Video capillaroscopy clarifies mechanism of the photoplethysmographic waveform appearance

Abstract

Photoplethysmography (PPG) devices are widely used in clinical practice but the origin of PPG signal is still under debating. The classical theory assumes that the PPG waveform stems from variations of blood volume in pulsating arteries. In this research we analysed high-speed video recordings of capillaries in a fingernail bed. It was found that speed of erythrocytes in capillaries has pronounced modulation in time, which follows variations of instantaneous blood pressure in arteries. However, the mean speed significantly differs even for neighbour capillaries whereas change of the speed occurs in phase for the most of capillaries. Moreover, the light intensity remitted from the papillary dermis is also modulated at the heartbeat frequency displaying significant correlation with waveforms of the RBC speed. Obtained results can hardly be explained by the classical theory of PPG signal formation. Shallow penetrating visible light acquires modulation of erythrocytes density in the capillary bed without interacting with deeper situated pulsating arteries. Therefore, the capillary bed could serve as a distributed sensor for monitor the status of deep vessels. Better understanding of the photoplethysmography basis will result in a wider range of applications of this fast growing technology in both medical and research practice.

Alterations of blood pulsations parameters in carotid basin due to body position change

“…Therefore, modulation of the erythrocytes speed in capillaries accurately enough describes the shape of the pulse wave in the point of measurements. In recent comparative measurements of the erythrocytes speed and PPG waveforms, the high correlation between these waveforms was reported 28 suggesting smallness of the delay between them. However, one may assume existence of additional time delay between erythrocytes speed and light intensity modulation due to still debatable mechanism of light intensity modulation in capillaries.…”

“…According to the recent model proposed by our group 28 , 29 , interaction of the polarized green light with skin integument allows assessment of the erythrocytes speed in capillaries of the papillary layer. Recordings of the pulsatile components originating from the pulse wave arrived to the facial area allows us mapping the spatial distribution of both the amplitude and dynamic parameters of the capillary blood flow 28 . Due to described technological advances, these maps were calculated with high spatial and temporal resolution.…”

“…We assume that the PPG waveform under green illumination originates from the upper capillary level 28 , 29 . Considering the small length of capillaries (about 1 mm), the time delay of the pulse wave between the arteriole and capillary is negligible compare to the time needed to arrive the arteriole.…”

Camera-based pulse-oximetry - validated risks and opportunities from theoretical analysis

“…The origin of the PPG signal is subject to debate recently for visible wavelengths [14,15]. Experiments using green resulted in observations which contradicted the commonly accepted volumetric model.…”

Dynamic evaluation of blood flow microcirculation by combined use of the laser Doppler flowmetry and high‐speed videocapillaroscopy methods

“…Moreover, the combination of LDF and high‐speed VCS is motivated by the involvement of two methods with different physical principles, so the results are independent of each other, and their comparison makes the results more reliable. It has been shown in previous studies that combined approach can be used to validate photoplethysmographic records . A single‐capillary estimate of RBCs velocity may also be of particular interest for many applications of LDF measurements.…”