What is the modulation ratio with regard to pulse oximetry? (R-Value)

In the context of pulse oximetry, the modulation ratio is a measure of the amplitude modulation of the light transmitted through the tissue due to the presence of pulsatile arterial blood. It is defined as the ratio of the AC (alternating current) component to the DC (direct current) component of the transmitted light signal

The modulation ratio is used in pulse oximetry to determine the accuracy and reliability of the measured oxygen saturation (SpO2) values. A higher modulation ratio indicates a stronger pulsatile signal and better signal-to-noise ratio, resulting in more accurate and reliable SpO2 readings.

According to a study by Allen et al. (2007), the modulation ratio is typically in the range of 1% to 3% for healthy adults. However, the modulation ratio may be lower in patients with low perfusion, weak pulse, or other physiological conditions that affect the pulsatility of arterial blood flow. In such cases, the accuracy and reliability of the SpO2 readings may be compromised.

Keywords: modulation ratio, R-value

Reference: Allen, J. (2007). Photoplethysmography and its application in clinical physiological measurement. Physiological Measurement, 28(3), R1-R39.