Intense Exercise Post-MI? Some Caution and Encouragement

August 15, 2014

BERKELEY, CA — Regular running as a form of exercise, even running at higher-gear competitive levels, is an admirable way to regain and even improve on one's physical status lost in the early days following an acute MI. But for some of those patients with damaged but recovering hearts, there may be a peak exercise threshold that preserves cardiovascular safety; higher than that threshold, CV mortality may start climbing again, suggests a cohort study[1].

Cardiovascular mortality over an average of 10 years fell consistently in post-MI high-level exercisers as their achieved MET-hours per day (by running or walking) climbed continuously from the lowest level up to 7.2 MET-h/day, mimicking a dose-response effect, after which the benefit seemed to reverse.

The prospective study "provides what is to our knowledge the first data in humans demonstrating a statistically significant increase in cardiovascular risk with the highest levels of exercise," the authors write. "These data were obtained in heart-attack survivors and require confirmation in cohorts of healthy subjects before concerns about excessive exercise can be extended to the general public."

In some ways "the bigger story" from the analysis, because it's more widely applicable, is that exceeding exercise levels recommended for post-MI patients, whether by a little or lot, "substantially reduces your risk even more than just meeting the exercise recommendations," according to first author Dr Paul T Williams (Lawrence Berkeley National Laboratory, Berkeley, CA). It's just that those benefits apparently reach a ceiling, above which risk starts to climb.

That ceiling of about 7.2 MET-h/day, at least in the current study, is "an energy expenditure approximately equivalent to running 30 miles or 50 km per week, or walking briskly 46 miles or 75 km per week," the group writes.

"I think pushing them to the energy equivalent of 25 miles per week is certainly achievable by most of these people, by walking or by running," Williams told heartwire .

Walker and Runner Cohorts

In 2377 self-identified MI survivors in the National Runners' Health Study (recruited from 1991 to 1994 and 1998 to 2001) and National Walkers' Health Study (recruited from 1998 to 2001), CV-related mortality—compared with the lowest exercise-level group (<1.1 MET-h/day)—decreased by:

  • 21% for those achieving 1.1 to 1.8 MET-h/day by running or walking (p=0.11).

  • 24% for those attaining 1.8 to 3.6 MET-h/day (p=0.04).

  • 50% for those achieving 3.6 to 5.4 MET-h/day (p=0.001).

  • As much as 63% for those reaching 5.4 to 7.2 MET-h/day (p<0.001).

But it rose 12% for those exceeding 7.2 MET-h/day (p=0.68).

"Fewer than 6% of [the current study's] the heart-attack survivors actually exceeded 7.2 MET-h/day," according to the authors, and that group consisted overwhelmingly of runners. "For the remaining 94% there was a graded reduction in CVD-related mortality with increasing exercise that extended substantially above the current recommendations of 1.07 to 1.8 MET-h/day."

That recommended exercise level corresponds to "150 minutes of moderately intense exercise or 75 minutes of vigorous exercise," according the report from Williams and Dr Paul D Thompson (Hartford Hospital, CT) published August 12, 2014 in Mayo Clinic Proceedings.

But, "Our analyses suggest that the benefits of running or walking do not accrue indefinitely and that above some level, perhaps 30 miles or 50 km per week [running], there is a significant increase in CVD mortality (p=0.009)."

Running tends to be far more vigorous than other forms of exercise in which MI survivors are likely to engage, setting this group apart from others, according to Williams. Whereas about 6% of them reached or surpassed the ceiling of benefit, "it might be only 2% or 3% of all MI survivors who exercise" who actually hit that level, he said.

Williams would expect to see the same exercise-risk effect, an inverse-J curve for events by METS achieved, among avid runners in the general population, "but the effect wouldn't be as pronounced." And his group's observed benefit ceiling would be reached by a much lower proportion, only 1%, of the general population. For that number, Williams referred to coverage of the current study in Runners World, which used National Health and Nutrition Examination Survey (NHANES) numbers to estimate that probably "well under 1%" of Americans run >30 miles per week[2].

"There may well be a subset, which is what we'd like to focus our attention on, of those who are really at risk," Williams said hopefully. "The ideal would be that we could figure out what it is, [identifying] some sort of marker that causes people to be more careful."

Unknown Mechanisms of Harm

There was a parallel but attenuated effect for all-cause mortality, Williams noted. Moreover, CV-mortality events were not related to arrhythmia per se but were associated with ischemic heart failure—odd, he said, because the most active runners in the cohort had better conventional coronary risk factor profiles than runners exercising less intensely: lower blood pressure, LDL-cholesterol, and body-mass index, for example, and less likely to be on antidiabetic agents even after adjustment for age and sex.

An accompanying editorial refers to "cardiac overuse injury" as the term for ill heart effects due to overexercising[3]. Dr James H O'Keefe (Saint Luke's Mid America Heart Institute, Kansas City, MO) et al note that many endurance athletes are subjected to orthopedic overuse injuries, but "cardiac overuse injury may be associated with more ominous outcomes, including threatening cardiac arrhythmias, accelerated coronary plaque formation, premature aging of the heart, myocardial fibrosis, plaque rupture and acute coronary thrombosis, and even sudden cardiac death."

The current study's results, they write, in many ways parallel to those in a 2014 study from Germany that saw a reverse J-curve for outcomes across a spectrum of activity levels in >1000 patients with stable CAD[4].

"Collectively, the data suggest that moderate doses of exercise are sufficient for conferring optimal CV and longevity benefits and very high doses of physical activity appear to offer little or no additional survival," they write.

"The hypothetical upper-dose limit for safe exercise remains unclear, but generally keeping the intensity, frequency, and duration of exercise all in the moderate ranges may be important for maximizing health and longevity benefits."

The study was supported by the National Heart, Lung, and Blood Institute. The authors had no disclosures.





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