• J R Dodam, K R Branson, M E Gross, and G F Petroski.
• Department of Veterinary Medicine, University of Missouri, Columbia 65211, USA.
• Vet Surg. 1999 Nov 1;28(6):506-12.

ObjectiveThe purpose of this study was to assess carbon monoxide (CO) exposure during equine anesthesia with either halothane (H) or isoflurane (I) delivered in a circle rebreathing system.Study DesignProspective clinical investigation.AnimalsFifty client-owned horses.MethodsHorses were randomly assigned for anesthetic maintenance with H (n = 26) or I (n = 24). Two large animal anesthetic machines were used and assigned to a single agent for 2-4 weeks at a time. Machines were disassembled and soda lime changed prior to switching anesthetic agents. Inhalant anesthetic concentration and CO concentration were measured in gas samples obtained from the inspiratory limb of the anesthetic circuit. Values were recorded at 15 minute intervals for 90 minutes. Soda lime status (new or used) and mode of ventilation (spontaneous or mechanical) were also recorded. Data were analyzed using a five-factor ANCOVA with repeated measures.ResultsInspired CO concentration for H and I increased from 1 +/- 3 and 6 +/- 11 ppm at baseline to 54 +/- 33 and 21 +/- 18 ppm at 90 min, respectively (mean +/- sd). H was associated with significantly greater CO concentrations than I at 30 to 90 min, although baseline CO was significantly greater 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 for H and I, respectively. There were no significant differences between groups for O2 flow at any time point. Neither mechanical ventilation nor new versus used soda lime affected CO concentration.ConclusionsSignificantly higher concentrations of CO were recorded during the administration of H than I.Clinical RelevanceLevels of CO observed during the administration of either H or I for 90 minutes to horses were not clinically significant.

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