Induction and maintenance of anesthesia in dogs by intravenous administration of methohexital

Objective - To devise and test an IV methohexital infusion regimen for induction and maintenance of surgical anesthesia in dogs from which they would rapidly recover. Design - Dose-response and plasma concentration-effect study. Animals - 11 clinically normal dogs. Procedure - Bolus methohexital pharmacokinetic variables were determined in ketamine- and pentobarbital-anesthetized dogs. Plasma methohexital concentrations required to inhibit purposeful movement in response to painful stimuli were determined during a stepped methohexital infusion in the same dogs on a second occasion. These pharmacokinetic/pharmacodynamic data were next used to design a bolus and two-stage infusion regimen that would result in stable plasma methohexital concentrations with prolonged infusion. This regimen was tested in a second group of dogs. Results - Mean steady-state volume of distribution of methohexital in the anesthetized dogs was 1.50 L/kg of body weight and mean elimination clearance was 10.2 ml/kg/min. Mean plasma concentrations required to prevent movement response to a noxious stimulus and at which the dogs could be extubated were 11.8 and 6.9 μg/ml, respectively. After a 6-hour infusion, recovery of airway reflexes sufficient to allow extubation required 67 minutes. Conclusions - An easily implemented IV methohexital infusion regimen for induction and maintenance anesthesia in dogs was developed. During a 6-hour infusion, hemodynamic variables did not change. Use of this regimen resulted in anesthesia of sufficient depth to prevent withdrawal in response to noxious stimuli and in reliable and acceptable emergence times for use in canine survival studies in a cost-effective manner.