Principle of pulse oximetry

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  • Kosaka Makoto
    Department of Anesthesiology, Showa University Northern Yokohama Hospital
  • Yoshida Ai
    Department of Anesthesiology, Showa University Northern Yokohama Hospital
  • Ohe Katsunori
    Department of Anesthesiology, Showa University Northern Yokohama Hospital

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  • パルスオキシメータの原理

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In the ICU, electrocardiogram (ECG) and SpO2 are required for patient monitoring. An ECG shows cardiac electrical activity, but pulse oximetry can measure the pulse rate and SpO2 using plethysmogram. Arterial blood drawing is not needed, and the SpO2 measurements are noninvasive, unlike SaO2 measurements. SpO2 is measured using a red light, an infrared light, and absorption spectroscopy. There are many factors affecting the measurement of SpO2. A low peripheral circulation and body movement can make the detection of the pulse wave difficult, greatly decreasing the accuracy of the SpO2 measurement. The solution for this disturbance to the measurement varies according to each maker. Understanding the principle of measurement is necessary for the interpretation of SpO2 values. There are two methods for measuring SpO2: transmittance and reflectance. The reflection type measures SpO2 at the supra-orbital artery, wherein arterial heartbeat is maintained at the time of a shock and the hypothermia. Moreover, the reflection type has a shorter discrimination time to SpO2 change than the finger of limb. This method is suitable for ICU patients with low peripheral perfusion and motion artifacts.

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