Safety of High- vs Moderate-Intensity Exercise for Cardiac Rehabilitation

Abstract & Commentary

By Michael H. Crawford, MD, Editor

Source: Rognmo O, et al. Cardiovascular risk of high- versus moderate-intensity aerobic exercise in coronary heart disease patients. Circulation 2012; 126:1436-1440.

Current guidelines recommend cardiac rehabilitation using moderate exercise programs for most ischemic heart disease (IHD) patients. Also, studies have shown that the intensity of exercise is directly related to the cardioprotective effects. However, there is concern that high-intensity exercise may be dangerous in IHD patients. Thus, these investigators from Norway studied almost 5000 IHD patients in cardiac rehabilitation units that employed both high-intensity interval exercise and moderate-intensity continuous exercise training sessions. High-intensity exercise was defined as achieving > 85% of maximum predicted heart rate (mpHR) at intervals and moderate as 60-70% of mpHR continuously. The sessions lasted 1 hour and the patients underwent an average of 37 sessions. The primary outcome measure was cardiac arrest or acute myocardial infarction (MI) during or within 1 hour after a session. There were > 129,000 moderate-intensity sessions and > 46,000 high-intensity sessions. There were two non-fatal cardiac arrests during high-intensity exercise and one fatal cardiac arrest in the moderate group. There were no MIs. The authors concluded that both types of exercise training are associated with a low incidence of events and that the presumed benefits of high-intensity exercise training outweigh these risks.


This study is in agreement with other studies of cardiac rehabilitation that showed very low event rates. What distinguishes this study is the use of high-intensity exercise in some of the patients. In their protocol, after a warm-up period, high-intensity exercise was done for 4 minutes and then the patients did low-level exercise until they were symptomatically recovered and were at 50-70% mpHR. This cycle was repeated for almost an hour ending with a cool-down, low-exercise period. The patients were carefully selected clinically and by formal exercise testing. Those with symptoms or signs of ischemia were excluded. Also, these were not all post-MI patients; only 7% were post-MI. Most were post revascularization patients. Thus, this was a low-risk group.

The major limitation of this study was that it was not randomized. However, given the low event rate observed, they estimated that for a randomized trial to show any difference in the two levels of exercise, more than 10,000 patients would be required in each group. Previous randomized trials were all underpowered, and observational studies have inherent biases. Thus, the issue of vigorous vs moderate exercise for cardiac rehabilitation remains somewhat controversial, especially in post-MI patients. My take is that in carefully selected patients, in a supervised program, vigorous interval exercise is safe. However, if patients are doing their own rehabilitation at home or in their gym, then I recommend they stick to moderate exertion.