Depth Selective Photoplethysmography-based Method for Pulse Transit Time Measurement at a Single Measuring Position (DeePPG)
Biomedical Systems Technology
Final Report Abstract
Pulse wave transit time (PTT) is a cardiovascular parameter used in various medical applications, with the future application for cuffless blood pressure measurement currently being the most challenging. The established method for measuring PTT is based on a combination of electrocardiography (ECG) and pho-toplethysmography (PPG). The time interval between the electrical excitation of the heart (by ECG) and the arrival of the (blood) pulse wave caused by the heart contraction at a perfused skin area (by PPG) is measured. PPG is an optical method that enables the measurement of perfusion in the skin through the combination of light emitter(s) and light detector(s). The aim of this project was to investigate whether a preparation-free, depth-selective PTT measurement is possible using local PPG at a single measurement position by means of parallel measurement with different wavelengths. Based on literature data on tissue composition, vascular content, and distribution, as well as related data on reflection, transmission and attenuation behavior, different layer models of the upper skin layers were created. These were used to calculate the light distribution in the crosssection of the region between light emitter and light detector by means of a single photon ray tracing method based on random processes using a Monte Carlo simulation approach. From this, the penetration behavior, or the average penetration depth of the photons as a function of the emitted wavelength spectrum of the LEDs used as light emitters in the later measurements and the emitter-detector distance could be derived, quantified and reproduced in computationally intensive simulation cycles. The simulations are still ongoing for further statements. Furthermore, various demonstrators of DeePPG measurement systems were iteratively developed for the subject experiments, with which a wide range of constellations of excitation wavelengths, spectral distributions, LED arrangements and scalable emitter-detector distances could be analyzed. The photocurrent of the detectors is load-decoupled via an amplifier chain, band-limited up to 10 kHz, amplified by 20 dB and digitized. Further measured value conditioning, feature extraction and indicator generation are carried out using Matlab (Mathworks). Measurements were carried out on three cohorts of test subjects in three development stages of the measurement system, each based on adapted study designs, with the most recent study being the most extensive with 47 test subjects. Based on the large number of measurement data, many characteristics could be identified, and indicators extracted and analyzed. However, not all the planned research questions could be answered conclusively at the time of writing, that of a generalizable algorithmic solution for local pulse transit time measurement.
Publications
-
Optimization of the deep selectivity of PPG signals in the context of PTT determination; Konferenzbeitrag und Abstract BMT September 2023, Duisburg; DGBMT
Hallekamp, Gabriel
