Spontaneous episodes of VT are preceded by changes in HRV in the frequency domain. Divergent dynamics of HRV before the onset of nonsustained and sustained VT episodes may reflect differences in factors that can facilitate the perpetuation of these arrhythmias.

ALTHOUGH the response of the human heart to exercise has been studied extensively, little information is available on the effects of utterly exhaustive exercise on cardiac performance in man. Marathon running is growing in popularity, however, and thousands of runners, more or less well trained, are taking part in various ultramarathon races (i.e., distances greater than the marathon) all over the world each year. It is well recognized that ultramarathon running is not without risk. In addition to casualties caused by exhaustion, heat stroke, renal failure, and sudden cardiac death," repeated hemoptysis and pulmonary edema due to left ventricular failure has been reported in two welltrained athletes during a 90 km run.5 However, subsequent clinical investigations in these athletes, including cardiac catheterization, revealed no apparent cardiac abnormalities, and it was postulated that some as yet unknown factor (or factors) could lie behind these observations. We had a unique opportunity to make echocardiographic and biochemical measurements on welltrained ultramarathon runners in connection with a competitive 24 hr run. Our results indicate a deleterious effect of such severe prolonged exercise on left ventricular performance. Methods

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