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Effects of a constant rate infusion of magnesium sulphate in healthy dogs anaesthetized with isoflurane and undergoing ovariohysterectomy


Eva Rioja, Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort, 0110, Pretoria, South Africa. E-mail:


Objective  To determine the effects of intravenous (IV) magnesium sulphate (MgSO4) as a bolus followed by a constant rate infusion (CRI) on anaesthetic requirements, neuroendocrine stress response to surgery, haemostasis and postoperative analgesia in healthy dogs undergoing ovariohysterectomy.

Study design  Blinded randomized clinical trial.

Animals  Sixteen female dogs.

Methods  After intramuscular premedication with acepromazine (0.05 mg kg−1) and morphine (0.3 mg kg−1), anaesthesia was induced with diazepam (0.2 mg kg−1) and propofol (2 mg kg−1) intravenously and maintained with isoflurane in oxygen in all dogs. Dogs were randomly assigned to two groups, M and C. Group M received MgSO4 (50 mg kg−1 over 15 minutes, followed by a 15 mg kg−1 hour−1 CRI). Group C received an equivalent bolus and CRI of lactated Ringer’s solution. In addition, all dogs received lactated Ringer’s solution (10 mL kg−1 over 15 minutes followed by 10 mL kg−1 hour−1). End-tidal isoflurane and carbon dioxide tensions, cardio-respiratory variables, arterial blood gases, electrolytes, ACTH and cortisol concentrations were measured at different time points. Thromboelastography (TEG) was performed pre- and post-anaesthesia. Postoperative pain was evaluated using the short form of the Glasgow Composite Pain Scale. Data were analysed with repeated measures anova and Mann–Whitney U tests (< 0.05).

Results  No statistically significant differences between groups were found in any of the measured variables. However, the alpha angle and maximal amplitude recorded by TEG in group M were significantly increased post-anaesthesia, but remained within the reference interval. One dog in Group M and two in Group C received rescue analgesia during recovery.

Conclusions and clinical relevance  As used in this study, MgSO4 failed to decrease isoflurane requirements, postoperative pain and stress hormone concentrations; however, it did not produce any cardio-respiratory or major haemostatic side effects. Administration of intravenous MgSO4 together with an opioid during ovariohysterectomy in dogs does not seem to provide any clinical advantage.