City Research Online

Abstract

Cardiovascular diseases (CVD) remain a major health challenge, affecting millions of individuals. The natural process of vascular ageing contributes to changes in the vascular system, potentially compromising cardiovascular health by causing arterial stiffness. This thesis investigates photoplethysmography (PPG) as a low-cost, non-invasive approach for vascular ageing assessment, with particular emphasis on its potential application in assessing arterial stiffness through waveform morphology and extracted features. A central contribution is the novel development of customisable vessel-tissue phantoms, mechanically characterised to replicate the properties of human femoral arteries across healthy and diseased states. These novel silicone-based phantoms were embedded within a controlled in vitro vascular system, providing a reproducible platform for evaluating PPG-derived markers of arterial stiffness.

The feasibility of PPG for assessing arterial stiffness was investigated through controlled experimental studies. For the first time, customisable vessel–tissue phantoms with tuneable mechanical properties were embedded within bilateral and single-branch in vitro vascular models to replicate vascular symmetry and progressive stiffness changes. Morphological waveform features were extracted from PPG recordings, with key metrics associated with arterial stiffness identified and analysed. A significant methodological contribution was the implementation of multimodal experiments integrating PPG and laser Doppler flowmetry (LDF) to examine the complementary discriminatory performance of combining optical modalities for vascular health assessment.

This work demonstrated the successful fabrication of novel vessel–tissue phantoms with mechanical properties representative of femoral arteries and surrounding tissue. Three phantoms were developed, for the first time, to model healthy, intermediate, and unhealthy vascular states, with Young’s modulus values of 0.82 MPa, 1.48 MPa, and 2.06 MPa, respectively. Bilateral experiments confirmed the repeatability of PPG signals across identical phantoms, with no statistically significant differences (p ≥ 0.05) in the majority of fundamental PPG features, apart from rise time. Four key waveform features, amplitude, area under the curve (AUC), median upslope–downslope ratio, and median end datum difference, showed strong correlation with arterial stiffness. Differentiation between healthy and unhealthy vessels was verified under hypertensive conditions, although classification of the intermediate state remained less distinct, with some features showing statistical similarity to the healthy state. The second derivative of the photoplethysmography signal (SDPPG) indicated that the b/a ratio showed the clearest separation between the healthy and unhealthy phantoms under hypertensive conditions (–2.13 to –2.06). A novel multimodal approach combining PPG and LDF demonstrated complementary strengths: PPG was most effective at distinguishing unhealthy vessels from healthy and intermediate states through pulsatile blood volume changes, while LDF better differentiated the health states via LDF-derived flux. A progressive reduction in direct current (DC) mean flux was observed across the healthy, intermediate, and unhealthy phantoms, confirming the sensitivity of LDF to vascular stiffening.

This research demonstrates the significant potential of PPG as a non-invasive method for vascular ageing assessment. While integration with complementary modalities such as LDF may enhance discriminatory capability, further optimisation and validation are required before clinical implementation.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > T Technology (General) T Technology > TA Engineering (General). Civil engineering (General)
Departments:	School of Science & Technology > Department of Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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