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

The research investigates the error in measuring isoflurane, a commonly used anaesthetic in horses, using short wavelength infrared (IR) gas analysis. It shows that the error rate is related to the speed of the fresh gas flow during anaesthesia and whether the horse was deprived of food before the anaesthesia.

Objective and Methodology

• The aim of the study was to understand the accuracy of isoflurane measurement error resulting from the use of short wavelength infrared (IR) anaesthetic gas analysis during low flow anaesthesia in horses.
• The researchers carried out a clinical study on 64 client-owned horses, which were categorised into four groups based on the length of time the horses were deprived of food before anaesthesia, and the speed of the fresh gas flow during anaesthesia.
• Isoflurane concentrations were simultaneously measured using a short wavelength (methane-sensitive) analyser and a long wavelength (methane-insensitive) analyser during a period of at least 60 minutes. The difference between the readings gave the isoflurane measurement error of short wavelength IR analysis, which was then used to calculate the circuit methane concentration.

Findings

• The study found that isoflurane measurement errors increased over time in the low flow groups, compared to the high flow group where the error remained constant after an initial rise in the first 15 minutes.
• Compared to the low flow-normal food deprivation period (FDP) group, the measurement error was significantly higher in the low flow – long FDP group and significantly larger in the low flow – no FDP group.

Conclusions

• The study concluded that the use of short wavelength IR absorption for measuring isoflurane is inaccurate. The rate of fresh gas flow and the duration of pre-anaesthetic food deprivation have an influence on the measurement error during anaesthesia in horses.
• In effect, short wavelength IR analysers may not be reliable for isoflurane measurement during low flow anaesthesia in horses. This has potential implications for the management of anaesthesia and surgery in horses.