Origin of Infrared Light Modulation in Reflectance-Mode Photoplethysmography.

We recently pointed out the important role of dermis deformation by pulsating arterial pressure in the formation of a photoplethysmographic signal at green light. The aim of this study was to explore the role of this novel finding in near-infrared (NIR) light. A light-emitting diode (LED)-based imaging photoplethysmography (IPPG) system was used to detect spatial distribution of blood pulsations under frame-to-frame switching green and NIR illumination in the palms of 34 healthy individuals. We observed a significant increase of light-intensity modulation at the heartbeat frequency for both illuminating wavelengths after a palm was contacted with a glass plate. Strong positive correlation between data measured at green and NIR light was found, suggesting that the same signal was read independently from the depth of penetration. Analysis of the data shows that an essential part of remitted NIR light is modulated in time as a result of elastic deformations of dermis caused by variable blood pressure in the arteries. Our observations suggest that in contrast with the classical model, photoplethysmographic waveform originates from the modulation of the density of capillaries caused by the variable pressure applied to the skin from large blood vessels. Particularly, beat-to-beat transmural pressure in arteries compresses/decompresses the dermis and deforms its connective-tissue components, thus affecting the distance between the capillaries, which results in the modulation of absorption and scattering coefficients of both green and NIR light. These findings are important for the correct interpretation of this widely used medical technique, which may have novel applications in diagnosis and treatment monitoring of aging and skin diseases.

Influence of a skin status on the light interaction with dermis.

We present experimental evidence that the parameters of green light remitted from a human tissue in-vivo strongly depend on skin contact status. In case when the skin is free of any contact, simultaneous recording of imaging photoplethysmogram (iPPG) and electrocardiogram revealed that contactless iPPG fails in correct estimates of the heart rate in almost half of the cases. Meanwhile, the number of successful correlations between ECG and iPPG is significantly increased when the skin is in contact with a glass plate. These observations are in line with the recently proposed model in which pulsatile arteries deform the connective-tissue components of the dermis thus resulting in temporal modulation of the capillary density interacting with slightly penetrating light.