Respiratory muscle strength and endurance is reduced in patients with congestive heart failure, making these patients susceptible to diaphragmatic fatigue during exercise.In order to determine whether or not contractile fatigue of the diaphragm occurs in patients with congestive heart failure following intense exercise, twitch transdiaphragmatic pressures (twitch Ptdi) were measured during unpotentiated and potentiated cervical magnetic stimulation (CMS) of the phrenic nerves before and at intervals after cycle endurance exercise.Ten patients aged 65.7 ¡ 6.0 yrs (mean ¡ SD) with an ejection fraction of 31.2 ¡ 9.8% performed a constant-load symptom-limited exercise test at 60% of their peak work capacity. Twitch Ptdi at baseline were 15.9¡6.3 cmH 2 O (unpotentiated CMS) and 28.8¡ 10.7 cmH 2 O (potentiated CMS) and at 10 min postexercise were 16.4¡4.7 cmH 2 O (unpotentiated CMS) and 27.6¡10.1 cmH 2 O (potentiated CMS). One patient demonstrated a sustained fall in twitch Ptdi of o15%, considered potentially indicative of diaphragmatic fatigue.Contractile diaphragmatic fatigue is uncommon in untrained patients with congestive heart failure following high-intensity constant-workload cycle exercise. Therefore, diaphragmatic fatigue is an unlikely cause of exercise-limitation during activities of daily living in heart failure patients.