Published Online
on February 27, 2006

A more recent version of this article appeared on April 1, 2006

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Submitted on October 4, 2005
Revised on October 22, 2005

Reactivity of Ambulatory Blood Pressure to Physical Activity Varies With Time of Day

Helen Jones^*; Greg Atkinson; Andrew Leary; Keith George; Michael Murphy; and Jim Waterhouse

From the Research Institute for Sport and Exercise Sciences (H.J., G.A., K.G., J.W.), Liverpool John Moores University, Liverpool, United Kingdom; Shandon Clinic (A.L.), Cork, Ireland; and Department of Pharmacology and Therapeutics (A.L., M.M.), National University of Ireland, Cork, Ireland.

^* To whom correspondence should be addressed. E-mail: spshjone{at}livjm.ac.uk.

Abstract--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.

Key words: exercise • blood pressure monitoring, ambulatory • heart rate • circadian rhythm

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