Analysis of the Pulse Transit Time Estimation of a Dual-PPG Setup

Non-invasive, continuous blood pressure monitoring is a crucial diagnostic tool for society, increasingly affected by cardiovascular diseases worldwide. One commonly used surrogate parameter for blood pressure estimation is the Pulse Transit Time (PTT). With the advent of wearable sensors, the dual-Photoplethysmography (dual-PPG) emerges as a promising approach for capturing the local PTT. Several approaches have already used the dual-PPG to estimate the peripheral blood pressure, although the approaches differ significantly in terms of the setup and the subsequent PTT calculation. To establish a more standardized approach, this paper will first introduce a new low-cost dual-Photoplethysmography setup for measuring the local PTT at the radial artery and evaluate it in a pilot study. Secondly, it analyzes the effects of the wavelength and the Fiducial Point (FP) selection on the resulting PTT. Thirdly, the paper investigates the capabilities of the proposed setup to track blood pressure changes by variations of the PTT. The results indicate that capturing the local PTT is still a non-trivial task. Signal quality, inter-individual differences and sensor placement all affect the reliability and outcome of the PTT estimation. The findings suggest that the pulse wave onset and the pulse wave turning point are better suited for PTT calculation than the commonly used systolic peak. Both methods yield similar PTTs, which fall within the expected value range for healthy participants. In contrast, using the systolic peak leads to highly variable and negative PTT values, making it an unsuitable basis for subsequent data processing and is therefore not recommended. Moreover, the results indicate that infrared light is the more robust choice compared to red light and is better suited for the dual-PPG measurement.