Aortic augmentation index and pulse wave velocity in response to head-up tilting: effect of autonomic failure

Background: Aortic augmentation index (AIx) but not carotid-femoral pulse wave velocity (cfPWV) has reported to decrease in response to vasodilators, which has been related to changes in the timing and/or intensity of wave reflection. Yet, recent evidence indicates that arterial reservoir pressure rather than wave reflection is the most important determinant of AIx. Methods: Using radial artery applanation tonometry and a general transfer function AIx, aortic pulse wave reflection time and cfPWV (foot-to-foot method) were determined in 10 patients with severe autonomic failure and in 14 healthy individuals during supine rest and graded head-up tilting. Results: During supine rest, mean blood pressure (BP) (127.6 +/- 21.5 and 97.5 +/- 9.4 mmHg), AIx (32.4 +/- 13.0 and 23.1 +/- 8.7%) and cfPWV (12.1 +/- 3.6 and 8.9 +/- 1.6 m/s) were higher in patients than in controls. In patients, BP decreased by 18.7 +/- 9.8 and 39.6 +/- 11.7%, AIx by 39.2 +/- 27.5 and 100.9 +/- 78.1% and cfPWV by 12.0 +/- 10.5 and 27.7 +/- 13.5% in response to 30 and 60 degrees head-up tilting. Decreases in AIx and cfPWV correlated with the BP fall (r = 0.67, P = 0.001 and r = 0.75, P < 0.001), but changes in AIx and cfPWV were unrelated. In controls, AIx during head-up tilting decreased despite increases in vascular tone and cfPWV. Aortic reflection time in patients and controls during tilting did not change. Stepwise regression analysis revealed that 68% of the variation in AIx could be explained by the BP fall and reflection time and 76% of the variation in cfPWV by the BP fall and sex. Conclusion: In a clinical model of autonomic failure, both AIx and cfPWV largely depend on instantaneous BP, but these two variables are unrelated, supporting the contention that aortic reservoir pressure rather than wave reflection is the main determinant of AIx.