Comparative study of three intramuscular anaesthetic combinations (medetomidine/ketamine, medetomidine/fentanyl/midazolam and xylazine/ketamine) in rabbits

Objective To compare the quality of surgical anaesthesia. and cardiorespiratory effects of three intra-muscular (IM) anaesthetic combinations in rabbits. Study design Prospective randomized cross-over experimental study. Animals Nineteen adult female chinchilla mixed-bred rabbits weighing 3.9 0.8 kg. Methods Rabbits were given one of three IM anaesthetic combinations: 0.25 mg kg(-1) medetomidine and 35.0 mg kg(-1) ketamine (M-K), 0.20 mg kg(-1) medetomidine and 0.02 mg kg(-1) fentanyl and 1.0 mg kg(-1) midazolam (M-F-Mz) and 4.0 mg kg(-1) xylazine and 50 mg kg(-1) ketamine (X-K). The effects of anaesthesia on nociceptive reflexes, circulatory and respiratory function were recorded. Statistical analyses involved repeated measures ANOVA with paired Student's t-test applied post hoc. P-values < 0.05 were considered as significant. Results Reflex loss was most rapid and complete in M-K recipients, whereas animals receiving M-F-Mz showed the longest tolerance of endotracheal intubation (78.1 +/- 36.5 minutes). Loss of righting reflex was significantly most rapid (p < 0.05) in the X-K group (114.7 +/- 24.0 minutes). Surgical anaesthesia was achieved in 16 of 19 animals receiving M-K, in 14 animals receiving M-F-Mz, and in seven animals with X-K, but only for a short period (7.1 +/- 11.6 minutes). This was significantly (p < 0.001) shorter than with M-K (38.7 +/- 30.0 minutes) and M-F-Mz (31.6 +/- 26.6 minutes). Heart rates were greatest in X-K recipients: lowest HR were seen in animals receiving M-F-Mz. Mean arterial blood pressure was significantly higher (about 88 mmHg) during the first hour in the M-K group. During recovery. the greatest hypotension was encountered in the X-K group; minimum values were 5 3 12 mmHg. Six of 19 animals in the Mz group showed a short period of apnoea (30 seconds) immediately after endotracheal intubation. Respiratory frequency was significantly lower in this group (p < 0.001). Highest values for arterial carbon dioxide partial pressures (PaCO2) (6.90 +/- 0.87 kPa: 52.5 +/- 6.5 mmHg) occurred after induction of anaesthesia in group M-F-Mz animals. There was a marked decrease in PaO2 in all three groups (the minimum value 5.28 +/- 0.65 kPa [39.7 +/- 4.9 mmHg] was observed with M-K immediately after injection). Arterial PO2 was between 26.0 and 43.0 kPa (196 and 324 mmHg) in all groups during O-2 delivery and decreased - but not < 7.9 8 kPa - on its withdrawal. Immediately after drug injection, pH. values fell in all groups, with lowest values after 30 minutes (7.23 +/- 0.03 with M-K, 7.28 +/- 0.05 with M-F-Mz. and 7.36 +/- 0.04 with X-K). The X-K animals showed significantly (p < 0.001) higher pH values than medetomidine recipients. During I hour of anaesthesia pH values in the medetomidine groups remained below those of the X-K group. Conclusions Surgical anaesthesia was induced in most animals receiving medetomidine-based combinations. Arterial blood pressure was maintained at baseline values for about I hour after M-K. Transient apnoca occurred with M-F-Mz and mandates respiratory function monitoring. Oxygen enrichment of inspired gases is necessary with all three combinations. Endotracheal intubation is essential in rabbits receiving M-F-Mz. Clinical relevance The quality of surgical anaesthesia was greatest with M-K. All combinations allowed recoveries of similar duration. It is theoretically possible to antagonize each component of the M-F-Mz combination.