24-Hour Variation in the Reactivity of Rate-Pressure-Product to Everyday Physical Activity in Patients Attending a Hypertension Clinic

The exercise-related response of the rate-pressure-product (RPP) is a prognostic marker of autonomic imbalance, cardiovascular mortality, and silent myocardial ischemia in hypertension. In view of the well-known 24h variation in out-of-hospital sudden cardiac events, our aim was to investigate whether the reactivity of RPP to everyday physical activities varies over the 24h. Ambulatory measurements of systolic blood pressure (BP) and heart rate were recorded every 20min for 24h in 440 diurnally active patients attending a hypertension clinic. Wrist activity counts were summed over the 15min that preceded a BP measurement. An RPP reactivity index was derived for each of twelve 2h data bins by regressing the change in RPP against the change in logged activity counts. The RPP showed 24h variation (p 0.0005), with a peak of 11,004 (95% CI = 10,757 to 11,250) beat center dot min-1 center dot mmHg occurring at 10:00h (2h after mean wake-time). The overall 24h mean of RPP reactivity was 477 beat center dot min-1 center dot mmHg center dot logged activity counts-1 (95% CI = 426 to 529). The largest increase in RPP reactivity occurred within the first 2h after waking (p 0.0005). There were no subsequent significant differences in RPP reactivity up to 14h after waking. The lowest RPP reactivity was found 18-20h after waking, with a peak-to-trough variation of 593 beat center dot min-1 center dot mmHg center dot logged activity counts-1 (95% CI = 394 to 791, p 0.0005). Although this variation was not moderated by BP status, age, or sex, less variability in RPP reactivity was found for the medicated individuals during the waking hours. These data suggest that under conditions of normal living, the reactivity of RPP to a given change in physical activity increases markedly during the first 2h after waking from nocturnal sleep, the time when out-of-hospital sudden cardiac events are also most common. Therefore, these data add weight to the notion that reactivity of RPP to physical activity could be a prognostic marker of autonomic imbalance and cardiovascular mortality, although more research is needed to assess the specific prognostic value of 24h ambulatory measurements of RPP and physical activity.