Insufficient anesthetic potency of nitrous oxide in the rat

Abstract

Nitrous oxide (N2O) is frequently used for maintenance of anesthesia in research animals because of its minimal effect upon circulatory variables and the ability to rapidly alter its anesthetic concentration. However, N2O's low anesthetic potency may result in inadequate levels of anesthesia under experimental conditions; thus, the ability of N2O to provide adequate anesthesia during tracheal stimulation in rats, following withdrawal of halothane was evaluated. Twenty rats of similar ages and weights were anesthetized with halothane (1.44% inspired) and their tracheas orally intubated. Ten minutes prior to the conclusion of a 90-min halothane stabilization period, either 70% N2O (n = 10) or 70% nitrogen (n = 10) was added to the inspired anesthetic gas mixture. The halothane was discontinued at the conclusion of the 90-min period and each rat was observed for spontaneous motor behavior while end-expiratory anesthetic gases were intermittently measured with mass spectrometry. The halothane concentrations present at the times of four specific, consistently occurring behaviors were measured, and comparisons were made between the N2O and nitrogen groups by a mean t test. There were no significant differences (P < 0.05) in the halothane concentration between the N2O and nitrogen groups in three of the four behaviors compared: 1) change in respiratory pattern, 2) purposeful movement of the torso, and 3) purposeful self-extubation. In the fourth behavior (purposeful movement of an extremity) the halothane concentration was higher in the N2O group than in the nitrogen group. A mean halothane concentration of 0.49% was required to prevent the purposeful bahaviors in the presence of N2O. Thus, under conditions of this study, when utilized with sub-MAC concentrations of halothane there is no difference between the anesthetic potency of N2O and nitrogen. When N2O (70%) is utilized with halothane concentrations of less than 0.50% during tracheal stimulation in the rat, insufficient anesthesia is present.