Inhaled carbon monoxide concentration during halothane or isoflurane anesthesia in horses

Objective-The purpose of this study was to assess carbon monoxide (CO) expo sure during equine anesthesia with either halothane (Ii) or isoflurane (1) delivered in a circle rebreathing system. Study Design-Prospective clinical investigation. Animals-Fifty client-owned horses. Methods-Horses were randomly assigned for anesthetic maintenance with H (n = 26) or I (n = 24). Two large animal anesthetic machines were used and ass igned to a single agent for 2-4 weeks at a time. Machines were disassembled and soda Lime changed prior to switching anesthetic agents. Inhalant anest hetic concentration and CO concentration were measured in gas samples obtai ned from the inspiratory limb of the anesthetic circuit. Values were record ed at 15 minute intervals for 90 minutes. Soda lime status (new or used) an d mode of ventilation (spontaneous or mechanical) were also recorded, Data were analyzed using a five-factor ANCOVA with repeated measures. Results-Inspired CO concentration for H and I increased from I +/- 3 and 6 +/- 11 ppm at baseline to 54 +/- 33 and 21 +/- 18 ppm at 90 min, respective ly (mean +/- sd). II was associated with significantly greater CO concentra tions than I at 30 to 90 min, although baseline CO was significantly greate r in the I group than the H group. Oxygen flow rates were 9.9 +/- 0.5 L/min at baseline for H and I, and 5.0 +/- 0.4 and 5.0 +/- 0.7 L/min at 90 min f or H and I, respectively. There were no significant differences between gro ups for O-2 flow at any time point. Neither mechanical ventilation nor new versus used soda lime affected CO concentration. Conclusions-Significantly higher concentrations of CO were recorded during the administration of H than I. Clinical Relevance-Levels of CO observed: during the administration of eith er H or I for 90 minutes to horses were not clinically significant. (C) Cop yright 1999 by The American College of Veterinary Surgeons.