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.
