Pulse Measurement Alters the True Pulse Signal in an Artery: A Coupled String-SDOF Model

This paper presents an analytical model for capturing the influence of pulse measurement on the true pulse signal in an artery, which poses challenges in accurately detecting arterial pulse signals. An artery is modeled as a vibrating-string, with its displacement representing the displacement of the arterial wall. Formed by overlying tissue and a sensor, a tissue-contact-sensor (TCS) stack transmits the true pulse signal in an artery to the sensor as a measured pulse signal and is modeled as a single-degree-of-freedom (SDOF) system. As a local disturbance, this SDOF system interacts with the vibrating-string, leading to a coupled string-SDOF model. The related numerical calculation is conducted in matlab, with three sensor types: tactile sensors, accelerometers, photoplethysmography (PPG) sensors, and ultrasound. With all sensor types and ultrasound, the extent of the true pulse signal being affected varies with the TCS stack and the artery (i.e., geometry and physical properties) and its pulse signals. While a tactile sensor and ultrasound can suppress the amplitude of a true pulse signal by about 50%, an accelerometer or a PPG sensor can amplify it by less than 10%. The obtained results reveal the need to quantitatively account for the TCS-stack-artery interaction, if not fully, at least to some extent, when high measurement reliability and discerning relatively fine differences between two true pulse signals are required. The coupled string-SDOF model provides an analytical framework for possibly quantifying such interaction in the future.