Pulse oximetry relies on the principles of spectrophotometry and photoplethysmography to non-invasively estimate the oxygen saturation of hemoglobin in arterial blood (SpO2).
Spectrophotometry
Spectrophotometry is the core principle behind pulse oximetry. It's based on the fact that oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) absorb light differently at different wavelengths.
- Red Light (around 660 nm): Deoxygenated hemoglobin (Hb) absorbs more red light than oxygenated hemoglobin (HbO2).
- Infrared Light (around 940 nm): Oxygenated hemoglobin (HbO2) absorbs more infrared light than deoxygenated hemoglobin (Hb).
The pulse oximeter emits both red and infrared light through a finger, toe, or earlobe. A photodetector on the opposite side measures the amount of each wavelength that passes through the tissue. By analyzing the ratio of red to infrared light absorption, the device can calculate the percentage of hemoglobin that is saturated with oxygen.
Photoplethysmography
Photoplethysmography (PPG) is a technique that detects changes in blood volume in the microvasculature. In pulse oximetry, PPG helps to distinguish between arterial blood (pulsatile flow) and venous blood, tissue, and bone (non-pulsatile flow). The pulsatile component, which reflects the arterial blood flow with each heartbeat, is used to determine the SpO2.
The pulse oximeter detects the pulsatile changes in light absorption caused by the arterial blood flow. By subtracting the baseline (non-pulsatile) absorption from the peak (systolic) absorption for both red and infrared wavelengths, the device isolates the arterial component. This is critical for accurate SpO2 measurement because it minimizes the interference from venous blood, tissue, and other factors.
Calculation of SpO2
The pulse oximeter calculates the SpO2 using the following steps:
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Measurement of Light Absorption: The photodetector measures the intensity of red and infrared light that passes through the tissue.
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Separation of Pulsatile and Non-Pulsatile Components: PPG is used to isolate the arterial component from the total light absorption.
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Calculation of the Ratio of Ratios (R): The ratio of red light absorption to infrared light absorption is calculated for both the pulsatile (AC) and non-pulsatile (DC) components:
R = (AC Red / DC Red) / (AC Infrared / DC Infrared)
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SpO2 Determination: The calculated ratio (R) is then used in an empirical formula or look-up table (stored in the oximeter's memory) that relates the ratio to the SpO2 value. This relationship is derived from studies in which arterial blood gas measurements were compared to pulse oximeter readings.
Factors Affecting Accuracy
Several factors can affect the accuracy of pulse oximetry readings:
- Poor Perfusion: Low blood flow (e.g., due to hypothermia, hypotension, or vasoconstriction) can reduce the pulsatile signal, leading to inaccurate readings.
- Motion Artifact: Movement of the patient or sensor can create noise in the signal, affecting the accuracy of the measurements.
- Ambient Light: Strong ambient light can interfere with the light emitted by the oximeter.
- Dyshemoglobinemias: Conditions like carbon monoxide poisoning (carboxyhemoglobin) or methemoglobinemia can cause falsely high or low SpO2 readings because these abnormal hemoglobin species absorb light differently than Hb and HbO2.
- Skin Pigmentation: Although newer devices are more accurate across different skin tones, very dark skin pigmentation may sometimes affect the accuracy of older models.
- Nail Polish: Dark nail polish can interfere with light transmission and affect the reading.
Clinical Significance
Pulse oximetry is a valuable tool for monitoring oxygen saturation in a variety of clinical settings, including:
- Anesthesia and Surgery: Monitoring patients' oxygenation during procedures.
- Intensive Care: Assessing respiratory status in critically ill patients.
- Emergency Medicine: Evaluating patients with respiratory distress.
- Pulmonary Function Testing: Monitoring oxygen saturation during exercise or other challenges.
- Sleep Studies: Detecting sleep apnea and other sleep-related breathing disorders.
