How does a pulse oximeter work?

What is a pulse oximeter, and how does it work?

A pulse oximeter is a noninvasive medical device that measures blood oxygen saturation (SpO2) in a matter of seconds. This small device is usually clipped onto a finger, but can also be attached to the ear or foot, particularly in infants. It works using spectrophotometry: the device emits beams of light (red and infrared) through the tissue and analyzes variations in light absorption based on the oxygen content of hemoglobin, the pigment in the blood that carries oxygen.

The device typically displays two values: SpO2 (percentage of hemoglobin saturated with oxygen) and heart rate. Normal oxygen saturation ranges from 95% to 100%. Values below 95% may warrant a medical consultation, particularly if they are accompanied by symptoms such as wheezing, unusual fatigue, or marked paleness—these signs may indicate a respiratory problem requiring prompt evaluation, especially in people withasthma or COPD.

In what situations is a pulse oximeter useful?

The pulse oximeter is useful in several clinical and routine monitoring situations:

• Chronic respiratory conditions: asthma, COPD, respiratory complications related to a history of smoking —regular monitoring of SpO2 helps detect significant changes
• Monitoring respiratory infections: It helps track the progression of viral infections affecting respiratory function; a sudden drop in saturation may signal a worsening condition requiring prompt medical attention
• Postoperative monitoring: monitoring oxygenation after surgery or during certain treatments
• High-altitude or high-intensity sports: Athletes use it to ensure their oxygen levels remain adequate during exercise and to adjust training intensity
• Newborns: With an appropriate sensor (usually attached to the foot), it allows for continuous monitoring of premature infants or those with medical conditions requiring oxygenation monitoring

There is an important distinction between this and a capnograph (CO₂ monitor), which is reserved for surgical settings and mechanical ventilation: the latter measures the concentration of carbon dioxide in exhaled air to assess the effectiveness of ventilation, whereas the oximeter is limited to measuring blood oxygen saturation.

What precautions ensure a reliable measurement?

Several factors can skew an oximeter reading and must be anticipated:

• Nail polish or artificial nails: interfere with light absorption—remove before measurement
• Excessive movement: Remaining still during the measurement prevents motion artifacts
• Bright ambient light: can interfere with the optical sensor
• Peripheral circulatory disorders or cold hands: reduce blood flow to the finger and may skew the reading

For maintenance, clean the sensor regularly with a soft cloth lightly moistened with isopropyl alcohol—never submerge the device in water or use strong solvents—and periodically check the condition of the batteries. In the event of an abnormal reading or concerning respiratory symptoms, a medical consultation is essential for the correct interpretation of the results—the pulse oximeter is a monitoring tool, never a standalone diagnostic tool.