By Michelle Rizzo

NEW YORK (Reuters Health) - Physical exertion may trigger the onset of heart attack in a "dose-response fashion", that is, the risk increases with the intensity of physical exertion, according to study published in the European Heart Journal.

Depending on the patient's initial risk, cardiac events can be triggered by transient exposure to stress, including physical, psychological, or chemical stressors," note Dr. Stephanie von Klotz, at the German Research Center for Environmental Health, Neuherberg, and colleagues.

The researchers examined factors associated with a sudden heart attack in 1,301 patients, who were interviewed regarding levels of activity and time spent outdoors on the day of the event and the 3 preceding days.

The risk of heart attack was increased 5.7-fold within 2 hours of an episode of strenuous exertion and 1.6-fold within 2 hours of moderate exertion, compared to very light or no exertion.

"This suggests a graded exposure-response relationship between physical exertion intensity and triggering of heart attack onset," von Klot told Reuters Health. However, she added, "Regular physical exertion was protective against the risk associated with moderate and strenuous exertion episodes."

Of interest, was that strenuous activity performed outside was associated with a 4-fold greater risk of heart attack symptom onset compared with activities performed indoors, the researcher noted. "This was not observed for moderate exertion and was not explained by outdoor temperature," she said.

The risk of heart attack onset with moderate exertion was "particularly elevated" in patients with high blood pressure or those who reported very low levels of physical activity, von Klot pointed out.

"These findings emphasize the importance of increasing endurance very carefully prior to introducing a cardio-protective training program to sedentary or hypertensive patients."

SOURCE: European Heart Journal, August 2008.

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