Photoplethysmography: Towards a non-invasive pressure measurement technique.

There is a need for non-invasive and continuous blood pressure measurements. This study explored the relationship between Photoplethysmography (PPG) and Pressure-Volume (P-V) changes in an in vitro model. We proposed and compared methods for an estimation of the pulsatile volume, termed as Normalized Pulse Volume (NPV) and Adjusted Pulse Volume (APV). To validate the methods, pressure, Red (R) and InfraRed (IR) PPG signals were recorded continuously in an arterial model utilizing a pulsatile pump. Flow rates were controlled by varying pumping frequencies at low and high stroke volumes. It was found that the optimum method for estimation of the pulsatile volume is through APV, which had a high correlation (r(2)=0.99, p<;0.001) with the assumed exponential P-V model. APV obtained a significantly better fit when compared to NPVIR (r(2)=0.73, z=25.85, p<;0.001) and NPVR (r(2)=0.95, z=12.26, p<;0.001). These preliminary findings suggest that APV could be used as a potential non-invasive continuous method of blood pressure measurement.