A sudden cardiac death, as the name implies, occurs quickly and without warning. Within an hour after the heart stops working properly, the person is dead. With an eye toward pinpointing who might be vulnerable, researchers in France sought to test the hypothesis that problems with the control of heart rate might lead to sudden cardiac death.

What the researchers wanted to know: Does the heart rate before, during, and after exercise predict sudden death in men without symptoms of heart disease?

What they did: The researchers used data from a long-term study of native Frenchmen who had worked for the Paris government in the late 1960s and early 1970s. When they joined the study, the men had physical exams and ekgs, gave blood samples for lab tests, and answered questions about their health. Each man's heart rate was measured at rest and during and after riding a bicycle. Then, for the next 30 years or so (until Jan. 1, 1994), the study team compiled information on the men's deaths. The study included more than 5,000 men, none of whom had apparent cardiovascular disease when the study started; of 1,516 deaths during the follow-up period, 400 were from cardiac causes, of which 81 were sudden cardiac deaths.

What they found: The higher a man's resting heart rate, the greater his risk of sudden cardiac death. Indeed, men whose resting heart rate was faster than 75 beats per minute had a risk that was 3.5 times as high as that of men whose resting heart rate was under 60 beats per minute. The researchers adjusted for body mass index and level of physical activity. A man also had a higher risk of sudden cardiac death if his heart rate didn't go up very much during exercise. Men whose heart rate went up by less than 89 beats per minute during exercise had 4 times the risk of sudden cardiac death as men whose heart rate went up by more than 113 beats per minute. Men whose heart rate returned to normal more slowly also had a higher risk of sudden cardiac death.

What the study means to you: A man's heart rate profile does seem to help predict his risk of sudden cardiac death, which suggests that the nervous system's control (or lack of control) over heart rate might be part of the reason why sudden cardiac deaths happen. But not every man who has this heart rate profile will die a sudden cardiac death. For example, while men with a resting heart rate of more than 75 beats per minute had a higher risk of dying from sudden cardiac death than men with a lower resting heart rate, 95 percent of them still died of something else.

Caveats: Because none of the people in this study were women, these results might not apply to women. Also, they were French, and their diets and alcohol intake were probably different from the average contemporary American's, which could change their profile for heart disease. Those different risk factors for heart disease could affect a person's risk of dying from sudden cardiac death.

Find out more: Read information on sudden cardiac death from the American Heart Association.

Check out a highly detailed and technical article on sudden cardiac death from eMedicine.com.

Read the article: Jouven, X. et al. "Heart-Rate Profile During Exercise as a Predictor of Sudden Death." New England Journal of Medicine. May 12, 2005, Vol. 352, No. 19, pp. 1951–1958.

Abstract online: http://content.nejm.org