<title>Time variable photoplethysmographic signal: its dependence on light wavelength and sample volume</title>

Abstract

ABSTRACTThe photoplethysmographic signal (PPG) from forearm skin and the ECG were recorded simultaneously fromhealthy subjects. The optical signal was derived with a fiber-optic probe which consisted of 61 fiber pairs. The peak-to-peak averaged AC-signal was calculated. The dependency of this signal on light wavelength and on sample volume wasstudied. The light intensities used at 560 nm were 0.015 and 0.029 mWmm2 and at 940 nm we used 0.029, 1.37 and2.77 mWmm2. A theoretical model for calculation of the sample volumes was also developed. The amplitude of theAC-signal at 560 nm is larger than at 940 nm at the same intensity (0.029 mWmm2). At 560 nm and small samplevolumes (7 fiber pairs) the amplitude did not increase with increased intensity. At 940 nm using 1 .37 and 2.77 mWmm2the AC-signal increased with the number of fiber pairs (volume).1. INTRODUCTIONThe most common clinical methods for heart rate monitoring are based on the electrocardiogram (ECG) and thephotoplethysmographic signal (PPG). Electrode-skin impedance instabilities and skin irritations caused by the electrodepaste are common problems, making the ECG less suitable for monitoring purposes.PPG was first used by Hertzman1 for studies of blood volume pulsations in the skin. Since then PPG has beenused for heart rate monitoring, perfusion monitoring and oxygen saturation measurements (pulse oximetry). Thetechnique is based on light impinging on the skin surface. The light is absorbed, scattered and reflected in tissue andblood. Perfusion and structural changes in the skin modulate the light which returns to the photodetector. The intensityvariations arise from perfusion changes, reorientation of the erythrocytes and vessel wall movements which aresynchronous with the heart rate1'2'3. By in vivo and in vitro studies of blood flow in arteries, D'Agrosa4 and Hertzman1concluded that the origin of the heart synchronous (AC) variations of the PPG-signal is not blood volumetric changes butrather reorientation and distribution changes of the erythrocytes. Lindberg and Oberg5 suggested that the origin of thesignal has different mechanisms; at 560