Photoplethysmography beyond Perfusion and Oxygenation Monitoring: Pulse Wave Analysis for Hepatic Graft Monitoring

Photoplethysmography is a technique widely used in monitoring perfusion and blood oxygen saturation based on the amplitude of the pulsatile signal at one or multiple wavelengths. However, the pulsatile signal carries in its waveform a substantial amount of information about the mechanical properties of the tissue and vasculature under investigation that is still yet to be utilized to its full potential. In this work, we present the feasibility of pulse wave analysis for the application of monitoring hepatic implants and diagnosing graft complications. In particular, we demonstrate the utility of computing the slope of the pulse during the diastole phase to assess compliance changes in tissue. This hypothesis was tested in a series of in vitro experiments using a polydimethylsiloxane based phantom mimicking the optical and mechanical properties of the portal vein. The emptying time decreased from 148.1 ms for phantoms with compliance of 12 KPa to 97.5 ms for phantoms with compliance of 61 KPa. These compliance levels mimic those seen for normal and fibrotic hepatic tissue respectively.