Objective: Augmentation index (AIx) is often used to quantify the contribution of wave reflection to central pulse pressure. Recent studies have challenged this view by showing how contractility-induced changes in the forward pressure wave can markedly impact AIx. We hypothesized that changes in preload will also affect AIx through changes in the forward wave and studied this in two experiments.
Methods: Noninvasively obtained aortic pressure was used to study central haemodynamics and wave morphology. In the first experiment, we examined the effects of head-up tilt with and without unilateral thigh cuff in 12 young healthy volunteers (mean age 26 years, 50% men). In the second experiment, we examined the effects of active standing in 31 middle-aged patients (mean age 57 years, 65% men) before and after phlebotomy.
Results: Head-up tilt or active standing significantly decreased AIx [-17.7 ± 10.4 percentage point (pp) in the young population, -4.7 ± 12.3 pp in the middle-aged population, both P < 0.05]. The fall in AIx was associated with increases in HR, diastolic pressure and systemic vascular resistance and a decrease in stroke volume (all P < 0.05). Inflation of a unilateral thigh cuff reduced the decrease in AIx by 10.7 pp, whereas 500 ml of blood loss augmented the fall in AIx by 5.9 pp (both P < 0.05). The changes in AIx were related to a preload-induced change in forward pressure wave shape (earlier peaking and steeper downstroke).
Conclusion: Next to inotropic and chronotropic effects, preload emerges as another myocardial factor that obscures the relation between wave reflection and AIx.
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