Abstract
Photoplethysmography (PPG) is a non-invasive optical method that can be used to detect blood volume changes in the microvascular bed of tissue. The PPG signal comprises two components; a pulsatile waveform (AC) attributed to changes in the interrogated blood volume with each heartbeat, and a slowly varying baseline (DC) combining low frequency fluctuations mainly due to respiration and sympathetic nervous system activity. In this report, we investigate the AC pulsatile waveform of the PPG pulse for ultimate use in extracting information regarding the biomechanical properties of tissue and vasculature. By analyzing the rise time of the pulse in the diastole period, we show that PPG is capable of measuring changes in the Young's Modulus of tissue mimicking phantoms with a resolution of 4 KPa in the range of 12 to 61 KPa. In addition, the shape of the pulse can potentially be used to diagnose vascular complications by differentiating upstream from downstream complications. A Windkessel model was used to model changes in the biomechanical properties of the circulation and to test the proposed concept. The modeling data confirmed the response seen in vitro and showed the same trends in the PPG rise and fall times with changes in compliance and vascular resistance.
Highlights
Photoplethysmography (PPG) is one of the commonly used methods to record the pulse noninvasively [1]
The phantoms have a Young’s modulus of 11.5 and 61 KPa which mimic the change from normal hepatic tissue to the last stage of fibrosis [40]
For our in vitro studies, phantoms were developed with Young’s Modulus (YM) of 11.7, 15 and 61 KPa and the analysis of the PPG rise time showed a significant decrease with the increase of the YM
Summary
Photoplethysmography (PPG) is one of the commonly used methods to record the pulse noninvasively [1]. PPG sensors illuminate tissue with various wavelengths of light and collect the remainder of the light after it travels through tissue. These sensors can be divided into two categories depending on the probe formation and the location of light collection. The first approach is transmission which uses a photodetector and a light source on opposite sides of the tissue to measure optical intensity after light has propagated through tissue. The pulsatile blood flow causes changes in the tissue blood volume. This leads to variation in optical absorption which modulates the intensity of the collected light. PPG has been used in various applications ranging from heart rate monitoring [2, 3] to non-invasive perfusion tracking and imaging [4, 5], but pulse oximetry remains the most common and widely adapted application of PPG [6, 7]
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