The incidence of primary cardiac arrest during vigorous exercise

To examine the risk of primary cardiac arrest during vigorous exercise, we interviewed the wives of 133 men without known prior heart disease who had had primary cardiac arrest. Cases were classified according to their activity at the time of cardiac arrest and the amount of their habitual vigorous activity. From interviews with wives of a random sample of healthy men, we estimated the amount of time members of the community spent in vigorous activity. Among men with low levels of habitual activity, the relative risk of cardiac arrest during exercise compared with that at other times was 56 (95 per cent confidence limits, 23 to 131). The risk during exercise among men at the highest level of habitual activity was also elevated, but only by a factor of 5 (95 per cent confidence limits, 2 to 14). However, among the habitually vigorous men, the overall risk of cardiac arrest--i.e., during and not during vigorous activity--was only 40 per cent that of the sedentary men (95 per cent confidence limits, 0.23 to 0.67). Although the risk of primary cardiac arrest is transiently increased during vigorous exercise, habitual vigorous exercise is associated with an overall decreased risk of primary cardiac arrest.