Figure 1.

A visualization of the methods multi-site photoplethysmography (PPG) to measure the pulse transit time (PTT). Multisite PPG can make use of twos PPG probes, one proximal and one distal, to measure the time it takes for the pulse wave to travel a distance. (A,B) Two-location dual PPG/iPPG systems. (C) iPPG system. (D) Single artery dual PPG system. (E) Multiple wavelengths system. ECG—electrocardiography, IPG—impedance plethysmography, BCG—ballistocardiography, PCG—phonocardiography, PPG—photoplethysmography, iPPG—image-based PPG, and MWPPG—multi-wavelength PPG.