Non-invasive characterisation of age-related changes in ascending aortic blood pressure and blood flow: from old concept to novel approach

The elucidation of mechanisms whereby arterial stiffness with increasing age alters the propagation of pressure (and flow ejection) waves generated by each heart beat is still evolving and sometimes debated. The research described in this thesis aims to better characterise the age-related changes in ascending aortic blood pressure and blood flow non-invasively. This thesis investigates four key areas:- (i) measurement of aortic stiffness with pulse wave velocity; (ii) non-invasive central arterial pressure measurements; (iii) characterisation of age-related changes in central aortic pressure and flow with non-invasive methodology; (iv) relationship of cerebral arterial pressure and flow with central aortic pressure and flow. Findings of this investigation highlight the clinical importance of non-invasive measurement of central aortic pressure and aortic flow to determine ascending aortic impedance as a measure of cardiac load. The physical relationship between aortic pressure, flow and arterial impedance is quantified in terms of age-related changes. Changes in aortic flow pattern and means to derive aortic flow waveform from aortic pressure waveform non-invasively are compared with other methods. Results show that changes in the cardiac ejection pattern with age are better determined using magnetic resonance imaging compared to Doppler ultrasound techniques. Initial investigation of the effect of arterial stiffening, as occurs with age, shows that there is a relationship between the higher pressure pulsations from the heart and the damage in the brain due to the higher flow pulsations being transferred to the cerebral circulation. This investigation confirmed the changes in central aortic blood pressure and flow ejection with aging. Derivation of the aortic flow pattern from central aortic pressure is feasible, through their relationship with aortic impedance. Increased pulsatility of arterial pressure and flow may cause damage to the microvasculature in the brain and may relate to the cognitive decline with aging and disease.