Night and day changes in heart rate and blood pressure fractal dimensions from 24-hour ambulatory blood pressure monitoring devices

Ambulatory blood pressure monitoring (ABPM) devices allow measuring changes of BP and heart rate (HR) by periodically inflating an arm cuff. Due to the low sampling rate, only simple indices of BP variability (V) have been considered so far, such as the standard deviation (SD) or the coefficient of variation (CV) of average BP levels over day and night periods. Our aim is to describe day-night changes in a specific feature of ABP complex dynamics: the fractal dimension (FD); and to evaluate whether FD provides complementary information to SD or CV in the description of 24h BPV. We obtained HR and systolic (S) and diastolic (D)BP time series from ABPM readings taken every 15´ during the day (6:00-22:00) and every 30´ at night (22:0006:00) in 67 hypertensive subjects, and compared daytime vs. nighttime values of SD, VC and FD. FD was estimated by the Higuchi´s algorithm (FDH) and by the recently proposed Katz´s corrected algorithm (FDc). From day to night, SD decreased significantly (p<;0.01) for SBP, DBP and HR, while VC decreased significantly (p<;0.05) for HR only. FD showed a different behavior, increasing significantly (p<;0.05) for DBP. Therefore FD provides additional information on night/day changes of ABP dynamics not included in SD or CV assessment.