Letter to the Editor
Response to Arterial Stiffness Index Is Not a Stiffness Parameter But a Ventriculo-Arterial Coupling Factor
We defined the ambulatory arterial stiffness index (AASI) as unity minus the regression slope of the brachial diastolic on systolic blood pressure.1,2 AASI, therefore, models the dynamic relation between diastolic and systolic pressure throughout the day. We concur with Westerhof et al3 that AASI is an indirect measure of arterial stiffness. Several hemodynamic factors influence AASI, including ventriculo-arterial coupling. In this regard, AASI does not differ from other measures of arterial stiffness, including pulse wave velocity, which many experts consider as the gold standard.
To further assess the physiological meaning of AASI, we partially implemented the proposal of Westerhof et al.3 We computed in our 348 Chinese subjects1 the decay time of aortic pressure during diastole (τ). We rewrote τ as [(60×mean arterial pressure)/(heart rate×pulse pressure). We averaged the τ values obtained from the blood pressure readings in each 24-hour ambulatory recording. Across our study sample,1 τ was normally distributed (Shapiro–Wilk’s W, 0.995; P=0.34), averaging (SD) 1.84 (0.29) seconds. We found an inverse association (r=−0.21; P<0.0001) between τ and AASI (Figure), which strengthens the concept that AASI is a measure of arterial stiffness. In the final formula, Westerhof et al3 defined the slope of diastolic on systolic blood pressure as the ratio of diastolic to systolic blood pressure and, therefore, assumed an intercept of 0. We did not force the regression line through the origin,1,2 because during diastole, when blood flow drops to 0, this is not the case for blood pressure.
The discussion on what AASI stands for should not detract attention from its prognostic significance. To date, 1 cross-sectional analysis4 and 3 prospective cohort studies2,5,6 demonstrated association of AASI either with signs of target organ damage in never-treated hypertensive patients4 or with the incidence of cardiovascular mortality and morbidity.2,5,6 AASI is particularly predictive of stroke,2,5,6 even at levels of blood pressure within the normotensive range.2,6 When adjusted for pulse pressure, AASI retained its predictive value.2,5,6 Currently ongoing analyses of the Copenhagen cohort showed that AASI predicts stroke over and beyond aortic pulse wave velocity.
Because AASI reflects more than just arterial stiffness in the narrow sense of the word, some experts proposed a name change. AASI should not belie its name. The rationale for a name change, in that AASI reflects more than just arterial stiffness, is equally applicable to most other measures of arterial function, including pulse wave velocity.
Li Y, Wang JG, Dolan E, Gao PJ, Guo HF, Nawrot T, Stanton AV, Zhu DL, O’Brien E, Staessen JA. Ambulatory arterial stiffness index derived from 24-hour ambulatory blood pressure monitoring. Hypertension. 2006; 47: 359–364.
Dolan E, Thijs L, Li Y, Atkins N, McCormack P, McClory S, O’Brien E, Staessen JA, Stanton AV. Ambulatory arterial stiffness index as a predictor of cardiovascular mortality in the Dublin Outcome Study. Hypertension. 2006; 47: 365–370.
Westerhof N, Lankhaar JW, Westerhof BE. The ambulatory arterial stiffness index is not a stiffness parameter but a ventriculo-arterial coupling factor. Hypertension. 2007; 49: e7.
Leoncini G, Ratto E, Viazzi F, Vaccaro V, Parodi A, Falqui V, Conti N, Tomolillo C, Deferrari G, Pontremoli R. Increased arterial stiffness index is associated with target organ damage in primary hypertension. Hypertension. 2006; 48: 397–403.
Kikuya M, Staessen JA, Ohkubo T, Thijs L, Metoki H, Asayama K, Obara T, Inoue R, Li Y, Dolan E, Hoshi H, Hashimoto J, Totsune K, Satoh H, Wang JG, O’Brien E, Imai Y. Ambulatory arterial stiffness index and 24-hour ambulatory pulse pressure as predictors of mortality in Ohasama, Japan. Stroke. In press.