Mortality reductions in patients receiving exercise-based cardiac rehabilitation: how much can be attributed to cardiovascular risk factor improvements?

BACKGROUND: It is unclear how much of the reduction in cardiac mortality in coronary heart disease (CHD) patients with exercise training is the result of direct effects on the heart and coronary vasculature, or to indirect effects, via primary risk factors.
OBJECTIVE: The aim of this article was to quantify the cardiac mortality benefits of exercise-based rehabilitation attributable to risk factor reductions versus the direct effects on the heart and vasculature.
METHODS: The IMPACT coronary heart disease model was used to examine the reduction in cardiac mortality attributable to changes in risk factors from a meta-analysis of cardiac rehabilitation randomized, controlled trials. Patients were receiving rehabilitation following an acute myocardial infarction, angina pectoris or revascularization. Outcomes considered were primary risk factors (total cholesterol, systolic blood pressure and smoking behaviour) and cardiac mortality.
RESULTS: Nineteen exercise-only cardiac rehabilitation trials (including 2984 patients) were identified. Across these trials, exercise training reduced pooled cardiac mortality by 28% (relative risk, 0.72, 95% confidence interval 0.55-0.95), with 30 fewer deaths than in the control group. Applying the CHD model, approximately 17 (58%) of these 30 fewer deaths were attributable to reductions in major cardiovascular risk factors: 7.1 deaths (minimum estimate 6.2, maximum estimate 9.5) attributable to an 18% reduction in smoking prevalence; 5.9 deaths (minimum -0.6, maximum 12.6) to a 0.11 mmol/l reduction in cholesterol, and 4.4 deaths (-1.0 minimum, 6.7 maximum) to a 2.0 mmHg reduction in systolic blood pressure.
CONCLUSIONS: Approximately half of the 28% reduction in cardiac mortality achieved with exercise-based cardiac rehabilitation may be attributed to reductions in major risk factors, particularly smoking.