Measurement of Pulse Transit Time (PTT) Using Photoplethysmography

An experimental setup for measuring pulse transit time (PTT) using two photoplethysmographic (PPG) signals is proposed. The pulse wave velocity is calculated from the measured PTT, for its potential use in monitoring arterial stiffness and continuous non-invasive arterial blood pressure. The PPG sensors are placed in-line separated by 1cm, over the arterial branch under consideration. PTT is determined by using the fact that there exists a time difference be-tween PPG signals obtained from the two sensors. The outputs from the sensors are bandlimited and amplified. The signals are then made TTL- compatible, to determine pulse transit time. The final output is in the form of a rectangular pulse whose width is equal to the PTT. A phase shifted sinusoid generated by an all-pass filter is used to validate the proposed experimental setup. Placing the sen-sors over the digital artery of the index finger, 1 cm apart, the PTT for 13 healthy volunteers was measured. PTT is averaged for 10 cardiac cycles to obtain mean PTT. From mean PTT the Pulse Wave Velocity (PWV) is calculated for a volunteer. The maximum of 2.6 and a minimum of 0.4 standard deviation was observed, for a single volunteer. The overall mean PTT was 10.1 ms and the mean PWV was 1.1 m/s.