Correction of carotid augmentation index for heart rate in elderly essential hypertensives

Abstract

Carotid augmentation index (AI) is used as a surrogate measure of arterial stiffness. Although arterial stiffness has been shown to either remain unchanged or increase with an increase in heart rate, AI decreases as heart rate increases. This study aimed to quantify this confounding effect of heart rate on AI. We investigated 873 hypertensives, mean age 72 ± 5 years, 44% men, mean brachial blood pressure 161 ± 21/82 ± 11 mm Hg. Carotid artery tonometry with simultaneous continuous wave Doppler measurement of ascending aortic blood flow was performed. AI was calculated from the carotid pressure waveform. Waveforms were decomposed into their forward and backward components and the time to reflection between the maxima of the forward and backward pressure waves was measured. AI showed a stronger ( P < .001) association with ejection time (r = 0.48, P < .001) than with heart rate (r = −0.28, P < .001). Although AI is strongly related to the time to reflection (r = −0.51, P < .001), only a weak association was seen between time to reflection and heart rate (r = 0.16, P < .001) or ejection time (r = −0.12, P < .001). Our analysis in an elderly cohort of patients with essential hypertension demonstrates that AI is related to the time to reflection. It also reiterates that AI is confounded by heart rate without any underlying heart rate-dependent change in wave reflection. In population-based studies the confounding effect of heart rate can potentially be corrected. AI remains strongly (r = −0.52) related to time to reflection after correction for the effects of ejection time on AI.