Isoflurane measurement error using short wavelength infrared techniques in horses: influence of fresh gas flow and pre-anaesthetic food deprivation

Abstract

ObjectiveTo quantify the isoflurane measurement error arising from the use of short wavelength infrared (IR) anaesthetic gas analysis during low flow anaesthesia in horses. Study designProspective clinical study. Animal populationSixty-four client-owned horses referred for elective or emergency surgery (age 1–16 years, body mass 400–650 kg). Materials and methodsHorses were divided into four groups based on duration of pre-anaesthetic food deprivation period (FDP) and fresh gas flow during anaesthesia: a high flow group with normal FDP (n = 16) and three groups with low flow and normal (n = 29), long (n = 5) or no (n = 14) FDP, respectively. Circuit isoflurane concentrations were measured simultaneously using a short wavelength (methane-sensitive) analyser (Datex Capnomac Ultima) and a long wavelength (methane-insensitive) analyser (Hewlett Packard M 1025 B) for at least 60 minutes. The difference between the readings of both analysers gave the isoflurane measurement error of short wavelength IR analysis, from which the circuit methane concentration was calculated. ResultsIn the low flow groups, isoflurane measurement error increased over time, whereas in the high flow group, error remained constant after an initial rise in the first 15 minutes. The isoflurane measurement error was significantly lower (p < 0.005) in the high flow group compared with the low flow-normal FDP group from 15 to 60 minutes. Compared to the low flow – normal FDP group, isoflurane measurement error was significantly smaller (p < 0.001, from 15 to 60 minutes) in the low flow-long FDP group and significantly larger (p = 0.016, at 60 minutes) in the low flow-no FDP group. Within the low flow-no FDP group, values in colic cases did not differ from those in noncolic cases (p > 0.7). ConclusionsIsoflurane measurement using short wavelength IR absorption is inaccurate. The fresh gas flow and duration of pre-anaesthetic food deprivation influence the isoflurane measurement error during anaesthesia in horses. Clinical relevanceShort wavelength IR analysers are not reliable for isoflurane measurement during (low flow) anaesthesia in horses.