Comparison of thiopentone/guaifenesin, ketamine/guaifenesin and ketamine/midazolam for the induction of horses to be anaesthetised with isoflurane

Forty-eight horses subjected to elective surgery were randomly assigned to three groups of 16 horses. After premedication with 0.1 mg/kg acepromazine intramuscularly and 0.6 mg/kg xylazine intravenously, anaesthesia was induc ed either with 2 g thiopentone in 500 ml of a 10 per cent guaifenesin solut ion, given intravenously at a dose of I ml/kg (group TG), or with 100 mg/kg guaifenesin and 2.2 mg/kg ketamine given intravenously (group KG), or with 0.06 mg/kg midazolam, and 2.2 mg/kg ketamine given intravenously (group Km ). Anaesthesia was maintained with isoflurane. The mean (sd) end tidal isof lurane concentration (per cent) needed to maintain a light surgical anaesth esia (stage III, plane 2) was significantly lower in group KM (0.91 [0.03]) than in groups TG (1.11 [0.03]) and KG (1.14 [0-03]). The mean (sd) arteri al pressure (mmHg) was significantly lower in group KG (67.4 [2.07]) than i n groups TG (75.6 [2.23]) and KM (81.0 [2.16]). There were no significant d ifferences in the logarithm of the heart rate, recovery time or quality of recovery between the three induction groups. However, pronounced ataxia was observed in the horses of group Km, especially after periods of anaesthesi a lasting less than 75 minutes.