BACKGROUND:Ambulatory Arterial Stiffness Index (AASI) has been proposed as an indirect and simpler method to estimate the Arterial Stiffness (AS). AASI, calculated from a set of data collected during a 24-hours ambulatory blood pressure monitoring (ABPM), is defined as 1 minus the regression slope of diastolic on systolic blood pressure (BP) values. For a given increase in diastolic BP, the increase in systolic BP is smaller in a compliant compared to a stiff artery; the stiffer the arterial tree, the closer AASI is to 1. AASI was demonstrated to predict cardiovascular mortality, cerebrovascular events and to be associated with target organ damage. Taking into account the almost complete absence of data regarding the ability of AASI to predict the different degree of AS when hypertensives are divided into four classes of dipping in relation to the extent of the nocturnal reduction of BP (extreme dippers, dippers, mild dippers and reverse dippers) aim to clarify the ability of AASI to estimate the different degree of AS of hypertensive subjects with different nocturnal BP profile and resulting in different extent of organ damage.MATERIALS AND METHODS:We enrolled 816 subjects (403 men and 413 women) with essential hypertension, referred to the U.O.C of Medicina Interna e Cardioangiologia of the University of Palermo; 173 subjects (71 men and 102 women, mean age 44.4 ± 14.6 years) without a history of hypertension were enrolled as controls.RESULTS:The analysis of data was performed by dividing the population into four categories in relation to the extent of the nocturnal decline of BP: 124 extreme dipper (mean age 54,8 ± 12,4 years, men 46.8 %); 287 dipper (mean age 55,9 ± 14,2 years, men 54,0 %); 271 mild dipper (mean age 61,5 ± 14,7 years, men 52,0 %); 134 reverse dipper (mean age 61,5 ± 14,7 years, men 33.6 %). The mean value of AASI was significantly higher for mild and reverse dippers versus control patients and versus the other categories of dipping. The multiple regression analysis with AASI as the dependent variable confirmed the significant association between AASI and nocturnal dip (p: 0.015). The Multinomial Logistic Regression Analysis, in which AASI values were adjusted for the main confounders (age, sex, Body Mass Index, 24h SBP, 24h DBP) showed that the association between AASI and dipping is maintained only for dipper and extreme dipper hypertensives, missing the significance for mild and reverse subjects.CONCLUSION:1) AASI levels are associated with night-to-day BP ratio; 2) Lower levels of AASI are significantly associated to extreme dipper and dipper BP nocturnal profile when compared to healthy controls. 3) After correction for the major confounding factors, the association between AASI and the high-damaged class of hypertensive subjects with lower or no nocturnal fall of BP is lost. Our findings support the hypothesis that AASI is unable to estimate AS of older hypertensive subjects with a high burden of organ and vascular damage and several comorbidities, probably because the nocturnal reduction of BP is the main determinant of AASI, being more powerful than AS itself.