It remains a rare phenomenon, but researchers have now documented 25 cases in which apparently healthy young athletes were killed instantly when hit in the chest while playing sports. Their deaths, researchers have concluded, resulted from a sharp blow during the split-second interval when the heart readies itself for the next beat.

In most cases, the athletes were struck by a baseball or a hockey puck. Four deaths resulted from a blow during a tackle, body check or karate kick, researchers reported in the New England Journal of Medicine. Children may be particularly vulnerable to this phenomenon, known as commotio cordis, or "concussion of the heart," even if they have no underlying heart abnormalities."Children and adolescents . . . have thin, compliant chests - a situation that enhances the transmission of force to the heart," noted Dr. Mark Estes, of Tufts University School of Medicine, in the accompanying editorial.

According to the researchers' hypothesis, commotio cordis can occur when the heart suffers a sharp blow during what cardiologists call the T-wave, the interval on an electrocardiogram when heart muscle cells are regrouping in preparation for the next contraction. Previous research has shown that such a disruption can cause the heart to go into repetitive activity, or ventricular fibrillation, which can lead to heart attacks.

Given the rarity of this phenomenon, Estes says, it would be inappropriate for parents to restrict athletic activities. And no one knows yet what kind of safety equipment would reduce or eliminate the risk of commotio cordis. Estes called for more research to prevent these deaths.

- Alison Bass