How dangerous is vigorous exertion?

Summarized by Robert W. Griffith, MD
December 15, 2000 (Reviewed: February 17, 2003)

Introduction

It is well known that sudden cardiac death occurs with increased frequency during or shortly after vigorous exercise - indeed, frequencies of 6% - 17% of all sudden deaths have been related to such exertion. However, it is also recognized that physical activity has beneficial effects on health, in particular being associated with a reduction in mortality from cardiac events. How can these conflicting truths be reconciled? Boston scientists have used a sampling of subjects from the Physicians' Health Study to determine if vigorous exertion triggers sudden death, and whether habitual vigorous exercise can diminish the risk.

Method

The Physicians' Health Study recruited over 22,000 male physicians in 1982. They were age 40 to 84, and had no history of cardiac events (myocardial infarction, stroke, or transient ischemic attacks) or cancer. They were assigned to receive aspirin, beta-carotene, or both, in a randomized placebo-control fashion. Questionnaires on known cardiovascular risk factors, as well as the frequency of physical vigorous exercise, were administered at baseline.

A 60-minute "hazard period" was established - 30 minutes during and 30 minutes after vigorous exertion - that allowed a prospective nested case-crossover analysis comparing the risk of sudden death during this period with that during periods of light or no exertion. The relation of vigorous exercise to overall risk of sudden death and non-sudden death from coronary heart disease was also assessed. To this end, the prospectively established habitual frequency of exertion for a subject served as control information for that subject.

To help define vigorous exertion at baseline, subjects were asked, "do you exercise vigorously enough to work up a sweat?" Subjects were asked at 36 months into the study to give the usual duration of their exercise sessions. The median duration reported - 30 minutes - was used for further calculations.

As an additional check, the specific activity undertaken at the time of sudden death and for 1 hour before was obtained from the medical record or in the next of kin. This was then quantified on a scale of 1 to 8 metabolic equivalents (MET; one MET is the energy expended during quiet seated rest, roughly 1 kcal per kg per hour). Vigorous exertion was assumed if the activity was estimated as 6 MET or more. A sudden death was defined as death within one hour after the onset of symptoms, or death after a witnessed cardiac arrest or abrupt collapse without preceding symptoms lasting one-hour or more. Anyone with evidence of circulatory collapse before disappearance of the pulse was excluded from the analysis.

The ratio of the observed frequency of sudden death in the 60-minute hazards period to that expected on the basis of the usual frequency of exercise provided an estimate of the relative risk (RR). Three categories for the frequencies of habitual exercise (less than once, 1-4 times, and 5 or more times per week) were examined by testing linear trend, and the effect of the time of day on the relative risk of sudden death was assessed by stratification.

Results

During the follow-up period of 12 years, there were 122 sudden cardiac deaths among the 21,481 participants. This yielded an overall incidence of sudden death per person-hour of one death for 19 million hours. The risk of sudden death associated with vigorous exertion was one death per 1.42 million person-hazard periods. The risk of sudden death during light or no exertion was one death per 23 million person hours. These data gave a relative risk of sudden death during the one-hour hazard period of 16.9 (95% confidence interval, 10.5 - 27.0, p<0.001). Expressed otherwise, there was one excess sudden death per 1.51 million episodes of vigorous exercise.

The RR for sudden death during the hazard period showed a significant trend, being lower among those who exercised more frequently. In men who exercised at least five times a week the RR was low (10.9), rising to 74.1 for those who exercised less than once a week.

When the relative risks of sudden death were examined according to the time of day, there was no significant modification in the findings. The RR associated with vigorous exertion remained significantly elevated at times other than the known circadian peak of cardiovascular risk (waking up until to noon).

The level of vigorous exercise reported at baseline was not significantly associated with the risk of subsequent sudden death. (This is at odds with findings reported in numerous other studies.) On the other hand, the risk of nonsudden death from coronary heart disease in these physicians was significantly lower in those who exercised vigorously at least once a week.

Comment

This study has put numbers on what has long been recognized intuitively - namely that regular vigorous exercise carries a small risk of increased sudden death during the process, but that, overall, the long-term benefit (decreased cardiac nonsudden death) outweighs this risk. Moreover, the frequency of exercising is relevant; habitually active men had a much lower risk than those who exercised less than once a week.

The authors of the study speculate on the likely mechanisms accounting for their findings. Doubtless these will be clarified in future studies. Meanwhile, they state: "these data should not discourage participation in an exercise program. The benefits ....clearly outweigh the small risks described...." So, keep exercising, at least once a week!

Source

• Triggering of sudden death from cardiac causes by vigorous exertion. CA. Albert, MA. Mittleman, CU. Chae, N Engl J Med , 2000, vol. 343, pp. 1355--1361

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