Isoflurane Anaesthesia Leaves Rabbit Gut Transit and Appetite Largely Unchanged

Domestic rabbits are among the most challenging patients in veterinary anaesthesia, with their uniquely sensitive digestive tracts and a documented risk of peri-anaesthetic gastrointestinal complications that far exceeds that seen in cats and dogs. One retrospective study found that 38 per cent of anaesthetic events in rabbits were attributed to peri-anaesthetic gastrointestinal complications. Yet despite the widespread use of isoflurane, the inhalation anaesthetic most commonly chosen for rabbit procedures, no study had ever directly measured what it does to the movement of food through the rabbit gut. A new controlled trial has now filled that gap, and its central finding is reassuring: short episodes of isoflurane anaesthesia produced no significant changes in gastric transit, hard faeces formation, daily feed intake or faecal output in healthy male rabbits.

The study, published in Veterinary Medicine and Science, enrolled six healthy adult male New Zealand White rabbits aged 8 to 12 months and weighing 3 to 3.5 kilograms. One rabbit was excluded after accidentally ingesting the nasogastric tube used in the procedure, leaving five animals for the final analysis. Each rabbit served as its own control in a repeated-measures crossover design, experiencing four experimental conditions separated by six-day washout intervals: an awake control session, and anaesthesia lasting 15, 30 or 60 minutes. The order of the anaesthetic conditions was randomised for each rabbit to minimise order effects. Food was not withheld before anaesthesia, and all sessions began at a strictly standardised 10:00 to control for the pronounced diurnal rhythm of rabbit gut motility, which alternates between hard faeces production and cecotrophy.

Anaesthesia was induced with 5 per cent isoflurane in 100 per cent oxygen using an induction chamber, and rabbits were removed once the righting reflex was absent. An uncuffed 2.5 mm endotracheal tube was inserted by blind orotracheal intubation, with correct placement confirmed by capnography. Maintenance used 1.5 to 3 per cent isoflurane in oxygen to achieve deep surgical anaesthesia, judged by muscle relaxation and the loss of palpebral and corneal reflexes. The experimental durations were counted from the establishment of that deep surgical plane. Heart rate, pulse rate, oxygen saturation, respiratory rate and end-tidal carbon dioxide were monitored throughout, and the rabbits received 100 per cent oxygen during recovery until coughing or chewing reflexes returned.

Gastrointestinal transit was measured with contrast radiography. The distance from the last rib to the nostril determined the nasogastric tube length, the nostril was numbed with 2 per cent topical lidocaine gel, and tube placement was verified on a lateral radiograph before a 30 per cent barium sulphate suspension at 12 mL/kg was administered. Radiographs were taken at 1, 15, 30, 60, 120, 240, 360 and 420 minutes after barium delivery, and the session ended when barium appeared in firm faecal matter within the colon. Three markers defined the measurements: gastric transit time, when barium first reached the small intestine; small intestinal transit time, when it first reached the caecum; and time to hard faeces formation, when barium was first detected in solid faeces in the colon. No systemic analgesics or tranquilisers were given, avoiding confounding effects on motility.

The results were strikingly stable in most respects. Gastric transit time was identical across all four conditions, with barium reaching the small intestine at one minute in every session, so no statistical comparison was possible. Time to hard faeces formation also did not differ significantly among conditions, with a Friedman test yielding a chi-squared of 5.286 across three degrees of freedom, p = 0.152. Feed intake and faecal output, recorded daily at 09:00 throughout each six-day interval, showed no significant main effects of anaesthesia group, time or their interaction in two-way repeated-measures ANOVA. Mean daily feed intake hovered around 99 to 100 grams across conditions, and faecal output around 49 to 52 grams, with the 60-minute group showing slightly wider confidence intervals but no statistically meaningful departure from the awake baseline.

Small intestinal transit time was the one measure where the overall comparison reached significance. The Friedman test gave a chi-squared of 10.41, p = 0.015, with Kendall’s W of 0.69. The median small intestinal transit time was 60 minutes for the awake, 15-minute and 30-minute groups, but 120 minutes for the 60-minute group. However, the crucial pairwise comparison between the 60-minute group and the awake control did not reach significance, at p = 0.083. The only significant pairwise difference was between the 30-minute and 60-minute groups, at p = 0.038 with an effect size r of 0.92, and the authors urge caution in interpreting this. The 30-minute group actually showed the shortest mean transit time, 48 minutes, which was 12 minutes faster than the awake control but not statistically significant at p = 0.157.

