Public accounts of this year’s fatalities indicate that the athletes seemed outwardly healthy, and in some cases autopsies turned up no obvious cause of death, such as blocked arteries.

The combination of apparent good health and a negative autopsy suggests a death caused by abnormal heart rhythms, said Dr. Pamela Douglas, a Duke University cardiologist who has studied triathletes.

Evidence suggests that swimming may trigger a certain type of cardiac arrhythmia caused by a genetic condition called long QT syndrome, said Dr. Michael Ackerman, a cardiologist and the director of the Windland Smith Rice Sudden Death Genomics Laboratory at the Mayo Clinic in Rochester, Minn. About 1 in 2,000 people are born with a heart condition that causes a glitch in the heart’s electrical system, and the most common of these is called long QT syndrome, after the tell-tale interval on an electrocardiogram.

The long QT heart recharges sluggishly between beats, setting up the potential for a skipped beat, Dr. Ackerman said. When the problem strikes, a heart’s electrical system can go haywire, degenerating into a possibly fatal arrhythmia.”