Effect of time of day on intraindividual variability in ambulatory blood pressure

The aim of this study was to determine whether intraindividual blood pressu re (BP) variability, measured by noninvasive ambulatory monitoring, differs between the active (daytime) and inactive (nighttime) periods of the day. We obtained ambulatory BP recordings in 143 healthy adults (95 men, 48 wome n) from Rochester, Minnesota. Readings were obtained every 10 min for a 24- h period. We calculated the standard deviation of each individual's BP read ings about the means for the active period and for the inactive period as m easures of intraindividual BP variability. In men, mean within-individual s tandard deviations for both systolic (SBP) and diastolic blood pressure (DB P) were significantly greater during the inactive period than during the ac tive period (for SEP: 10.3 +/- 2.1 nu 11.9 +/- 2.7, P < .0001; for DBP: 8.8 +/- 2.0 <nu> 9.7 +/- 2.5, P = .0027). In women, the mean within-individual standard deviation for SEP did not differ significantly between the active and inactive periods (9.7 +/- 2.2 nu 10.3 +/- 2.4, P = 0.225) but for DBP was significantly greater during the inactive period than during the active period (8.1 +/- 2.0 nu 9.2 +/- 2.3, P = .020). Statistically significant p redictors of intraindividual BP variability included measures of age and bo dy size, metabolic traits, neuroendocrine traits, erythrocyte cation traits , and renal function traits. This study demonstrates that intraindividual B P variability, as measured by noninvasive ambulatory monitoring, is as grea t or greater during the inactive period as during the active period of the day. Am J Hypertens 2000;13:1203-1209 (C) 2000 American Journal of Hyperten sion, Ltd.