A 2-DOF Model of the Artery-Sensor System for Interpreting Variability in Measured Arterial Pulse Waveform

This study presents a two degree-of-freedom (2-DOF) model of the artery-sensor system that accounts for contact pressure, sensor type, and overlying tissue above the artery in measured arterial pulse waveform (APW). While the arterial wall is treated as a one degree-of-freedom (1-DOF) unit, the tissue-contact-sensor (TCS) unit, formed by overlying tissue, sensor, and contact interface, is treated as another 1-DOF unit. Measured APW from three types of sensors: a photoplethysmography (PPG) sensor, an accelerometer, and a tactile sensor, is theoretically related to dynamic parameters of the TCS unit. With pulsatile pressure at the radial artery (RA) as the true APW, the measured APW is calculated with different values of the dynamic parameters of the TCS unit. Due to its non-negligible damping and inertial terms, the TCS unit serves as a harmonics-dependent transfer function from the true APW to the measured APW. By determining dynamic parameters of the TCS unit, contact pressure, sensor type, and overlying tissue affect measured APW, and consequently the foot, peak, and other key features of measured APW. The natural frequency of the TCS unit plays a dominant role in affecting measured APW, as compared to its damping and spring stiffness. The myth of the influence of transmural pressure on arterial wall elasticity stems from negative pulsatile forces from the TCS unit. Motion artifacts introduce not merely additive noise (or baseline drift) but also multiplicative noise to measured APW, and thus minimizing motion artifacts is crucial for improving accuracy in measured APW.