The effect of volatile anaesthetics on the relative sensitivity of facial and distal thoracic limb muscles to vecuronium in dogs

To compare n. facialis-m. nasolabialis (nF-mNL) and n. ulnar-mm. carpi flexorii (nU-mCF) sensitivity to vecuronium during halothane or isoflurane anaesthesia. Randomized, prospective, experimental study. Forty-four client-owned dogs (19 male, 25 female) undergoing surgery; mean age: 5.0 years; mean body mass: 24.7 kg. Thirty minutes after acepromazine (0.05 mg kg(-1)) and morphine (0.5 mg kg(-1)), anaesthesia was induced with intravenous (IV) thiopental and maintained with either halothane (n = 22) or isoflurane (randomly allocated) in oxygen. The lungs were mechanically ventilated and end-tidal inhaled anaesthetic (Fe'(IAA)) maintained at 1.2 x MAC values. Neuromuscular transmission at nF-uNL and nU-mCF was monitored using the train of four count. Vecuronium (50 mu g kg(-1) IV) was injected (t = 0) after 15 trains, 50-60 minutes after inhalational anaesthesia began, when Fe'(IAA) had been constant for > 15 minutes. Times of the disappearance (-) and reappearance (+) of the fourth (T4) and first twitch (T1) were recorded allowing the calculation of: latent (t = 0 to T4-) and manifest onset times (t = 0 to T1-) duration of blockade (T1- to T1+) and drug effect (T4- to T4+) and recovery time (T1+-T4+). Student's paired t-test was used to compare simultaneous responses at nF-uNL and nU-mCF. An unpaired t-test was used to compare anaesthetic effects. Latent and manifest onset times were significantly (p <0.05) briefer, blockade and drug effects were significantly longer and recovery from blockade were significantly slower in the nF-mNL unit in both halothane and isoflurane recipients. Profound block duration and drug action were significantly longer and recovery from blockade were significantly slower in halothane recipients at both nerve-muscle units. The nF-mNL was more sensitive than nU-mCF to vecuronium, particularly in halothane-anaesthetized dogs.