Chronotropic incompetence in heart failure.

The relationship between heart rate (HR) during exercise and exercise capacity in patients with heart failure due to left ventricular systolic dysfunction remains unclear. The accepted diagnostic criterion for chronotropic incompetence (CI), a maximum agepredicted heart rate (MA-PHR) 85%, is seen in 50% of patients with severe heart failure on contemporary medical therapy (1). Whether the lesser HR rise is the cause of the exercise impairment is less certain: as HR increases as a function of exercise load, it is inevitable that HR at peak exercise will be lower in subjects with worse exercise capacity. Tse et al. (2) are to be congratulated in trying to explore these issues in their study of patients with cardiac resynchronization devices. However, some important issues were raised by their suggestion that rate-adaptive pacing might be more widely used in patients with resynchronization devices. In patients with the most severe CI, defined as failure to reach 70% of age-predicted maximum HR (n 11), Tse et al. were able to demonstrate an increase in exercise capacity with rate-adaptive pacing. There are no published data on the incidence of their chosen value of 70%, although it was seen in 28% of patients in our dataset (1). At first sight, this seems a useful incremental benefit. However, using the more widely accepted definition of failure to achieve 85% of age-predicted maximal HR (3), there was no benefit of rate response pacing. In fact, in one-third of patients with less severe CI (MA-PHR 70% to 85%), there was a reduction in exercise capacity with rate response pacing. Although CI in chronic heart failure (CHF) patients not taking beta-blockers predicts a worse prognosis (1), lower HRs at rest are associated with improved outcomes in beta-blocker–treated individuals (4). We have also previously demonstrated that HR lowering by aggressive use of beta-blockers does not reduce exercise capacity (1,5). Hence, the issue of CI in patients with severe CHF is not resolved. Furthermore, the long-term effects of rate response pacing and the consequent higher average HR on mortality and left ventricular function are unknown. It is too early to suggest that patients undergoing CRT should have their device’s rate response function active.