Reactivity of ambulatory blood pressure to physical activity varies with time of day

Blood pressure ( BP) fluctuates over a 24-hour period, but it is unclear to what extent this variation is governed completely by changes in physical activity. Our aim was to use a BP "reactivity index" to investigate whether the BP response to a given level of physical activity changes during a normal sleep-wake cycle. Hypertensive patients ( n = 440) underwent simultaneous 24-hour ambulatory BP, heart rate ( HR), and activity monitoring. BP and HR were measured every 20 minutes. Actigraphy data were averaged over the 15 minutes that preceded a BP measurement. Individual BP and HR reactivity indices were calculated using least-squares regression for twelve 2-hour periods. These indices were then analyzed for time-of-day differences using a general linear model. Systolic BP and HR were generally more reactive to physical activity than diastolic BP. The highest reactivity of systolic BP ( mean +/- SE = 4 +/- 1 mm Hg per logged unit change in activity) was observed between 8: 00 AM and 10: 00 AM ( P = 0.014). Between 10: 00 AM and 12: 00 PM, BP reactivity then decreased ( P = 0.048) and showed a secondary rise in the early afternoon. These 24-hour changes in BP reactivity did not differ significantly between groups formed on the basis of early and late wake times ( P = 0.485), medication use, age, and sex ( P > 0.350). In conclusion, under conditions of normal living, the reactivity of BP and HR to a given unit change in activity is highest in the morning and shows a secondary rise in the afternoon.