Photoplethysmography for an independent measure of pulsatile pressure under controlled flow conditions

Noninvasive continuous blood pressure measurements are desirable for patients and clinicians. This work proposes and validates a method for transmural pressure measurement using photoplethysmography (PPG) in an in vitro setup that allows control of pressure and flow conditions. The optimum pulsatile volume measure is obtained by comparing parameters extracted from the photoplethysmographic signal (AC amplitude, normalized pulse volume (NPV) and adjusted pulse volume (APV)). Pulsatile volume can then provide pressure measurements using the exponential pressure-volume (P-V) relationship and validated using the gold standard catheter pressure measurement. Pressure, red (R) and infrared (IR) PPG signals were recorded continuously in two arterial models with different cross-sectional areas (Model 1 and Model 2) utilising a pulsatile pump. Flow rates were controlled by varying pumping frequencies at low and high stroke volumes. The optimum method for estimation of the pulsatile volume is through APV, which had a highly significant correlation (r(2) = 0.99, p < 0.001) for Model 1 and (r(2) = 0.98, p < 0.001) for Model 2. APV obtained a significantly better fit when compared to NPVIR (r(2) = 0.73, z = 25.85, p < 0.001), NPVR (r(2) = 0.95, z = 12.26, p < 0.001), IRAC (r(2) = 0.52, z = 28.29, p < 0.0001) and RAC (r(2) = 0.92, z = 15.27, p < 0.0001) in Model 1, and when compared to NPVIR (r(2) = 0.92, z = 10.23, p < 0.0001), NPVR (r(2) = 0.96, z = 5.08, p < 0.001) IRAC (r(2) = 0.63, z = 22.47, p < 0.0001) and R-AC (r(2) = 0.92, z = 17.70, p < 0.0001) in Model 2. These preliminary findings suggest that APV could be used as a potential non-invasive continuous method of blood pressure measurement at different flow conditions.