Efeitos da infusão prolongada de fentanil, isoladamente ou associado à atropina, sobre a concentração alveolar mínima do isofluorano em cães

This study assessed whether isoflurane MAC changes over a prolonged infusion of fentanyl and if concurrent atropine administration influences isoflurane MAC in dogs. Six adult bitches were anesthetized with isoflurane, under IPPV, normocapnia and normothermia. The animals were instrumented for measurements of HR and invasive blood pressure. Each animal received, on different occasions, a fentanyl bolus (5 μg/kg, IV) and constant rate infusion (CRI, 0.15 μg/kg/min) alone (FEN) or combined with atropine (bolus 0.02 mg/kg and CRI at 0.04 mg/kg/h - FEN+AT). The MAC and other variables were determined before (baseline) and at 2 and 5 hours after initiation of the fentanyl CRI. Variables were compared by ANOVA followed by Bonferroni correction or Tukey test (p<0.05). Baseline MAC values for FEN and FEN+AT treatments were 1.38 ± 0.16% and 1.39 ± 0.14%, respectively. Fentanyl significantly decreased MAC by 50% ± 9 and 49 ± 13% after 2 hours and 51 ± 14% and 51 ± 9% after 5 hours, respectively for FEN and FEN+AT treatments. Compared to baseline, HR decreased significantly in the FEN treatment by 35 and 43% at 2 and 5 hours, respectively. HR did not change significantly over time in the FEN+AT treatment, but was higher than in the FEN treatment at all time points. In both groups, SAP increased from baseline after fentanyl. Under the conditions of this study, it was concluded that atropine does not influence isoflurane MAC and that, during a 5 hours CRI of fentanyl, isoflurane MAC remains stable suggesting no cumulative effect of the opioid.