Several technical factors make that single significant pairwise result difficult to interpret. Radiographs were taken at fixed intervals, so a recorded transit time of 60 minutes means barium reached the caecum at some point between the 30-minute and 60-minute films rather than precisely at 60 minutes. This discrete ordinal data, standard in rabbit contrast radiography, reduces measurement precision and may have driven the apparent difference between the 30- and 60-minute groups rather than any real change in gut physiology. With only five animals, no formal a priori power analysis, and substantial inter-individual variability, the authors conclude that their findings do not provide sufficient evidence that isoflurane causes a clinically relevant delay in small intestinal transit. More frequent radiographic sampling, fluoroscopy or ultrasonography would be needed to resolve the question.

The study’s non-surgical design is a notable strength. Pain itself is a potent modulator of gastrointestinal motility, and by avoiding tissue trauma and postoperative inflammation the protocol also eliminated the need for analgesic drugs that could confound the results. Nevertheless, the authors acknowledge two related concerns: whether the six-day washout was adequate to prevent carry-over, and whether repeated handling, anaesthesia and radiography induced stress that could independently influence motility. Stress is a well-established trigger of gut stasis in rabbits, suppressing appetite and slowing the gut, but no formal behavioural assessments or objective stress markers such as faecal glucocorticoid metabolites or heart rate variability were employed. From a pharmacokinetic standpoint the washout seemed adequate, since isoflurane undergoes minimal biotransformation and is rapidly eliminated primarily through exhalation. Only one rabbit, in the 60-minute group, showed a transient reduction in feed intake on the first post-anaesthetic day, which resolved spontaneously.

The population studied also limits generalisability. Only healthy male rabbits were included, because female rabbits in a pilot study exhibited hair-pulling behaviour that could interfere with radiographic assessment and potentially affect transit through hair ingestion. Clinical patients typically face additional risk factors absent here, including surgical trauma, inflammatory responses, opioid analgesics with known gut-inhibiting effects, longer anaesthetic durations, pre-existing gastrointestinal compromise and concurrent systemic disease, any of which could potentiate isoflurane’s effects or lower the threshold for clinical stasis. Only isoflurane was tested, so comparative studies with agents such as sevoflurane would be needed to determine whether the findings are agent-specific or common to volatile anaesthetics. Monitoring was also limited to heart rate, pulse rate, oxygen saturation, respiratory rate and end-tidal carbon dioxide, without blood pressure or body temperature.

Despite these constraints, the study offers the first empirical assessment of isoflurane’s gastrointestinal effects in rabbits and a foundation for future adequately powered work. The authors recommend proactive monitoring of appetite and faecal output in clinical patients, particularly after prolonged anaesthetic episodes, while noting that their own data showed no clinically evident impairment of feed intake or faecal output following isoflurane anaesthesia for up to 60 minutes under the conditions studied. Larger cohorts with more precise transit measurement methods are needed to determine whether isoflurane produces clinically meaningful alterations in rabbit gut motility, but for now the evidence suggests that short isoflurane episodes leave the rabbit digestive system essentially undisturbed.

Subject of Research: The effects of isoflurane anaesthesia on gastrointestinal transit time, feed intake and defecation in rabbits

Article Title: Evaluation of the Effects of Isoflurane Anaesthesia on Gastrointestinal Transit Time, Feed Intake and Defecation in Rabbits

Article References: Zarein, A., Rostami, A., Masoudifard, M., Farzad‐Mohajeri, S., Bahonar, A., Liyeh, H. N., & Sohani, Z. (2026). Evaluation of the Effects of Isoflurane Anaesthesia on Gastrointestinal Transit Time, Feed Intake and Defecation in Rabbits. Veterinary Medicine and Science, 12(6), Article e71272. https://doi.org/10.1002/vms3.71272

Image Credits: AI Generated

DOI: 10.1002/vms3.71272

Keywords: rabbits, isoflurane, anaesthesia, gastrointestinal transit, veterinary medicine, gut motility, feed intake, faecal output, contrast radiography, small intestine, New Zealand White rabbits, Evaluation