DIAPHRAGMATIC ACTIVITY DURING ISOFLURANE ANESTHESIA IN DOGS

The effect of isoflurane administration on diaphragmatic activity was investigated in six anaesthetized mechanically ventilated dogs. Diaphr agmatic strength was assessed by measuring the transdiaphragmatic pres sure (Pdi) generated during supramaximal stimulation of both cervical phrenic nerves at frequencies of 0.5, 10, 20, 50 and 100 Hz under part ially isometric conditions at 1, 1.5 and 2 minimum alveolar anaestheti c concentrations (MAC), after maintaining 1 h of stable conditions. Pd i measurements were made at the start of the stimulation (initial) and at the end of a 2-s period (2-s). The force-frequency relationship wa s compared at each anaesthetic level. For single twitch (0.5 Hz) stimu lation, the time constant of diaphragmatic relaxation was also assesse d. The sequence of changing anaesthetic depth was altered in random fa shion between animals. Pdi amplitude at single twitch stimulation was unchanged at the three anaesthetic concentrations. There was no signif icant difference in initial Pdi at various stimulus frequencies with i ncreasing depth of isoflurane anaesthesia. In addition, no change in 2 -s Pdi during low frequency stimulation (10 and 20 Hz) was noted durin g any of the three levels of anaesthesia. By contrast, 2-s Pdi with 50 Hz stimulation during 2 MAC isoflurane exposure decreased significant ly below Pdi levels seen at 1 and 1.5 MAC (P<0.01). Furthermore, 2-s P di at 100 Hz stimulation decreased significantly in a dose-dependent f ashion. From these results, we conclude that isoflurane reduces diaphr agmatic activity at higher stimulation frequencies of 50 and 100 Hz.