Studies on Inhalation Anesthesia in Horses

II .Inhaled Concentration of Halothane for Induction and Maintenance, and the Amount of Halothane Used

Abstract

Until recently intravenous anesthesia had been an almost exclusive type of equine general anesthesia. Equine inhalation anesthesia, however, has been used again in practice since ten years ago with the advancement of the premedication method and the development of powerful inhalation anesthetics, such as halothane. The practical use of halothane as anes thetic has been reported by many investigators, but no data are available on the inhaled concentration of halothane-oxygen which is used for induction and maintenance. Experiments were carried out with 30 Thoroughbred horses to determine the concent ration and amount of halothane and the anesthetic period for the practical use of this drug for induction and maintenance. An anesthetic apparatus under the closed circuit cycle system was employed. The inhaled concentration was measured by a “Fluothane-meter” throughout the anesthetic period. Every horses was administered with diazepam and atropine sulfate as a premedicant. Succinylcholine chloride was used to cast the horse. An endo-tracheal tube with a cuff was inserted through the oral cavity as soon as the horse lay down. The following results were obtained. 1. Anesthesia was induced with halothane-oxygen from 6 to 12 minutes, or its mean was 8 minutes. 2. The peak concentration of halothane was between 1.80 and 3.15 percent, or 2.45 percent on the average, during the induction period. The amount of halothane used during this period was from 18 to 47 ml, or 31.6 ml on the average. For the maintenance of anesthesia it was required to use a period of 47-140 minutes, 15-58 ml of halothane, and a halothane concentration of 0.8-2.0 percent, or 76.2 minutes, 31.2 ml, and 1.23 percent, respectively, on the average. 4.The recovery period varied from 10 to 85 minutes ; its mean was 31 minutes. The length of anesthesia had influence upon that of the recovery period. 5.For the maintenance of anesthesia it was required to administer 2.81-8.33 ml of halothane per 100 kg of body weight. The amount of halothane to be used per 100 kg of body weight decreased according to an increase in body weight and maintenance period.