Severely blunted early heart rate response during treadmill exercise is associated with above average exercise capacity

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.