Response to Ambulatory Arterial Stiffness Index Is Not a Specific Marker of Reduced Arterial Compliance
We thank Gavish et al1 for their comments on our study,2 in which we found that ambulatory arterial stiffness index (AASI), a recently described parameter derived from 24-hour blood pressure (BP) monitoring, is not a reliable index of arterial stiffness in hypertension. We also demonstrated that, given the mathematical definition of AASI, those individuals with an elevated AASI also tend to have a flattened day-night BP profile, and this spurious association may contribute to explain many of the reported adverse consequences of AASI. We proposed that, in a standard (or “asymmetrical”) regression model, such as the one used to derive AASI,3 the variation range of the dependent variable (in this particular case, diastolic BP) unavoidably influences the regression slope. In other words (see Figure 3 in our article2), the regression coefficient B tends to 0 for narrow ranges of the dependent variable, and 1 minus B, or AASI, tends to increase.
While agreeing with the findings of our article, Gavish et al1 suggest that the relation between AASI and reduced day-night BP difference might be expressed in a slightly different way. They propose that a low correlation coefficient between systolic and diastolic BP values over the 24 hours tends to artificially decrease B (and increase AASI) when a standard asymmetrical regression is used, whereas no such artifactual relationship is found when AASI is derived from a regression model, which handles both variables in a symmetrical way.
It should be recognized that the approaches developed by us and by Gavish et al1 essentially unravel the same mathematical relation. Indeed, nocturnal declines in systolic and diastolic BP are fairly well correlated with each other, and, therefore, individuals with a large nocturnal diastolic BP fall also tend to have a large systolic BP fall. This leads both to a high correlation coefficient between systolic and diastolic BP and a steep regression slope of systolic on diastolic BP (and a lower AASI). On the contrary, nondippers have a lower correlation between systolic and diastolic BP and higher AASI values.
We concur with Gavish et al1 that a symmetrical regression model might be more adequate than the standard regression proposed by Li et al3 in estimating slope-related parameters. Still, it remains to be established whether AASI values obtained with the former approach keep the clinical and prognostic impact, which have been attributed to AASI, calculated with standard regression. Our demonstration2 that nondipping, a powerful negative prognostic factor in hypertension,4,5 is strongly associated with high AASI values suggests that this may not be the case. At the least, the degree of nocturnal BP reduction should always be properly considered in future studies, which will examine the clinical and prognostic values of AASI.
Gavish B, Ben-Dov IZ, Bursztyn M. Ambulatory arterial stiffness index is not a specific marker of reduced arterial compliance. Hypertension. 2007; 50: e18.
Schillaci G, Parati G, Pirro M, Pucci G, Mannarino MR, Sperandini L, Mannarino E. Ambulatory arterial stiffness index is not a specific marker of reduced arterial compliance. Hypertension. 2007; 49: 986–991.
Li Y, Wang J-G, Dolan E, Gao P-J, Guo H-F, Nawrot T, Stanton AV, Zhu D-L, O’Brien E, Staessen JA. Ambulatory arterial stiffness index derived from 24-hour ambulatory blood pressure monitoring. Hypertension. 2006; 47: 359–364.
Verdecchia P, Porcellati C, Schillaci G, Borgioni C, Ciucci A, Battistelli M, Guerrieri M, Gatteschi C, Zampi I, Santucci A, Santucci C, Reboldi G. Ambulatory blood pressure: an independent predictor of prognosis in essential hypertension. Hypertension. 1994; 24: 793–801.(Correction. 1995;25:462.)
Staessen JA, Thijs L, Fagard R, O’Brien ET, Clement D, de Leeuw PW, Mancia G, Nachev C, Palatini P, Parati G, Tuomilehto J, Webster J, for the Systolic Hypertension in Europe (Syst-Eur) Trial Investigators. Predicting cardiovascular risk using conventional vs ambulatory blood pressure in older patients with systolic hypertension. JAMA. 1999; 282: 539–546.