Hemoglobin Oxygen Saturation Simulation

The Effects of Altitude

What is a Pulse Oximeter?

A pulse oximeter is a small plastic device that is typically clipped to a fingertip or an earlobe. 

The top of the device contains a light source that emits red and infrared light, while the bottom has a sensor that detects the amount of light absorbed by oxygenated hemoglobin. 

A small digital display screen integrated into the device shows the user's peripheral blood oxygen saturation percentage (SpO₂) and pulse rate (PR).

How Pulse Oximeters Work

:: Light Emission and Detection

Embedded on one side of the oximeter are two light-emitting diodes (LEDs); one releases red (~660 nm) and the other infrared (~940 nm) light. These specific wavelengths are used because the absorption of red and infrared light differs between oxygenated and deoxygenated hemoglobin. Oxygenated hemoglobin absorbs more infrared light and less red light, while deoxygenated hemoglobin absorbs more red light and less infrared light.

On the other side of the oximeter is a photodetector that measures the amount of light absorbed by the constant and variable components that make up the body part in the oximeter. The constant components include skin, connective tissue, bone, and nonpulsating blood. In comparison, the variable component consists of pulsating arterial blood caused by each heartbeat.

:: Calculation of SpO₂

A pulse oximeter calculates the oxygen-carrying capacity of all circulating hemoglobin molecules. It does this by projecting red and infrared light through pulsating arteries in peripheral blood and comparing the light absorption ratio. This calculation relies on algorithms programmed into the device, which consider the typical absorption characteristics of oxyhemoglobin and deoxyhemoglobin.

The results of this calculation are displayed as SpO₂ % or percentage of oxygenated hemoglobin in peripheral blood. When the SpO₂ reading is less than 100%, the hemoglobin in the peripheral blood carries less than the maximum oxygen it could potentially carry. In other words, not all of the hemoglobin molecules are fully saturated or bound to 4 O₂  molecules.

For example, a SpO₂ reading of 85% indicates that the hemoglobin in circulation carries 85% of its theoretical maximum oxygen capacity because many hemoglobin molecules are less than fully saturated.

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:: Pulse Rate

In addition to measuring oxygen saturation, the pulse oximeter measures a subject's pulse rate. As the heart pumps blood through the arteries, the amount of light absorbed by the blood changes. The pulse oximeter detects the frequency of these cyclical changes in light absorption and uses them to calculate the patient's pulse rate, which is also displayed on the digital screen.

Assessment

1. What is a pulse oximeter, and where is it typically placed on the body?
2. What is displayed on the digital screen of a pulse oximeter?
3. What wavelengths of light are used in pulse oximetry?
4. How does the absorption of these light wavelengths differ between oxygenated and deoxygenated hemoglobin?
5. What does SpO₂ represent, and how does the pulse oximeter calculate it?
6. How does a pulse oximeter measure a patient's pulse rate?
7. Based on your understanding of how an oximeter operates, describe three factors that may cause inaccurate oximeter readings.

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