DIFFERENTIAL EFFECTS OF VECURONIUM ON DIAPHRAGM AND GENIOHYOID MUSCLE IN ANAESTHETIZED DOGS

We have examined the sensitivity of the geniohyoid, an upperairway dilating muscle, to vecuronium in 12 anaesthetized dogsundergoing mechanical ventilation of the lungs and comparedit with that of the diaphragm. Dogs were allocated randomlyto two groups: pentobarbitone alone (group 1, n = 7); pentobarbitonecombined with 0.2 MAC (0.44%) of enflurane anaesthesia (group2, n = 5). Supramaximal single twitch stimulations (0.1 Hz)were applied to the phrenic nerves in the upper thorax and thegeniohyoid branches of the hy-poglossal nerves at the neck.The evoked responses were assessed by the transdiaphragmaticpressure (Pdi) and the isometric force of the geniohyoid muscles(Tgh) until complete recovery of these variables after i.v.administration of vecuronium 0.02 mg kg^–1. In both groups,the magnitude of the depression of twitch response was greaterand time required to reach control amplitude was longer in thegeniohyoid than the diaphragm. The depression of Tgh. was significantlygreater in group 2 than in group 1, whereas no change was observedin Pdi between the two groups. We conclude that the geniohyoidis more sensitive to vecuronium than the diaphragm and the differentialeffects of vecuronium are facilitated by a low concentrationof enflurane.