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 t hey would rapidly recover. Design - Dose-response and plasma concentra tion-effect study. Animals - 11 clinically normal dogs. Procedure - Bo lus methohexital pharmacokinetic variables were determined in ketamine - and pentobarbital-anesthetized dogs. Plasma methohexital concentrati ons required to inhibit purposeful movement in response to painful sti muli were determined during a stepped methohexital infusion in the sam e dogs on a second occasion. These pharmacokinetic/pharmacodynamic dat a were next used to design a bolus and two-stage infusion regimen that would result in stable plasma methohexital concentrations with prolon ged infusion. This regimen was tested in a second group of dogs. Resul ts - Mean steady-state volume of distribution of methohexital in the a nesthetized dogs was 1.50 L/kg of body weight and mean elimination cle arance was 10.2 ml/kg/min. Mean plasma concentrations required to prev ent movement response to a noxious stimulus and at which the dogs coul d be extubated were 11.8 and 6.9 mu g/ml, respectively. After a 6-hour infusion, recovery of airway reflexes sufficient to allow extubation required 67 minutes. Conclusions - An easily implemented IV methohexit al infusion regimen for induction and maintenance anesthesia in dogs w as 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.