15 June 2005

Volume 35, Issue 12, Pages 1-12

Consistent Exercise Protects the Heart

BRUCE JANCIN (Denver Bureau)

NEW ORLEANS -- A little bit of exercise can be a dangerous thing for a woman.

New data from the Nurses’ Health Study show the risk of exercise-triggered sudden cardiac death is higher in occasional exercisers than in women who are total couch potatoes—but that this risk is markedly diminished by regular exercise, William Whang, M.D., said at the annual meeting of the Heart Rhythm Society.

Indeed, the risk of exercise-associated sudden cardiac death was significantly lower in women who engaged in about 4 or more hours of moderate to vigorous physical activity per week than in less frequent exercisers or sedentary women, according to Dr. Whang of Massachusetts General Hospital, Boston.

Nurses’ Health Study investigators defined moderate to vigorous physical exertion as involving at least five metabolic equivalents. Examples include jogging, swimming, brisk walking, tennis, bicycling, aerobics, and lawn mowing.

Exercise is known to transiently increase the risk of ventricular arrhythmias, although experts agree this is greatly outweighed by the many well-documented health benefits of physical activity.

The Nurses’ Health Study is the first large study to focus on exercise as a possible triggering event for sudden cardiac death in women.

Although the study began in 1976, self-reported assessments of physical activity weren't included for the first time until 1986. Every 2–4 years since, participants have updated their physical activity level as part of detailed follow-up questionnaires.

For this analysis, Dr. Whang reported on 69,901 women, mean age 57 years at baseline, who were free of a history of cardiovascular disease, stroke, or cancer. There were 151 sudden cardiac deaths in this cohort between 1986 and 2004.

Information on the circumstances surrounding these deaths was gathered from three sources: medical records, next of kin, and autopsy reports.

Investigators defined sudden cardiac death occurring during or within 1 hour after moderate or vigorous physical activity as exercise related, Dr. Whang said.

Using a case-crossover study design in which the study participants served as their own controls, the relative risk of sudden cardiac death occurring during or shortly after exercise was increased 20.9-fold over physically inactive periods among the women who exercised less than 2 hours per week, compared with just a 3.2-fold increase in the relative risk of sudden cardiac death in women who exercised at least 2 hours weekly.

As the amount of self-reported weekly exercise increased, the overall risk of sudden cardiac death—exercise related or otherwise—decreased, even after investigators controlled for hypertension, body mass index, hyperlipidemia, and diabetes. (See graph left.)

“The inflection point seems to be about 4 hours per week,” the physician noted. Women who engaged in at least that amount of moderate to vigorous physical activity had a substantially lower overall long-term risk of sudden cardiac death than those who exercised less frequently or who did not exercise.

Dr. Whang stressed that despite the transient increase in sudden cardiac death risk that occurred during exertion, the absolute risk of such an event remained extremely low. There were only nine such deaths during follow-up.

The overall risk of sudden cardiac death in the study population was one case per 73.2 million hours of follow-up.

There was one case per 21.9 million hours of moderate to vigorous physical exertion.

At baseline—well before the U.S. surgeon general's groundbreaking report on the importance of physical exercise began to reshape American attitudes—fully 54% of participants in the Nurses' Health Study reported getting less than 1 hour per week of moderate to vigorous exercise. Only 31% of participants reported 2 or more hours weekly.