Physical Exertion, Exercise, and Sudden Cardiac Death in Women
Abstract & Commentary
By John P. DiMarco, MD, PhD. Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville. Dr. DiMarco is a consultant for Novartis, and does research for Medtronic and Guidant.
Synopsis: These prospective data suggest that sudden cardiac death during exertion is an extremely rare event in women.
Source: Whang W, et al. Physical Exertion, Exercise, and Sudden Cardiac Death in Women. JAMA. 2006;295:1399-1403.
Whang and colleagues performed an analysis of the relationship between acute physical exertion, routine exercise, and sudden cardiac death in women. Whang and colleagues analyzed data from the Nurses' Health Study. This is a large NIH-funded, longitudinal, epidemiologic study which involved over 120,000 female registered nurses who were between age 30 and 55 at the time of enrollment. The Nurses' Health Study began in 1976 and has provided important data about the epidemiology of cardiac and other diseases in women. For the purposes of this report, data from questionnaires obtained every 2 years about physical activity were collected. This information was then correlated with the occurrence of sudden cardiac death, defined as death within one hour of the onset of symptoms. For this study, most deaths during sleep were not counted as sudden deaths.
Among those who died suddenly, the medical records and reports from next of kin were analyzed, and the degree of physical exertion around the time of death was quantified on a scale of 1 to 8 metabolic equivalents. Moderate-to-vigorous exertion was estimated to be 5 or more metabolic equivalents. During the more recent years of the study, questionnaires included information about the average time spent in routine exercise counting lower intensity exercise, as well as moderate-to-vigorous activity. A number of different statistical methods were then used to analyze the relationship between chronic exertion and sudden death and the relative risk for sudden death during periods of high exertion. Proportional hazard models were used to adjust for other variables that might be related to sudden death. Patients who had a prior history of cardiac disease, stroke, or cancer prior to 1986 were excluded. This report covers approximately 79,000 women with data concerning their physical activities.
In this cohort, there were 288 sudden deaths that occurred over 1.93 million person years of follow-up. Of these, only 9 deaths occurred during an episode of moderate-to-vigorous exertion and, of these, only 3 were during voluntary exercise. The overall risk of a sudden death episode during moderate or vigorous exertion was estimated to be one per 36.5 million hours of exercise compared with an instance of one sudden death per 59.4 million hours with less or no exertion.
Using a case crossover analysis, the relative risk of sudden death during moderate-to-vigorous exertion was modestly elevated at 2.38 compared with risks at other time points. It was, however, noted that this slight elevation in risk was smaller and no longer significant among women who exercised 2 or more hours per week. Supporting this observation were data which indicated that women who reported exercising 4 or more hours per week had a lower absolute risk of sudden cardiac death. The trend indicating a protective effect of chronic exercise was still present, although of lower magnitude, even after multiple clinical factors which might affect the incidence of sudden death in the proportional hazards model.
Whang et al conclude that sudden cardiac during exertion is a rare event in women, and regular exercise for at least 4 hours per week can significantly minimize overall risk of sudden death.
There has been much interest in the role of exercise as both a trigger and a predictive factor for sudden death. This paper from the Nurses' Health Study supports previous data observed in men. Although there is an increase in the relative risk for death during exercise, the risk is low and chronic regular exercise serves to protect patients from this risk. Therefore, even though death may occur during exercise, patients should be encouraged to exercise on a regular basis. Routine exercise seems to be protective.