The use of photoplethysmography for assessing hypertension

Abstract

The measurement of blood pressure (BP) is critical to the treatment and management of many medical conditions. High blood pressure is associated with many chronic disease conditions, and is a major source of mortality and morbidity around the world. For outpatient care as well as general health monitoring, there is great interest in being able to accurately and frequently measure BP outside of a clinical setting, using mobile or wearable devices. One possible solution is photoplethysmography (PPG), which is most commonly used in pulse oximetry in clinical settings for measuring oxygen saturation. PPG technology is becoming more readily available, inexpensive, convenient, and easily integrated into portable devices. Recent advances include the development of smartphones and wearable devices that collect pulse oximeter signals. In this article, we review (i) the state-of-the-art and the literature related to PPG signals collected by pulse oximeters, (ii) various theoretical approaches that have been adopted in PPG BP measurement studies, and (iii) the potential of PPG measurement devices as a wearable application. Past studies on changes in PPG signals and BP are highlighted, and the correlation between PPG signals and BP are discussed. We also review the combined use of features extracted from PPG and other physiological signals in estimating BP. Although the technology is not yet mature, it is anticipated that in the near future, accurate, continuous BP measurements may be available from mobile and wearable devices given their vast potential.

Keywords: Data integration; Diagnostic markers; Electrocardiography - EKG; Predictive markers; Statistical methods.

Fig. 1

Key features of blood pressure estimation using PPG and other physiological signals. (i) Using PPG signal and its derivative, (ii) using ECG and PPG signals, (iii) using BCG signals and PPG signals, and (iv) using PCG and PPG signals. Here, PPG photoplethysmogram, APG acceleration photoplethysmogram, BCG ballistocardiogram, PCG phonocardiogram, STT slope transit time, PTT pulse transit time, PEP pre-ejection period, PAT pulse arrival time, TD time interval between the J peak in the BCG signal and the systolic peak in the PPG signal, VTT vascular time interval between the first heart sound S1 and the systolic peak in the PPG signal, S1 first heart sound, S2 second heart sound

Fig. 2

Difference between pulse arrival time (PAT) and pulse transit time (PTT). The PAT is defined as the time taken from the pulse waveform to traverse from the heart to a distal site. The PTT is defined as the period from relatively proximal site (e.g., arm) to a distal site (e.g., finger) or between two distal sites (e.g., figure and toe)

Fig. 3

Filter impact on PPG morphology. The left figure shows the impulse response difference between the Butterworth (red line) and ChebyshevII (black line) filters. The right figure shows the Butterworth bandpass filtered (red line) and the ChebyshevII bandpass (black line) filtered PPG signals of the raw PPG signal (blue line). It is clear that the ChebyshevII filter is able to emphasize the difference between the systolic and diastolic waves, compared to the Butterworth filter. PPG photoplethysmogram, dB Decibel, GHz Gigahertz

Fig. 4

Pulse arrival time in different hypertension stages. PAT pulse arrival time, ECG electrocardiogram, ABP arterial blood pressure, PPG photoplethysmogram