Assessment of maropitant citrate effectiveness as an intraoperative analgesic through monitoring parasympathetic tone activity in female dogs undergoing ovariohysterectomy

Maropitant has been proposed as an adjunct for pain relief in dogs undergoing surgeries like ovariohysterectomy (OVH), but its effectiveness has not yet been definitively proven. This study aimed to assess the intraoperative analgesic effect of intravenously administered maropitant citrate at a constant rate infusion through monitoring parasympathetic tone activity in female dogs undergoing OVH. Thirty healthy females of various breeds, with an average age of 3.8 ± 2.7 years, an average weight of 16.75 ± 10.68 kg, were randomly assigned to two treatment groups. The group receiving maropitant (GMaro, n = 15) was given a 1 mg kg⁻¹ maropitant bolus intravenously (IV), followed by a continuous infusion of 100 mcg kg⁻¹ min⁻¹. The lidocaine group (GLido, n = 15) received a 2 mg/kg lidocaine IV bolus, with subsequent infusion at 50 mcg kg⁻¹ min⁻¹. Cardiorespiratory variables and the PTA index were evaluated at 11 anesthetic time points. Overall, cardiovascular variables such as Heart Rate (HR) and systolic arterial pressure (SAP) significantly decreased during anesthesia induction in the GMaro (p = 0.0001; p = 0.01, respectively) and in GLido (p = 0.01). Differences between groups during induction were observed in HR (p = 0.03), SAP (p = 0.04), and mean arterial pressure (MAP) (p = 0.03). MAP showed significant changes from baseline at the start of surgery and during clamping in both GMaro (p = 0.03) and GLido (p = 0.003). Regarding ventilatory variables—pulse oximetry (SpO₂), respiratory rate (RR), inspired oxygen fraction (FiO₂), end-tidal CO₂ (EtCO₂)—no group differences were found, but RR (GMaro; p = 0.001, GLido; p = 0.0001) and SpO2 (GMaro; p = 0.004, GLido; p = 0.04) differed significantly from baseline due to the controlled clinical setting. During anesthesia maintenance, end-tidal isoflurane (ETIso) increased significantly in the GLido (p = 0.009), with no difference between groups (p = 0.94). Finally, only the PTA energy variable showed a significant decrease in the GMaro (p = 0.0006), and a significant difference in this parameter was observed during right ovarian pedicle manipulation between groups (p = 0.02). In conclusion, continuous intravenous infusion of maropitant citrate at 100 mcg kg⁻¹ h⁻¹ effectively reduced the sympathetic response related to nociception, similar to lidocaine, in healthy female dogs undergoing OVH.