Feasibility of Remote Pulse Transit Time Estimation Using Narrow-band Multi-wavelength Camera Photoplethysmography

Contact-free remote sensing gained much traction in the past decade. While such monitoring of some vitals (heart rate) is approaching clinical levels of performance, others remain difficult to estimate (blood pressure) while being very valuable. In this paper we investigated the feasibility of estimating pulse transit time (PTT) - a marker known to be highly correlated with blood pressure - in a remote way from a single measuring site, using just a single modified RGB camera. The replacement of infrared (IR) filter with a narrow band triple bandpass filter allowed us to remotely measure the PTT between traditional wavelengths (green) and infrared (NIR) using a regular RGB camera. We measured PTT leveraging the fact that different wavelengths penetrate to different skin depths. Use of such a filter minimizes the inter-channel influence and band overlap and leverages NIR information not traditionally available from consumer RGB cameras. This way we obtained slightly delayed photoplethysmograms corresponding to each wavelength and skin depth. In our initial experiments with 5 subjects we observed relatively consistent temporal delays between waveforms from different wavelengths (especially near-infrared and green) in accordance with expectations and related work. These early results show promising fundamentals for further research in remote multi-wavelength PTT and blood pressure estimation, while also highlighting important fundamental and technical challenges to be considered.