Photoplethysmography for the Assessment of Arterial Stiffness

Abstract

This review outlines the latest methods and innovations for assessing arterial stiffness, along with their respective advantages and disadvantages. Furthermore, we present compelling evidence indicating a recent growth in research focused on assessing arterial stiffness using photoplethysmography (PPG) and propose PPG as a potential tool for assessing vascular ageing in the future. Blood vessels deteriorate with age, losing elasticity and forming deposits. This raises the likelihood of developing cardiovascular disease (CVD), widely reported as the global leading cause of death. The ageing process induces structural modifications in the vascular system, such as increased arterial stiffness, which can cause various volumetric, mechanical, and haemodynamic alterations. Numerous techniques have been investigated to assess arterial stiffness, some of which are currently used in commercial medical devices and some, such as PPG, of which still remain in the research space.

Keywords: CVD; PPG; arterial stiffness; cardiovascular disease; photoplethysmography; vascular ageing.

Figure 1

Comparison of healthy elastic vessel (left image) with an aged, unhealthy stiff vessel (right image). The unhealthy vessel comprises thicker walls and a uniform flow due to the stiffer walls. Meanwhile, the elasticity of the healthy vessel allows for an increased pulsatile flow. Modified from LaRocca et al. [7].

Figure 2

Flow diagram from PRISMA [9] illustrating the search strategy for the literature review procedure. The PRISMA flow diagram facilitated study selection. It was used to present the review clearly, mapping the number of papers and eliminating duplicates in databases. Records deemed irrelevant based on title or abstract were excluded from the screened total. Subsequently, 64 papers underwent eligibility assessment, all of which have been included in the review.

Figure 3

Bar chart showing the mean number of publications released per decade for the years between 2000 and 2023 that met the inclusion criteria. The total number of publications has been split between PPG-derived methodologies and other techniques used. No relevant studies were found for the years between 1990 and 1999.

Figure 4

An example photoplethysmogram depicting the systolic and diastolic peaks with a dicrotic notch [45].

Figure 5

PPG sensor configuration modes. Transmittance mode configuration (top image) shows light source (LED) and photodetector (PD) placed on opposite sides of the body site. In reflectance mode (bottom image), the light source (LED) and photodetector (PD) are adjacent.