Submaximal heart rate response to exercise testing: independent predictor of mortality or inadequate test?

Inability to reach 85% of the predicted maximal heart rate for age during exercise stress testing (chronotropic incompetence) has been considered indicative of an uninterpretable or inadequate test. A new study from the Cleveland Clinic suggests that a submaximal heart rate response to exercise may be a significant independent risk factor for all-cause and cardiovascular death in individuals with known or suspected coronary artery disease (CAD). A "low-risk" group of 2953 consecutive patients with known or suspected CAD (36% women; mean age, 58 years) underwent symptom-limited treadmill thallium testing using a standard Bruce protocol. Participants were excluded if they had a prior history of coronary angiograms or invasive procedures, congestive heart failure, valvular disease, congenital heart disease, pre-excitation syndrome, or B-blocker use. Chronotropic incompetence was assessed if individuals failed to reach at least 85% of their age-predicted maximal heart rate or had a chronotropic index <0.8. The chronotropic index uses directly measured variables to correct for age, physical fitness, and resting heart rate, all of which can affect maximal heart rate response. It is unaffected by exercise protocol or exercise time. Chronotropic index at peak exercise was determined with the use of the following formula, where HR represents heart rate: