Possible isoflurane effect observed in isotope ratio mass spectrometry from a large animal model

Abstract

Currently breath test samples are not routinely collected whilst patients or animals are under general anesthesia as there are no easy methods of sample collection, however it is increasingly being performed. In a previous animal liver function study, it was observed that the 13CO2 quality control (QC)was unstable and trended away from its known value. Nine pathogen free female pigs were used with a mean weight of 45.2 kg. Six pigs were anesthetized by an inhalational dose of isoflurane in oxygen. To determine if isoflurane was the sole issue, the remaining group of three pigs was anesthetized via continuous intravenous administration of alfaxalone, diazepam and ketamine. All pigs were administered with 13C-methacetin (2 mg/kg) and breath samples were collected every two minutes for a total 40 minutes after each dose. Breath samples were analyzed via isotope ratio mass spectrometry. A quality control sample of 5% 13CO2 was tracked with the pig breath samples. The quality control 13CO2 gradually drifted from its known mean (S.E.) of -29.1 (0.22) ‰ in the first group of six pigs. After more pig breath samples that contained isoflurane were analyzed, the QC drifted further and more quickly from its known value. It was hypothesized that isoflurane was adsorbed by the gas chromatography column consequently causing the retention time of 13CO2 to decrease. The three pigs in the follow-up study, without isoflurane, showed no significant difference within the QC samples, suggesting that isoflurane has an effect on the chromatography process during analysis. It is recommended that in any future stable isotope breath testing studies that require sedation, only intravenous general anesthetics should be used to minimize the risk of perturbations in sample chromatography.