Pulse oximeters have become a standard medical device, especially for monitoring respiratory health. They provide a quick, non-invasive way to estimate arterial oxygen saturation, or SpO2. The core technology behind this measurement is based on a simple yet powerful principle: light absorption by hemoglobin. Understanding this allows us to appreciate how a small clip on a finger can offer vital insights.
The principle relies on the different light absorption properties of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb). Hemoglobin is the protein in red blood cells responsible for carrying oxygen. When hemoglobin binds with oxygen molecules in the lungs, it becomes bright red. When it releases oxygen to the tissues, it becomes a darker, bluish-red. These two forms absorb light differently, particularly in the red and infrared spectra.
A pulse oximeter typically contains two light-emitting diodes (LEDs). One LED emits red light at a wavelength around 660 nanometers, and the other emits infrared light at a wavelength around 940 nanometers. Deoxygenated hemoglobin absorbs more red light, while oxygenated hemoglobin absorbs more infrared light. The device shines these two specific wavelengths through a thin, translucent part of the body, usually a fingertip or earlobe.
On the opposite side of the sensor, a photodetector measures the amount of each wavelength of light that passes through the tissue without being absorbed. The total absorption is not just from the blood; it also includes absorption by skin, bone, muscle, and venous blood. This is where the "pulse" in pulse oximetry becomes crucial. The device distinguishes between constant absorption by non-pulsatile tissues and the varying absorption caused by the pulsatile arterial blood flow. Every time the heart beats, a surge of arterial blood enters the tissue, temporarily increasing the amount of blood in the light path. The oximeter measures the difference in light absorption between the baseline state and the peak of the pulse. This pulsatile component corresponds specifically to arterial blood, which is what we need to measure for SpO2.
Using the ratio of the absorption of red light to infrared light in this pulsatile component, the device calculates the percentage of oxygenated hemoglobin in the arterial blood. A high ratio of infrared absorption to red absorption indicates that most hemoglobin is saturated with oxygen, resulting in a high SpO2 reading, such as 98%. Conversely, a lower ratio suggests more deoxygenated hemoglobin and a lower SpO2 reading.
This method, however, has limitations. Factors like poor peripheral circulation, motion artifacts, ambient light interference, carbon monoxide poisoning, or nail polish can distort the readings. Despite these limitations, the technology remains a critical monitoring tool. It allows healthcare providers to quickly assess oxygen levels during surgeries, in emergency rooms, or for patients with chronic conditions like COPD. In everyday use, a healthy individual typically has an SpO2 reading between 95% and 100%. A reading below 90% is often considered low and warrants medical attention. In essence, the pulse oximeter transforms a physical property of hemoglobin—how it interacts with light—into a vital sign that can save lives by providing an early warning of respiratory compromise.