Evaluation of different intramuscular injectable anesthetic combinations in rabbits: Impact on anesthetic depth, physiological parameters, and EEG recordings

Abstract

Rabbits have a high anesthesia-related mortality rate because of their small size, high metabolic rate and challenging airway management. This study aimed to investigate different combinations of intramuscularly administered anesthetics in New Zealand White rabbits, focusing on their effects on anesthetic depth, physiological parameters, and electroencephalogram (EEG) recordings. Defined doses ketamine (K), esketamine (SK), medetomidine (M), dexmedetomidine (D), midazolam (Mi), and butorphanol (B) were investigated and compared in five different combinations: KM (25/0.25 mg/kg), SKM (17/0.25 mg/kg), SKD (17/0.15 mg/kg), MMiB (0.25/1/0.2 mg/kg), and DMiB (0.15/1/0.2 mg/kg). For 60 minutes, the anesthetic depth was assessed using an anesthesia score, and physiological parameters, including heart rate (HR), respiratory rate (RR), oxygen saturation and blood pressure were recorded. The study also assessed the latencies to loss and recovery of reflexes after targeted antagonization, and EEGs were measured. The rabbits were not intubated and were supplied with oxygenated air via nasal probes. All anesthetic combinations achieved anesthesia with surgical tolerance, with significant intergroup differences in HR, RR, blood pressure and EEG power spectra. The KM group demonstrated the most stable anesthesia and rapid recovery, while SKD and SKM groups showed prolonged recovery times. Oxygen saturation remained consistently high across all groups, obviating the need for intubation. All rabbits recovered fully after anesthesia. In conclusion, intramuscular administered anesthetic combinations can provide effective anesthesia with surgical tolerance for short procedures in rabbits. Monitoring circulatory parameters during and after anesthesia and adequate pain management pre-, inter-, and postoperatively are essential. Considering these criteria, the KM group presented the best results compared with the other groups.

Fig 1. Loss of the righting reflex and anesthesia scores.

A. Time from anesthesia induction to LOR, presented as medians ±  95% confidence intervals. Statistical analysis was performed using the Kruskal-Wallis test and Dunn’s multiple comparisons test for group-wise differences (p <  0.05). B. Anesthesia scores from 10 to 55 minutes post induction, analyzed using robust linear mixed models. Group comparisons were conducted after model fitting, with estimated marginal means and Tukey’s p value correction for multiple comparisons. Significant differences (p <  0.05) are indicated by superscripts within a time point. n =  7 per group.

Fig 2. Physiological parameters over time.

A. Mean HR (bpm), B. MAP (mmHg) and C. Mean RR (breaths per minute) with estimated means ±  95% confidence intervals. Robust linear mixed models were used for analysis, with Tukey’s p-value correction for multiple comparisons. Superscripts indicate significant differences (p <  0.05) between groups. The p-values for MAP and RR are provided in the supplements (S2, S3 Tables). n =  7 per group.

Fig 3. Group comparisons of EGG recordings.

Group comparisons of EGG recordings were analyzed 30 minutes after anesthesia induction over a 2- minute interval. Statistically significant differences were assumed if the AUC 95% confidence interval was ≤  0.5.

Fig 4. Spectrograms and AUC comparison.

Spectrograms and AUCs analyzed 30 minutes post induction for 2 minutes, significant differences were assumed if the AUC 95% confidence interval was ≤  0.5.

Fig 5. Recovery phase.

A. Anesthesia scores were evaluated every five minutes after antagonization (↑) until all reflexes were regained. Robust linear mixed models analyzed group differences, with Tukey’s p-value correction applied. Superscripts indicate significant differences (p <  0.05) between groups. B. Time (in minutes) until ASW recovery, analyzed using a Kruskal-Wallis test with Dunn’s multiple comparisons test for median contrasts. Values are medians ±  95% confidence intervals (n =  7 per group).

Fig 6. Regain of Reflexes.

Time until full reflex recovery varied significantly among groups.