Evaluation of the isoflurane-sparing effects of a constant rate infusion of remifentanil undergoing mastectomy in dogs

The aims of this study were to evaluate the effects of two constant rate infusions of remifentanil on end tidal isoflurane (ETiso) in dogs undergoing mastectomy surgery. Eighteen bitches, 12+2 years of age, weighing 15+5 Kg were randomized into 3 groups (n=6/group) and underwent unilateral mastectomy due to mammary neoplasia. All animals received the premedications of morphine (0.5 mg Kg-1) and acepromazine (0.03 mg Kg-1) by intramuscular route (IM). Propofol dose-effect was used for induction of anesthesia. The animals were intubated and connected to a circle breathing system, and IPPV (intermittent positive pressure ventilation) was used to maintain normocapnia with an oxygen flow rate of 2 L/min and FiO2 100%. Anesthesia was maintained with isoflurane and saline solution (control group [GCON], n=6) or intravenous infusion of remifentanil at a rate of 0.15 μg Kg-1min-1 (REMI 0.15 n=6) or 0.3 μg Kg-1min-1 (REMI 0.3 n=6). Cardiopulmonary variables and ETiso were monitored before and every 15 minutes after the start of surgery. The data were analyzed by ANOVA with multiple repetitions between moments and ANOVA followed by the Student Newman Keuls test (=0.05) for comparisons between groups. Heart rate was lower at all moments in the REMI 0.15 and REMI 0.3 groups than in the GCON, and heart rate was not significantly different between the two remifentanil infusion groups. Additionally, the arterial blood pressure values (SAP, MAP and DAP) were not different between all groups. Baseline values (before surgery) of ETiso were not different between the 3 groups. After the start of surgery, ETiso ranged from 1.37±0.3 to 1.05±0.19 in the control group; in the REMI 0.15 and REMI 0.3 groups, ETiso was 36.5% and 65.7% lower than in the control group (M15). Remifentanil infusion reduced ETiso in a dose-dependent manner in animals undergoing radical mastectomy without causing significant cardiopulmonary alterations.