reports a number of additional factors that may alter your normal pulse oximetry results. These include body movements during testing, the presence of bright light in the immediate environment, the presence of abnormal hemoglobin in your bloodstream, pulse oximeter failure, recent exposure to carbon monoxide and abnormal amounts of carbon dioxide in your bloodstream. A single pulse oximetry reading may also mislead if it records temporary changes in your blood oxygen that do not reflect your actual daily oxygen levels. To overcome potential errors in this area, health care professionals will typically place your pulse oximetry results in the context of other information regarding your current state of health.