Non-invasive baroreceptor function measurement in humans and the interaction with large artery stiffness
With age, the capacity of the large arteries to stretch decreases, and baroreceptor function is impaired. As baroreceptors are stretch receptors, these two phenomena may be mechanistically linked. This thesis investigates the correlation between these two parameters. A systematic review of the literature showed that in 35 studies that investigated the correlation between arterial stiffness and baroreceptor sensitivity (BRS), 89% showed the correlation to be significant. All but one study attempted to assess BRS only under resting BP conditions. There is scope for future investigations to investigate the wider BRS function curve and its association with arterial stiffness. A pilot study into arterial stiffness and BRS correlation was conducted. The hypothesis was that BRS and arterial stiffness are correlated, and baroreceptor modulation on blood pressure (BP) dampens as large arteries like the carotid artery and aorta become stiffer. BRS was assessed in 10 healthy participants under resting BP conditions and during BP altered by manoeuvres (leg ischemia, head-up tilt test, deep breathing test, handgrip exercise, and cycling ergometer exercise). Continuous finger BP and heart rate (HR) were measured non-invasively to estimate BRS. BRS from the finger BP was quantified by sequence and spectral methods. BRS from the calculated aortic BP was quantified using the sequence method only. Arterial stiffness was measured by carotid-femoral pulse wave velocity (cfPWV), carotid compliance, and carotid distensibility. BRS averaged across all manoeuvres was not correlated with arterial stiffness. Removing manoeuvres that caused BRS resetting (head-up tilt, exercise), the study was suitably powered to find that BRS was correlated with cfPWV (R2=0.7788, p=0.022) after adjusting for age and sex but not carotid compliance (R2=0.870, p=0.336). The conflicting result emphasises the need for a purpose-designed study on the topic, preferably using manoeuvres or vasoactive drugs to investigate BRS across a wider BP range. Sample size calculations showed that a study of 100 participants would allow investigation of all permutations of the correlation between carotid compliance, distensibility, and BRS calculated using peripheral and aortic BP.