The fundamental principle of an oximeter is spectrophotometry, which is used to measure the percentage of hemoglobin in your blood that is saturated with oxygen (SpO2). This involves shining light through the blood and measuring how much light is absorbed.
How Oximetry Works: Spectrophotometry Explained
Pulse oximetry relies on two key principles:
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Spectrophotometry: Different substances absorb light at different wavelengths. Oxygenated hemoglobin (hemoglobin carrying oxygen) and deoxygenated hemoglobin (hemoglobin not carrying oxygen) have distinct absorption spectra. The oximeter shines red and infrared light through the tissue (typically a fingertip) and measures the amount of light that passes through. By analyzing the absorption patterns, the oximeter can determine the relative proportions of oxygenated and deoxygenated hemoglobin.
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Photoplethysmography: This technique detects the pulsatile changes in blood volume caused by the heartbeat. The oximeter uses these pulsations to isolate the arterial blood signal, which is the blood of interest for measuring oxygen saturation. This ensures that the measurement is primarily from arterial blood and not from venous blood or other tissues.
Key Components and Process
Here's a breakdown of the process:
- Light Emission: The oximeter emits two wavelengths of light: red (around 660 nm) and infrared (around 940 nm).
- Light Transmission/Absorption: These light beams pass through the tissue (e.g., fingertip, earlobe). Hemoglobin absorbs red light more strongly when it is deoxygenated and infrared light more strongly when it is oxygenated.
- Light Detection: A photodetector on the opposite side of the tissue measures the amount of red and infrared light that passes through.
- Signal Processing: The oximeter processes the detected signals, taking into account the pulsatile nature of arterial blood flow, to isolate the arterial blood signal.
- SpO2 Calculation: Using the ratio of red to infrared light absorption, the oximeter calculates the percentage of oxygenated hemoglobin (SpO2). This value is then displayed on the oximeter screen.
Factors Affecting Oximeter Accuracy
Several factors can affect the accuracy of oximeter readings, including:
- Poor perfusion: Low blood flow to the measurement site (e.g., cold hands) can lead to inaccurate readings.
- Motion artifact: Movement during measurement can interfere with the signal.
- Ambient light: Strong ambient light can interfere with the detector.
- Nail polish or artificial nails: These can affect light transmission.
- Certain medical conditions: Anemia, abnormal hemoglobin levels (e.g., carboxyhemoglobin in carbon monoxide poisoning), and certain dyes in the blood can also affect accuracy.
In summary, pulse oximetry is a non-invasive method for estimating arterial oxygen saturation by using spectrophotometry and photoplethysmography to analyze the absorption of red and infrared light by hemoglobin in pulsating arterial blood.
