Objective: A major proportion of RR interval variability in long-term recordings is due to slow (< 0.03 Hz) fluctuations, which seem to be a good predictor of survival after myocardial infarction, whose origin remains unclear.
Methods: To study the effect of physical activity we compared by spectral analysis of the RR interval in 10 healthy human subjects (aged 28[s.e. 2] years) during 1-h periods each of rest (no activity), alternating rest and mild exercise (rhythmic activity), and normal spontaneous (random) activity.
Results: Compared to rest, during both random and rhythmic activities, the RR variance increased significantly (from 5802 to 13388 ms2, P < 0.05, and to 24959, P < 0.001) due to an increase in power below 0.03 Hz (from 3017 to 9606 ms2, P < 0.01, and to 21 103 ms2, P < 0.001) which explained 55.4, 73.2 and 86.1% of total RR variance, respectively.
Conclusions: The amount of RR variability and its slower fluctuations largely depend on physical activity, regardless of its regular or irregular occurrence. Attempts to predict cardiovascular prognosis on the basis of RR fluctuations should therefore take account of the confounding effect of physical activity since healthier subjects would probably be more active.