The effect of some local anaesthetics upon the electrical and mechanical activity of respiratory muscles in cats during pentobarbital anaesthesia.

No thorough investigation has been made of the effect of small intravenous doses of local anaesthetics upon respiration, although those agents are extensively used for clinical purposes. This prompted a study of the effects of rapid intravenous injections of 2 and 4 mg/kg lidocaine, 3 and 7 mg/kg mepivacaine and 0.4 and 0.8 mg/kg bupivacaine upon the electrical and mechanical activity of the respiratory muscles. Studies were also made of the effects of 3 mg/kg lidocaine when gas mixtures of different carbon dioxide and oxygen content were bein g breathed. The experimental animals were 43 cats, anaesthetized with 35 mg/kg pentobarbital intraperitoneally. Recordings were made of the number of impulses and the maximal impulse frquency from the external intercostal muscles, the diaphragm and the phrenic nerve, together with the tidal volume, minute ventilation, breathing frequency, duration of inspiration, and, during occluded breaths, the maximal pressure and the pressure impulses generated by the inspiratory muscles. It was found that all of the agents investigated had a clear effect upon respiration. The most marked changes became apparent during the first few minutes following the injections, and the effects lasted 10-20 minutes, as judged from changes observed in the recorded parameters. No differences could be seen in the effects of lidocaine and mepivacaine upon respiration, except as regards the effect upon inspiratory time. However, both of these agents differed from bupivacaine in the r effects. Lidocaine and mepivacaine depressed the activity of the external intercostal muscles, whereas the changes recorded from the diaphragm were much less marked...