A comparison of medetomidine and its active enantiomer dexmedetomidine when administered with ketamine in mice
… NY, USA). The hair of the right thigh was clipped and a pulse oximeter (MouseOx®,
Starr Life Sciences Corp., Oakmont, PA, USA) probe was placed to monitor pulse rate
(PR) and arterial haemoglobin saturation (SpO2). In addition … Background: Medetomidine-ketamine (MK) and dexmedetomidine-ketamine (DK) are widely used to provide general anaesthesia in laboratory animals, but have not been compared directly in many of these species, includingrodents. This study aimed to compare the onset and depth of anaesthesia, and changes in vital signs, after intraperitoneal (IP) or subcutaneous (SC) administration of ketamine (75 mg kg-1) combined with medetomidine(1 mg kg-1) or dexmedetomidine (0.5 mg kg-1) using a randomised semi-crossover design with ≥ 48 hours between treatments in 10 male and 10 female mice. Each mouse was anaesthetised twice using the same administration route (IP or SC): once with each drug-ketamine combination. Anaesthetised mice were monitored on a heating pad without supplemental oxygen for 89 minutes; atipamezole was administered for reversal. The times that the righting reflex was lost post-injection and returned post-reversal were analysed using general linear models. Tailpinch and pedal reflexes were examined using binomial generalized linear models. Pulse rate (PR), respiratory rate(fr), and arterial haemoglobin saturation (SpO2) were compared using generalized additive mixed models. Results: There were no significant differences among treatments for the times taken for loss and return of therighting reflex, or response of the tail-pinch reflex. The pedal withdrawal reflex was abolished more frequently with MK than DK over time (P = 0.021). The response of PR and SpO2were similar among treatments, but fr was significantly higher with MK than DK (P ≤ 0.0005). Markedly low SpO2 concentrations occurred within 5 minutes post-injection (83.8 ± 6.7%) in all treatment groups and were most severe after 89 minutes lapsed (66.7 ± 7.5%). No statistical differences were detected in regards to administration route (P ≤ 0.94). Conclusions: This study failed to demonstrate clinical advantages of the enantiomer dexmedetomidine over medetomidine when combined with ketamine to produce general anaesthesia in mice. At the doses administered, deep surgical anaesthesia was not consistently produced with either combination; therefore, anaesthetic depth must be assessed before performing surgical procedures. Supplemental oxygen should always be provided during anaesthesia to prevent hypoxaemia.