Magnesium sulfate does not reduce propofol requirements but contributes to lower opioid consumption in bitches undergoing ovariohysterectomy.

This study aimed to evaluate the effects of magnesium sulfate on propofol requirements and perioperative analgesia in dogs undergoing ovariohysterectomy. Fourteen female dogs (2 ± 1 years, 17 ± 5 kg) were enrolled in this randomized and blinded clinical trial. Anesthesia was induced and maintained with continuous rate infusion (CRI) of propofol, titrated to achieve an adequate depth of anesthesia. After anesthetic induction, dogs were randomly assigned to receive either magnesium sulfate (GM; bolus 50 mg/kg IV over 15 min followed by CRI at 80 mg/kg/h) or an equal volume of 0.9% saline (GS). A nociceptive response was defined as a ≥ 20% increase in systolic arterial pressure, treated with fentanyl (2 µg/kg IV). Cardiopulmonary variables, propofol infusion rates, intraoperative fentanyl requirements, and postoperative pain scores were measured. Propofol CRI did not differ significantly between groups (GM: 0.43 ± 0.07 mg/kg/min; GS: 0.38 ± 0.07 mg/kg/min; p = 0.1907). Kaplan-Meier survival analysis demonstrated a longer time to analgesic rescue and a higher proportion of dogs remaining without intraoperative fentanyl rescue in GM compared with GS (log-rank test, p = 0.0308). No differences were observed in cardiopulmonary parameters, surgery duration or time to extubation. Postoperative pain scores remained below analgesic rescue thresholds in both groups. Magnesium sulfate CRI did not reduce propofol requirements in dogs undergoing ovariohysterectomy but was associated with a reduction in intraoperative analgesic rescues. Magnesium sulfate may therefore provide some intraoperative analgesic benefits, such as reducing opioid consumption, without influencing anesthetic requirements or postoperative analgesia.