The arterial reservoir pressure increases with aging and is the major determinant of the aortic augmentation index

Abstract

The augmentation index predicts cardiovascular mortality and is usually explained as a distally reflected wave adding to the forward wave generated by systole. We propose that the capacitative properties of the aorta (the arterial reservoir) also contribute significantly to the augmentation index and have calculated the contribution of the arterial reservoir, independently of wave reflection, and assessed how these contributions change with aging. In 15 subjects (aged 53 +/- 10 yr), we measured pressure and Doppler velocity simultaneously in the proximal aorta using intra-arterial wires. We calculated the components of augmentation pressure in two ways: 1) into forward and backward (reflected) components by established separation methods, and 2) using an approach that accounts for an additional reservoir component. When the reservoir was ignored, augmentation pressure (22.7 +/- 13.9 mmHg) comprised a small forward wave (peak pressure = 6.5 +/- 9.4 mmHg) and a larger backward wave (peak pressure = 16.2 +/- 7.6 mmHg). After we took account of the reservoir, the contribution to augmentation pressure of the backward wave was reduced by 64% to 5.8 +/- 4.4 mmHg (P < 0.001), forward pressure was negligible, and reservoir pressure was the largest component (peak pressure = 19.8 +/- 9.3 mmHg). With age, reservoir pressure increased progressively (9.9 mmHg/decade, r = 0.69, P < 0.001). In conclusion, the augmentation index is principally determined by aortic reservoir function and other elastic arteries and only to a minor extent by reflected waves. Reservoir function rather than wave reflection changes markedly with aging, which accounts for the age-related changes in the aortic pressure waveform.

Fig. 1.

Illustration of the 3-stage process used to separate measured pressure into its reservoir, forward, and backward pressure components. Reservoir pressure is calculated from measured pressure and velocity. Wave pressure is calculated by subtracting reservoir pressure from measured pressure. Wave pressure is separated into forward and backward pressure components using velocity data.

Fig. 2.

Impact of accounting for the arterial reservoir pressure on the calculated forward- and backward-traveling pressure waves. The pressure wave is separated into its constituent components ignoring (left) and then accounting for (right) the aortic reservoir. When the aortic reservoir is ignored, both the forward and backward peak pressures are substantial and during diastole, they are equal and decrease slowly and continuously. When the reservoir pressure is taken into account, the backward-traveling (reflected) wave is significantly reduced and forward and backward wave pressures are absent following closure of the aortic valve.

Fig. 3.

Calculation of the components of augmentation pressure ignoring (left) and accounting for (right) the aortic reservoir pressure. Pressure was separated first using conventional separation technique, which ignores the reservoir pressure, and then using the wave reservoir technique, which accounts for aortic reservoir pressure. Augmentation pressure was calculated as the rise in pressure between the inflection point and peak pressure. With the use of the wave-only analysis (left; which ignores the aortic reservoir), the augmentation pressure is composed predominantly from backward-traveling pressure with a small contribution from the ongoing forward-traveling pressure. When the reservoir is accounted for (right), pressure augmentation primarily arises from reservoir pressure, with a far smaller contribution arising from backward-traveling pressure. The forward-traveling pressure was found to no longer contribute.

Fig. 4.

Relationship between separated pressure components and aging after accounting for the aortic reservoir pressure. The pressure waveform was separated into forward (A) and backward (B) and reservoir (C) pressure components. With increasing age, both forward and arterial reservoir pressure increased. However, the backward pressure component was not significantly correlated with age.