Behavioural and cardiorespiratory effects of a constant rate infusion of medetomidine and morphine for sedation during standing laparoscopy in horses.

REASONS FOR PERFORMING STUDY: Standing surgical procedures are performed commonly in horses under sedation. This approach minimises the morbidity/mortality risks associated with general anaesthesia. The use of a medetomidine and morphine combination has not been investigated in horses despite the usefulness of each drug individually.

OBJECTIVE: To determine the efficacy of a medetomidine and morphine combination to produce standing sedation with minimal cardiorespiratory changes and adequate analgesia for exploratory laparascopy in mature horses.

HYPOTHESIS: The combination of medetomidine and morphine will induce reliable sedation with minimal cardiorespiratory changes.

METHODS: Medetomidine (5 microg/kg bwt i.v.) followed in 10 min by morphine (50 microg/kg bwt i.v.) and 10 min later by a constant rate infusion (CRI) of medetomidine and morphine (5 and 30 microg/kg bwt/h, respectively) was administered in 7 horses undergoing standing exploratory laparoscopy. Quality of sedation and cardiorespiratory function were assessed.

RESULTS: Sedation was satisfactory after the medetomidine and morphine bolus. The CRI of both drugs enhanced sedation and ataxia. Mean visual analogue scores on a scale of 10 varied between 7.8 and 8.8 and were similar between anaesthesiologists and surgeons. Heart rate, respiratory rate and packed cell volume (PCV) decreased significantly after medetomidine and, at some periods, during the CRI. Blood pressure only increased significantly at 5 min and arterial O2 decreased significantly at 10 min post medetomidine. PCV remained significantly lower and total protein decreased post morphine and CRI administration. Arterial CO2 increased towards the end of the CRI. Cardiac output did not change significantly over time.

CONCLUSION AND CLINICAL RELEVANCE: The combination of medetomidine and morphine results in reliable sedation and stable cardiorespiratory function in horses undergoing exploratory laparascopy.