Efeito da Administração Perioperatória do Sulfato de Magnésio em Cadelas Anestesiadas com Propofol e Fentanil Para Ovariohisterectomia

Abstract

Balanced anesthesia involves administering different agents to create an optimal anesthetic state. This approach avoids exclusive dependence on opioids to control intraoperative nociception and postoperative pain. Magnesium sulfate (MgSO4) prevents central sensitization caused by peripheral nociceptive stimulation by acting as an NMDA receptor antagonist, its analgesic and anesthetic-reducing effects have been demonstrated in animal and human models. Thus, the aim of the present study was to evaluate the effect of magnesium sulphate administration on the anesthetic requirement of propofol and fentanyl, on the response to nociceptive stimulus during the intraoperative period and on the immediate postoperative analgesia of bitches undergoing ovariohysterectomy. Thirty-two bitches sedated with acepromazine 0.02 mg/kg intramuscularly, were randomly divided into four groups: PMFA - MgSO4 and high dose fentanyl; PFA - 0.9% NaCl and high dose fentanyl; PMFB - MgSO4 and low dose fentanyl; PFB - 0.9% NaCl and low dose fentanyl. Anesthesia was induced with propofol (1 mg/kg/min) to effect and maintained with continuous rate infusion of selected treatments. Propofol rate was titrated to maintain cardiorespiratory variables within normal range and an adequate anesthetic plan. If a 20% increase in HR, RR or SAP were noted, 2.5mcg/kg fentanyl was administered. The total dose of propofol necessary for anesthetic induction and maintenance was evaluated. Intraoperative antinociceptive effect was assessed through cardiorespiratory variables and the requirement of additional doses of fentanyl. Other variables such as quality of induction, intubation and intraoperative degree of muscle relaxation were evaluated. Time to extubation, head support and ability to reach sternal position were recorded. Following the recovery to sternal recumbency pain evaluation was assessed using the short form of the Glasgow Composite Pain Scale, and if analgesia was required methadone was administered IV (0.1 mg/kg). Serum magnesium concentrations were measured at three different time points (pre-, intra- and postoperatively).The present study didn’t demonstrate any significant difference among the groups regarding variables evaluated. Anyway, the number of bitches that needed additional doses of fentanyl in the intraoperative period were lower in group PMFA (4/8) PFA (4/8), and PMFB (5/8) than group PFB (7/8). There was no significant difference in pain assessment using the Glasgow pain scale in the postoperative period, however, more bitches in the PFB group required analgesic rescue (2/8). The mean serum magnesium concentration was higher in the postoperative than its preoperative values (p <0.001) in the PMFA and PMFB groups. This study did not find clear clinical benefits in administering magnesium sulfate in bitches sedated with acepromazine and anesthetized with fentanyl and propofol undergoing ovariohysterectomy.