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(Hypertension. 2006;47:778.)
© 2006 American Heart Association, Inc.

Original Articles

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

Helen Jones; Greg Atkinson; Andrew Leary; Keith George; Michael Murphy; 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.

Correspondence to Helen Jones, Research Institute for Sport and Exercise Sciences, Liverpool John Moores University, Henry Cotton Campus, Webster St, Liverpool L3 2ET, United Kingdom. E-mail spshjone{at}livjm.ac.uk

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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