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|Title:||Non-invasive wave intensity analysis in common carotid artery of healthy humans|
|Publisher:||Brunel University London|
|Abstract:||The study of arterial wave propagation is essential to understand the physiopathology of the cardiovascular system, as waves carry clinically relevant information. Impedance analysis was used for such type of studies, where results were presented in the frequency domain, but it was difficult to relate specific events to time points within the cardiac cycle. Therefore, a mathematical tool called wave intensity analysis was developed, initially using measurements of pressure and velocity (PU approach). However, the need to acquire such measurements in a non-invasive, direct and simultaneous fashion led to the development of the DU approach, a type of wave intensity analysis carried out using vessel diameter and flow velocity waveforms, thus giving up the pressure measurement. It is the only available technique, at present, able to extract wave intensity information without relying on distally recorded pressure measurements and on non-simultaneous recordings. Due to its non-invasive nature for collecting the required measurements, this technique has a potential use in clinical and research settings to investigate physiological changes under rapid perturbations, such as the ones introduced by exercise. In this thesis, the DU approach is performed by only using an ultrasound device and to extract information about cardio-arterial interaction in the human common carotid artery. In the first experimental chapter of this thesis, a reproducibility study of common carotid DU-derived wave intensity parameters was conducted on a healthy young cohort, both at rest and during exercise (semi-recumbent cycling). Carotid diameter and blood flow velocity features, as well as wave intensity parameters such as forward compression, backward compression and forward expansion wave peaks and energies, were overall fairly reproducible. In particular, diameter variables exhibited higher reproducibility and lower dispersion than corresponding velocity variables, whereas wave intensity energy variables exhibited higher reproducibility and lower dispersion than corresponding peaks. Local wave speed, calculated via lnDU-loop, a technique based only on local measurements of diameter and velocity and often associated with the DU approach, was also reproducible. It is possible to conclude that the DU-derived wave intensity analysis is reliable both at rest and during exercise. In a subsequent study, DU-derived wave intensity analysis was performed on a young trained cohort to investigate the contribution of cardiac and peripheral vascular alterations to common carotid wave intensity parameters, under rapid physiological perturbations, such as semi-recumbent cycling at incremental workrates, and subsequent recovery. Judging by the increase in local wave speed, the common carotid artery stiffened substantially as workrate increased whilst peak and energy of the forward and backward compression waves also increased, due to enhanced ventricular contractility, which was associated with larger reflections from the cerebral microcirculation and other vascular beds in the head. However, the reflection indices remained unchanged during exercise, highlighting that the increased magnitude of reflections is mainly due to the enhanced contractility, rather than changes in vascular resistance, at least at the carotid artery in young healthy individuals. The forward expansion wave increased during exercise, as the left ventricle actively decelerated blood flow in late systole, potentially improving filling time during diastole. In the early recovery, the magnitude of all waves returned to baseline value. Finally, the X wave, attributed to the reflection of the backward compression wave, had a tendency to increase during exercise and to return to baseline value in early recovery. A further development of wave intensity analysis came with the reservoir-wave approach, able to separate, from the pressure and velocity waveforms, the component solely due to the reservoir volume, for the correct evaluation of backward- and forward-travelling waves. A number of issues, however, still remains, involving specifically the lack of consensus over the fitting technique and over the value of the asymptotic pressure value (P ∞),used for the determination of the reservoir waveform. Therefore, to give a contribution to the debate involving the more correct model for the pressure and velocity reservoir-wave approach, a study aimed to investigate various common carotid hemodynamic and wave intensity parameters, using different fitting techniques and values of P ∞ currently available in literature, was performed and described in the last chapter of this thesis. The study demonstrated that different fitting method and values of P ∞ could bring significant variations in values and trends of hemodynamic and wave intensity parameters. However, despite the changes in the shape of the reservoir pressure waveform, its peak and integral with respect to time tended to remain constant. This is an important feature, because both reservoir peak pressure and its integral have been used in clinical settings for the calculation of diagnostic indicators. The reservoir and excess velocity peaks, instead, changed more significantly. This outcome, together with the concomitant substantial change in excess pressure peak and integral, may greatly affect wave intensity parameters. Wave intensity parameters were, in fact, significantly more sensitive to fitting techniques and values of P ∞ than pressure parameters. Finally, the wave speed did not substantially change, leading to the conclusion that the calculation of local vessel distensibility and/or compliance, when calculated from the excess components of the waveforms, seemed insensitive to fitting techniques and values of P ∞|
|Description:||This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University London|
|Appears in Collections:||Brunel Institute for Bioengineering (BIB)|
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