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

脉搏传导时间（PTT）是用于各种医疗应用的心血管参数。目前最具挑战性的是无袖带血压测量的应用。用于测量PTT的所建立的方法是基于心电图（ECG）和光电体积描记（PPG）的组合。它测量心脏的电激励（ECG）和由心脏收缩引起的（血液）脉搏波到达灌注皮肤区域（PPG）之间的时间差。这需要身体上的多个分布的测量点，因此需要受试者的预先准备。PPG是一种光学方法，其能够通过光发射器和光接收器的组合来测量皮肤中的血液和血流参数。在本项目中，应进一步开发这种方法，以便在单个测量位置记录本地PTT。因此，不需要使用ECG。为此，使用皮肤的层结构和血液和组织的光学性质的波长依赖性。研究应表明是否可以通过不同波长的平行测量开发深度选择性PPG方法。利用该深度选择性，应分析垂直于皮肤表面的脉搏波传播并确定局部PTT。为此，基于皮肤微循环的模型动态地模拟生理-物理关系，并将其与真实的测量结果相关联。对于模拟，皮肤的一维光学模型将与由传导理论调制的脉搏波传播相结合。因此，测量体积中的光学相互作用的动态模拟变得可能。为了验证模拟结果，应该开发一种测量装置，该装置考虑到特殊的边界条件（例如，皮肤的非均匀结构）。基于以这种方式获得的知识，将开发一种方法，该方法能够仅在身体上的一个测量位置处测量PTT。最后，将在已建立的参考测量方法的基础上验证与新方法的相关性。

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.