Measurement of pulse wave velocity. The delay time between the two pressures is the transmission time (T), and the distance (L) between the two pressure recording sites divided by T is the pulse wave velocity
Pulse wave velocity elevates with aging (Avolio et al. )
Intriguingly, it is reported that the PWV in aneurysmal aorta in patients with tetralogy of Fallot is elevated , although the PWV is inversely proportional to the diameter of the vessel. This phenomenon could imply that the damage of the aortic wall precede the dilatation (Fig. 7.3).
Pulse wave velocity on the ascending aorta in repaired and unrepaired patients with tetralogy of Fallot increases in comparison with that in control (a). There is no difference on the pulse wave velocity on the descending aorta between the groups (b). PWV, pulse wave velocity (Saiki et al. )
7.2 Augmentation Index
Augmentation index (AI) is the index of the aortic pressure wave reflection. Aortic pressure waveform is composed of two pressure waveforms: the forward pressure wave and the reflected pressure wave (Fig. 7.4). The forward pressure wave is generated by the systemic ventricular ejection, and the backward pressure is the sum of the pressure wave reflections. The pressure wave reflections arise from any discontinuity in the elastic properties along the arterial tree in which there is a change (or mismatch) in impedance . In normal aortic tree, the reflecting point that represents the integrated pressure wave reflections exists in the region of the aortic bifurcation . Because of the slow PWV, the reflected pressure wave returns to the heart during diastole in young people, it means the timing after closure of aortic valve (Fig. 7.4b). Therefore, it enhances the coronary perfusion by pushing the aortic blood stored during systole. With aging, the PWV gradually increases. It means the early return of the reflected pressure wave (in systole) which impairs the arterial and ventricular functions. The opposite directional reflected pressure wave that returns to the heart during systole interferes with the systemic ventricular ejection and increases the workload of the systemic ventricle (Fig. 7.4a). To evaluate the degree of the pressure wave reflection, AI is calculated. The AI is defined as the ratio of the augmentation pressure to the pulse pressure (Fig. 7.5) (5). The inflection point is defined as the timing with the peak flow velocity or obtained by the fourth derivative of the original pressure waveform (Fig. 7.6) (6).
Aortic pressure waveforms. A solid line means the forward pressure wave and dotted line stands for the reflected pressure wave. Early return of reflected pressure wave (a), namely, returns before closure of aortic valve, applies load to the systemic ventricle. On the other hand, late return of reflected pressure wave (b) can enhance coronary perfusion
Schematic representation of augmentation index. The augmentation index is the ratio of the augmentation pressure to the pulse pressure (Murakami et al. )
Determination of the inflection point. The inflection point is obtained by the fourth derivatives of the original pressure waveform (Murakami et al. )
As the PWV increases with aging, the AI also rises (Fig. 7.7) (7). Moreover, it is well known that other factors could influence on the aortic pressure wave reflection. The distance from the heart to the inflection point is one of the important determinants of AI. It is reasonable that the short distance means early return of the reflected pressure wave. Practically, it is reported that the AI is high and inversely related to the body height in children (Fig. 7.8) , although their PWV is low (Fig. 7.2). When there is a strong inflection point nearer to the heart than the normal inflection point (aortic bifurcation), the AI could elevate. In patients after repair of aortic coarctation, the repaired site generates a new pressure wave reflection resulting in the elevation of the AI (Fig. 7.9) .