Introduction: Chronotropic response with exercise is evaluated by peak heart rate (HR) achieved. Since most of the exercise-related chronotropic response occurs early after exercise is initiated, we investigated whether the HR achieved with a standard dose of exercise (Bruce stage 2) is associated with exercise capacity. We hypothesized that those with a blunted or disproportionate HR response at this exercise dose would have reduced exercise capacity compared to those with a typical HR response.
Material and methods: We reviewed 3,084 consecutive normal maximal treadmill stress echocardiographic reports acquired from individual adults over a 1.5-year period. We examined for association between stage 2 Bruce HR with age and sex-adjusted exercise capacity.
Results: After adjustment for age and sex, Bruce stage 2 HR was inversely associated (β = -0.08, p < 0.01) with exercise duration. Thus for every additional 10 beats per minute achieved in stage 2, exercise duration was generally shortened by about 45 s. Most of the subjects (92%) who had a stage 2 Bruce HR response below the 10th percentile had above average or average exercise capacity for their age and sex.
Conclusions: Lower Bruce stage 2 HR was associated with increased exercise capacity. Severely blunted HR response was associated with above average exercise capacity. Caution should therefore be exercised in attributing exercise intolerance to a blunted HR response when making a diagnosis of chronotropic incompetence.
Keywords: chronotropic incompetence; exercise capacity; peak heart rate.