The pulse oximeter detects a patient's heart rate by analyzing the changes in light absorption caused by the pulsatile flow of arterial blood in the finger (or other extremity).
Here's a more detailed explanation:
The Process Explained
Pulse oximeters use a combination of light and a light sensor to determine heart rate and oxygen saturation. The process involves the following steps:
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Light Emission: The oximeter emits two wavelengths of light: red light (around 660 nm) and infrared light (around 940 nm) through the patient's finger, toe, or earlobe.
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Light Absorption: Hemoglobin in the blood absorbs these lights differently depending on whether it's carrying oxygen (oxyhemoglobin) or not (deoxyhemoglobin). Arterial blood volume fluctuates with each heartbeat. This pulsatile flow causes corresponding changes in light absorption.
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Light Detection: A photodetector on the other side of the finger measures the amount of red and infrared light that passes through.
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Signal Processing: The pulse oximeter's internal processor analyzes the rhythmic changes in light absorption associated with each pulse of arterial blood. These changes correspond to the heart's pumping action.
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Heart Rate Calculation: By measuring the frequency of these pulsatile changes in light absorption, the device can accurately calculate and display the patient's heart rate (reported as pulses per minute or BPM). It identifies the peaks in the absorption pattern and calculates the time interval between them.
Key Components and Their Roles
| Component | Role |
|---|---|
| Red LED | Emits red light (660 nm) that is absorbed differently by oxyhemoglobin and deoxyhemoglobin. |
| Infrared LED | Emits infrared light (940 nm) that is also absorbed differently by oxyhemoglobin and deoxyhemoglobin, providing another data point for analysis. |
| Photodetector (Photocell) | Measures the amount of red and infrared light that passes through the tissue, providing the raw data for the algorithm. |
| Microprocessor | Analyzes the detected light signals, filters out noise, and calculates both heart rate and oxygen saturation (SpO2). It distinguishes between arterial and venous blood flow and other tissue absorption characteristics. |
Accuracy Considerations
The accuracy of the heart rate measurement by a pulse oximeter can be affected by several factors, including:
- Poor Circulation: Low blood flow to the extremities can make it difficult to detect pulsatile changes.
- Motion Artifact: Movement can introduce noise into the signal.
- Ambient Light: Excessive ambient light can interfere with the photodetector.
- Skin Pigmentation: Can affect light absorption
In summary, a pulse oximeter derives a patient's heart rate from the rhythmic fluctuations in light absorption caused by the pulsatile flow of arterial blood, using red and infrared light and a photodetector, which are then processed to isolate and quantify these fluctuations.
