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

Abstract

ObjectiveTo compare the quality of surgical anaesthesia and cardiorespiratory effects of three intramuscular (IM) anaesthetic combinations in rabbits. Study designProspective randomized cross-over experimental study. AnimalsNineteen adult female chinchilla mixed-bred rabbits weighing 3.9 ± 0.8 kg. MethodsRabbits 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. ResultsReflex 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 53 ± 12 mmHg. Six of 19 animals in the M–F–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 O2 delivery and decreased – but not <7.98 kPa – on its withdrawal. Immediately after drug injection, pHa 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 1 hour of anaesthesia pH values in the medetomidine groups remained below those of the X–K group. ConclusionsSurgical anaesthesia was induced in most animals receiving medetomidine-based combinations. Arterial blood pressure was maintained at baseline values for about 1 hour after M–K. Transient apnoea 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 relevanceThe 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.