Tonometric Multi-Wavelength Photoplethysmography for Studying the Cutaneous Microvasculature of the Fingertip

Microcirculation is a key compartment in the human cardiovascular system due to its vital roles in providing oxygen and nutrients to tissue, removing metabolic byproducts and regulating blood flow in organs. This network of small blood vessels, known as microvasculature, has been shown to have a link to many cardiovascular diseases. This work presents a method capable of extracting information from different depths of the cutaneous vasculature, including the microvasculature, of the fingertip using an optical sensor with controllable external compression force. Our experiments show that the optical channels can be used to estimate blood pressure at different depths of the tissue, including shallow depths with microvascular blood vessels. Additionally, we show that shorter-wavelength optical signals (465 nm, 515 nm and 590 nm) are more sensitive to pressure-induced vasodilation than longer-wavelength (640 nm and 880 nm) signals. The results indicate that the method can obtain signals originating from different depths of the cutaneous vasculature, from large arteries deep in the skin to arterioles and capillaries close to the surface of the skin. The experimental results are discussed with the help of a Monte Carlo model that simulates photon propagation in tissue under different compression scenarios.