Depth Selective Photoplethysmography-based Method for Pulse Transit Time Measurement at a Single Measuring Position (DeePPG)

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The Pulse Transit Time (PTT) is a cardiovascular parameter used in various medical applications. The most challenging is currently the application of cuff-less blood pressure measurement. The established method for measuring PTT is based on the combination of electrocardiography (ECG) and photoplethysmography (PPG). It measures the time difference between the electrical excitation of the heart (ECG) and the arrival of the (blood) pulse wave caused by the heart contraction at a perfused skin area (PPG). This requires several distributed measuring points on the body and thus a prior preparation of the subject. The PPG is an optical method which enables the measurement of blood and blood flow parameters in the skin through the combination of an optical transmitter and an optical receiver. In this project, this method should be further developed in such a way that a local PTT can be recorded at a single measuring position. Thereby, the usage of an ECG is not required. For this purpose, the layer structure of the skin and the wavelength dependence of the optical properties of blood and tissue are used. Investigations should show whether a depth-selective PPG method can be developed by parallel measurement with different wavelengths. With this depth selectivity, the pulse wave propagation perpendicular to the skin surface shall be analyzed and a local PTT determined. For this purpose, the physiological-physical relationships are dynamically simulated on the basis of a model of the microcirculation of the skin and correlated with real measurements. For the simulation, a one-dimensional optical model of the skin is to be combined with the pulse wave propagation modulated by the conduction theory. Thereby, a dynamic simulation of the optical interaction in the measuring volume becomes possible. To validate the simulation results, a measuring device should be developed which takes the special boundary conditions (e.g. the inhomogeneous structure of the skin) into account. Based on the knowledge gained in this way, a method will be developed which enables the measurement of PTT at only one measuring position on the body. Finally, the correlation with the new method will be validated on the basis of established reference measurement methods.