A preliminary study on multi-wavelength PPG based pulse transit time detection for cuffless blood pressure measurement

Abstract

Pulse transit time (PTT) has been widely studied as an index of blood pressure (BP) changes. In recent years, some prototypes of PTT-based wearable BP measurement devices have been developed, which can relieve users from the discomfort caused by the inflating cuff used in auscultatory and oscillometric BP measurement techniques. However, in the common practice for PTT detection, multi-site sensor implementation on human body is required, making it difficult for the integration of wearable devices. Since multi-wavelength (MW) photoplethysmogram (PPG) signals carry blood pulsation information of different blood vessels embedded in different skin depths, the time difference between different wavelength PPG signals collected at the same body site can be treated as a special PTT on a short length of blood vessels beneath the skin. In this work, the time difference between MW PPG, denoted as PTT_MW, was explored to track BP changes as a substitute of infrared (IR) PTT_EP. (PTT_EP is the time interval between electrocardiogram (ECG) and IR PPG.) Ten healthy adult subjects participated in the experiment, and their continuous BP, ECG and fingertip MW PPG signals generated from blue, green, yellow and IR light were recorded after 2-minute static handgrip exercise at 33% maximal voluntary contraction. The results showed that the correlation between Systolic BP (SBP) and IR-Blue PTT_MW (|r|= 0.52) was comparable to the correlation between SBP and IR PTT_EP (|r|= 0.59). Moreover, we optimized the wavelength combination of PTT_MWs for each subject and found the average value of optimal correlation between SBP and PTT_MW reached 0.76, which was significantly (p<;0.01) higher than the correlation between IR PTT_EP and SBP. This study reveals that the time difference between MW PPG can be potentially used as PTT for cuffless BP measurement with its unique advantage in simple sensor implementation at only one body site.

Novel Polydimethylsiloxane (PDMS) Pulsatile Vascular Tissue Phantoms for the In-Vitro Investigation of Light Tissue Interaction in Photoplethysmography

“…In-vitro investigations also have many advantages over in-vivo studies, where control over physiological parameters are impractical. Many groups are currently researching the potential use of PPG for non-invasive continuous measurement of blood pressure [14,15]. Varying patient's blood pressures is a major limiting factor in in-vivo studies (especially in studies using healthy volunteers), so the employment of an advanced in-vitro rig where physiological parameters such as blood pressure can be precisely controlled could be an advantage.…”

Design of Multi-Wavelength Optical Sensor Module for Depth-Dependent Photoplethysmography

“…Generally, it is known that the light penetration depth deepens as the wavelength increases [4]. In recent years, multi-wavelength PPG has been proposed for analyzing pulse waves in blood vessels of different depths using light sources of various wavelengths [5][6][7][8][9][10]. The most representative application using multi-wavelength PPG is blood pressure estimation that is based on local pulse wave velocity.…”

“…The most representative application using multi-wavelength PPG is blood pressure estimation that is based on local pulse wave velocity. There are studies to measure local pulse wave velocity in microvascular structure and apply it to blood pressure estimation from the premise that pulse wave velocity is related to blood pressure [5][6][7][8]. In general, it is known that wavelengths in the range of 390−600 nm penetrate to the superficial tissue, and longer wavelengths in the range of 600−1100 nm, which penetrate further ( Figure 1) [11].…”

“…Based on these characteristics, attempts have been made to obtain PPG measurements from various blood vessel depths while using different wavelength light sources. Previous research by Spigulis et al, and Liu et al, demonstrated the feasibility of multi-wavelength PPG for measuring arterial blood volume changes at different depths [5][6][7][8][9][10]. However, the sensors that were proposed in the previous studies require a complicated setup, including an optical fiber and spectrometer [9,10], thus it has a limitation in portable or daily purposes.…”

“…In other words, a sensor module having a simpler structure is required in order to be used as a wrist watch or band form factor, which is frequently used for health care currently. Moreover, most of existing sensors had non-uniform distances between the light sources of each wavelength and the photodetector [5][6][7][8]. Further, since the light source was only located in one direction of the photodetector, it could not reflect the blood volume change in a common vascular bed while considering the topological and geometric irregularities of a microvascular network [12], which is a significant limitation.…”