How Many Measurements Are Needed to Provide Reliable Information in Terms of the Ambulatory Arterial Stiffness Index? The Ohasama Study

The aim of this study was to investigate how frequent ambulatory blood pressure (ABP) readings need to be obtained to reproduce the ambulatory arterial stiffness index (AASI) and pulse pressure (PP) without loss of information. We compared concordance from full and reduced ABP recordings. We recorded 24-h ABP at 30-min intervals in 1542 residents of Ohasama, Japan (baseline age, 40-93 years; 63.4% women). We randomly excluded up to 16 readings per recording or we selected readings at fixed 1- or 2-h intervals. Using full recordings as reference, we computed for the reduced recordings repeatability coefficient by Bland and Altman's approach. By Cox regression, we also calculated multivariate-adjusted hazard ratios for cardiovascular mortality. The median number of ABP readings per recording was 46. Randomly excluding more readings reduced the concordance of AASI, but not PP. Selecting blood pressure readings at 1- or 2-h intervals produced mean values of AASI and PP, which significantly differed from those in full recordings. During follow-up (median, 13.3 years) 126 cardiovascular deaths occurred. Across quartiles, AASI significantly predicted cardiovascular mortality in a U-shaped manner. AASI lost its prognostic significance when the number of randomly excluded readings increased from 8 to 16 or when the interval between readings was 1 h or longer. Compared with PP, AASI is less reproducible when the number of readings in ABP decreases, but this does not affect the predictive accuracy of AASI for cardiovascular mortality, until the median number of readings per ABP recording is less than ∼35.