Formation and informative content of photoplethysmographic signal under the conditions of controllable external compression

Abstract

The physical nature and quantitative characteristics of the photoplethysmographic (PPG) signal under conditions of external compression of the studied tissue area are theoretically investigated. Dependences of the constant and variable components of the PPG signal on the applied pressure are revealed, which have maxima at certain compression values. The physical model of the PPG signal formation process takes into account the optical and mechanical properties of biological tissues. The model is based on the consideration of the diffuse propagation of light in a two-component scattering medium (blood + surrounding tissues) and employs the Kubelka – Munk two-stream theory; in this case, the amount of blood in the tissues is determined by the conditions of mechanical equilibrium and the laws of tissue deformation.
 It is shown that the developed model correctly describes the observed qualitative features of the PPG signal, and the condition for the best agreement between theoretical and experimental curves allows real-time determination of the peripheral microcirculation quantitative characteristics (mean value and amplitude of the blood pressure fluctuations in precapillaries, erythrocyte concentration, average degree of blood oxygenation, the vessel walls elasticity, etc.). The developed methods for determining the blood circulation parameters can be used as the basis for inexpensive and effective express methods of research and medical diagnostics, which are especially useful in mass surveys or in environmental monitoring.