Two large new studies have provided the strongest evidence yet that a sudden burst of physical activity can set off a heart attack. But even more important, the studies showed a strong protective effect of regular physical activity.
Over all, the risk of a heart attack during or just after heavy physical exertion is two to six times greater than the risk during less strenuous activities or no activity, the studies showed. But regular physical activity diminished the added risk to practically none at all.
Normally sedentary people who try something strenuous like shoveling snow, sprinting to catch a bus, playing tennis or pushing a car out of a snowdrift are especially at risk.
The findings, published in today's issue of the New England Journal of Medicine, suggest that sedentary people, particularly those with other risk factors for heart disease like smoking, high blood pressure or diabetes, might be wise to hire a young neighbor to shovel the walk or have the driveway plowed.
"Better yet," said Dr. Gregory Curfman, an associate editor of the journal, who wrote an editorial about the two reports, "would be for people who are sedentary to gradually get themselves into shape."
For, he said, another important finding of the two studies was that regular physical activity strongly protected people against an exertion-related heart attack.
While many people have believed that intense activity can precipitate a heart attack, this had never been clearly established because early studies were smaller and less statistically certain. The new findings "really settle the question that intense activity can trigger a heart attack," Dr. Curfman said in an interview.
Among the cardiovascular benefits of regular exercise, he said, are a diminished tendency of the blood to form clots, an improved cholesterol profile, more efficient use of oxygen by the muscles, a larger volume of blood pumped with each heartbeat, and, during periods of exertion, greater dilation of the arteries, lower heart rate and lower blood pressure.
In one of the studies, directed by Dr. Murray A. Mittleman of Deaconess Hospital and Harvard Medical School in Boston, the chance of a sedentary person suffering a heart attack during or just after heavy exertion was nearly 50 times that faced by people who usually exercise five or more times a week.
In fact, exercising just one or two times a week cut a person's risk by more than 80 percent.
"Heavy exercise can trigger a heart attack. But more importantly, regular exercise can lower the probability," says Dr. Mittleman.