How Do Pulse Oximeters Work? An In-Depth Guide

Pulse oximeters are non-invasive devices used to measure the oxygen saturation level, SpO2, in a person's blood. These devices are widely used in various medical settings, including hospitals and clinics, as well as for home monitoring. This article provides a detailed explanation of how pulse oximeters function, their key components, working principles, and the advantages and limitations of their use.

Key Components

Pulse oximeters consist of several key components that work in harmony to provide accurate readings. These include light-emitting diodes (LEDs), photodetectors, and algorithms for processing the data collected.

Light Emitting Diodes (LEDs)

Pulse oximeters typically use two LEDs that emit light at different wavelengths. The most common wavelengths are red (around 660 nm) and infrared (around 940 nm). These LEDs shine light through the tissue to the photodetector on the other side.

Photodetectors

A photodetector is a sensor that measures the amount of light that passes through the tissue. This sensor plays a crucial role in determining the oxygen saturation level.

Working Principle

The functioning of a pulse oximeter involves several steps, including placement, light absorption, pulse detection, and calculation of oxygen saturation.

Placement

The pulse oximeter is placed on a thin part of the body, such as a fingertip or earlobe. The LEDs shine light through the tissue to the photodetector on the opposite side.

Light Absorption

During the measurement process, light absorption plays a vital role. Oxygenated hemoglobin (HbO2) absorbs more infrared light and allows more red light to pass through. Conversely, deoxygenated hemoglobin (Hb) absorbs more red light and allows more infrared light to pass through.

Pulse Detection

The device captures data during the pulsatile flow of blood, which is caused by the heartbeat. It distinguishes between the light absorption of arterial blood, which varies with each heartbeat, and non-pulsatile blood (venous blood), which does not.

Calculating Oxygen Saturation

The ratio of the absorbed red and infrared light is calculated to determine the ratio of oxygenated to deoxygenated hemoglobin. This ratio is then converted into a percentage, indicating the oxygen saturation level in the blood (SpO2).

Display and Interpretation

The results are displayed as a percentage on the screen of the pulse oximeter and are often accompanied by a heart rate reading. Normal SpO2 levels typically range from 95 to 100. Levels below 90 may indicate a need for medical attention.

Advantages and Limitations

Advantages:

Non-invasive Quick and easy to use

Limitations:

Poor circulation, skin pigmentation, nail polish, or external light interference can affect accuracy

Applications

Pulse oximeters are widely used in various medical settings, including hospitals, clinics, and for home monitoring, particularly for patients with respiratory or cardiovascular conditions.