Chronotropic incompetence. The implications of heart rate response to exercise (compensatory parasympathetic hyperactivity?)

The report in this issue by Lauer et al1 provides more fascinating data on the possible predictive value of a reduced response of the sinus node to exercise. They exploited the opportunity to study this important response in 1575 healthy participants in the Framingham Offspring Study who had exercise tests on entry in the 1970s. The mortality and morbidity of the men in this study were extracted from the Framingham follow-up data as well as during follow-up examinations 8, 12, and 16 years after their entry. These investigators correlated the mortality and incident coronary disease with the subject’s ability or failure to reach 85% of maximum predicted heart rate, actual increase in heart rate from rest to maximum, and a ratio of heart rate to metabolic reserve achieved by stage 2 of the Bruce exercise protocol. This ratio compensates for the differences in age and physical fitness and provides a chronotropic index. During a mean follow-up of 7.7 years, the inability to achieve the target heart rate in 21% of the subjects was correlated with total mortality and with the incidence of coronary disease including myocardial infarction. Similar correlations occurred with a reduced heart rate increase with exercise and with the chronotropic response index calculated as previously mentioned. The data suffer from the fact that exercise was arbitrarily terminated at 85% of age-predicted heart rate, a highly artificial value that had considerable adherence in the 1970s. However, previous work suggests that if their patients had been exercised to a symptom-limited end point, the correlations with coronary events would have been even stronger. The reason the magnitude of the increase in heart rate during an exercise test would be predictive of subsequent events and mortality is an intriguing one. Lauer and colleagues’1 report reaches the same …