Non-invasive reservoir pressure parameters : measurement and clinical relevance

High blood pressure (BP) is the single greatest risk factor for death from cardiovascular diseases (CVD). High BP is conventionally defined by the systolic BP and diastolic BP, which are the peak and nadir points on the BP waveform, but cannot comprehensively represent systemic arterial haemodynamics. The reservoir-excess pressure model, which was proposed to analyse the BP waveform, provides potentially new information regarding arterial haemodynamics. The reservoir-excess pressure model interprets the BP waveform into a reservoir pressure (RP), which is related to the buffering capacity of elastic arteries, and into an excess pressure (XSP), which is ascribed to wave propagation in the arterial system. Furthermore, reservoir pressure parameters (e.g. RP, XSP and the associated systolic rate constant) have been shown to predict cardiovascular events independent of standard BP and other conventional cardiovascular risk factors. However, non-invasive measurement of reservoir pressure parameters is technically challenging, which limits the widespread application of reservoir pressure parameters. Thus, the overall aims of this research programme were to determine whether reservoir pressure parameters could be non-invasively measured in the human upper arm using an oscillometric cuff device, and further to determine whether cuff device-measured reservoir pressure parameters were clinically relevant ‚Äö- this was assessed by association with cardiovascular risk markers. In chapter 2, the change in reservoir pressure parameters from the aorta to the brachial and radial arteries was invasively investigated in 51 participants undergoing coronary angiography. A relatively constant RP and an amplified XSP were observed from the aorta to the brachial and radial arteries. These observations provide a new understanding on arterial reservoir pressure parameters and large artery BP physiology. In chapter 3, the performance of an oscillometric cuff device for measuring the central BP was investigated in 182 people with treated hypertension. The central BP parameters derived from the cuff device were substantially equivalent to the central BP parameters measured using the non-invasive reference standard (radial tonometry) method. This finding is the basis of accurately deriving the reservoir pressure parameters from cuff-based device-measured central BP waveforms. In chapter 4, whether reservoir pressure parameters could be non-invasively derived from the cuff device-measured brachial or central BP waveform was examined in comparison to true invasive aortic measures among 163 participants undergoing coronary angiography. The brachial-cuff method estimated reservoir pressure parameters had higher concordance with the intra-aortic measures than did the central-cuff method estimated reservoir pressure parameters. In chapter 5, brachial-cuff reservoir pressure parameters were applied in a large population of Australian adults (n=1874) to examine the potential clinical relevance. Brachial-cuff reservoir pressure parameters were significantly associated with cardiovascular risk markers, indicating their potential clinical significance for predicting cardiovascular risk. In summary, this thesis determined that reservoir pressure parameters could be reliably estimated on the brachial artery using the non-invasive cuff device, and that these cuff reservoir pressure parameters were related to cardiovascular risk markers. Overall, this research program provides novel information that increases understanding of the reservoir-excess pressure model in humans.