Prolonged exercise alters cardiac chronotropic responsiveness in endurance athletes.

OBJECTIVE: To determine whether exhaustive exercise alters cardiac adrenergic chronotropic responsiveness in endurance-trained athletes.
METHODS: Fifteen athletes were studied prospectively 2-4 days before and within 3.3 hours after completing the Hawaii Ironman Triathlon (3.9 km swim, 180.2 km bike, 24.2 km run). Increasing intravenous boluses of isoproterenol were given until the rise in heart rate was > 30 bpm (n = 3-6 doses). A log dose heart rate response curve was constructed, and the dose required to increase heart rate by 15 and 25 bpm estimated. Left ventricular size and function were also assessed by echocardiography.
RESULTS: After race finish, left ventricular volume (98 vs 83 cc), ejection fraction (56 vs 46%) and diastolic filling (3.86 vs 3.12 edv/sec) were reduced (all p < 0.01). Resting heart rate rose from 54 +/- 7 bpm to 70 +/- 10 bpm. The isoproterenol dose required to increase heart rate by 15 bpm rose from 0.6 to 1.7 micrograms by 25 bpm rose from 1.8 to 4.0 micrograms, both p < 0.01. The linear relationship between change in heart rate and log isoproterenol dose was preserved.
CONCLUSIONS: Cardiac chronotropic responsiveness is reduced following an Ironman triathlon.