[Diaphragmatic fatigue and its recovery are influenced by markedly decreased cardiac output: possible involvement of neuromuscular junction]

Effects of highly decreased cardiac output on the development and recovery of diaphragmatic fatigue were studied in dogs. The fatigue was induced by supramaximal electrical stimulation (20 Hz) of phrenic nerves for 30 minutes. Cardiac output was reduced during this period to 30% of control value by mechanically obstructing inferior vena cava with a balloon in animals with lowered cardiac output group (lowered Qt group). Cardiac output was maintained at normal value throughout the experiment in animals of the other group (control group). The animals were observed for recovery for 60 minutes after induction of the fatigue. The standardized transdiaphragmatic pressure (Pdi/Pdi 100) and integrated electromyographic activity (Edi/Edi 100) elicited by electrical test stimuli (20, 100 Hz) were significantly lower in lowered Qt group (P less than 0.01) during entire fatigue and recovery period. The decrease of Pdi/Pdi 100 and Edi/Edi 100 at high frequency test stimuli (100 Hz) was observed only in lowered Qt group. These results suggest that when cardiac output is severely decreased the diaphragm is more susceptible to fatigue, and that this may be caused by a failure of neuromuscular junction as well as by an impaired excitation-contraction coupling.