Different effects of halothane on diaphragm and hindlimb muscle in rats

The effects of halothane administration on diaphragm and tibialis anterior (TA) muscle were investigated in 30 anesthetized mechanically ventilated rats. Diaphragmatic strength was assessed in 17 rats by measuring the abdominal pressure (Pab) generated during supramaximal stimulation of the intramuscular phrenic nerve endings at frequencies of 0.5, 30, and 100 Hz. Halothane was administered during 30 min at a constant minimum alveolar concentration (MAC): 0.5, 1, and 1.5 MAC in three groups of five rats. For each MAC, Pab was significantly reduced for all frequencies of stimulation except at 100 Hz during 0.5 MAC halothane exposure. The effects of halothane (0.5, 1, and 1.5 MAC) on diaphragmatic neuromuscular transmission were assessed in five other rats by measuring the integrated electrical activity of the diaphragm (Edi) during electrical stimulation of the phrenic nerve. No change in Edi was observed during halothane exposure. In five other rats TA contraction was studied by measuring the strength of isometric contraction of the muscle during electrical stimulation of its nerve supply at different frequencies (0.5, 30, and 100 Hz). Muscle function was unchanged during administration of halothane in a cumulative fashion from 0.5 to 1.5 MAC. These results demonstrate that halothane does not affect hindlimb muscle function, whereas it had a direct negative inotropic effect on rat diaphragmatic muscle.