EFFECT OF FENTANYL ON THE MINIMUM ALVEOLAR CONCENTRATION OF ISOFLURANE IN SWINE

Background: Fentanyl is used in anesthetic protocols for swine, but th ere are no reports on its potency in this species, This study measured the extent to which fentanyl reduces the minimum alveolar concentrati on of isoflurane (MAC(ISO)) in swine. Methods: Sixteen swine were rand omly assigned to four groups, For each group, baseline MAC(ISO) was de termined, and three groups received two of three fentanyl infusions as follows: 50 mu g . kg(-1). h(-1) intravenously followed by 100 mu g . kg(-1). h(-1), 50 mu g . kg(-1). h(-1) followed by 200 mu g . kg(-1). h(-1), or 100 mu g . kg(-1). h(-1) followed by 200 mu g . kg(-1). h(- 1) (n = 8 for each dosage). A loading dose of fentanyl preceded each i nfusion. Each infusion was maintained for 60 min before initiating min imum alveolar concentration determination, The infusions were maintain ed throughout the period of minimum alveolar concentration determinati on, Plasma fentanyl samples were obtained after 30 min of each infusio n, and plasma fentanyl and hemodynamic parameters were obtained immedi ately before stimulating swine for the final isoflurane concentration used in determining minimum alveolar concentration, A fourth group, co ntrol animals, received saline infusions. After each infusion, the MAC (ISO) was redetermined. Minimum alveolar concentration was determined using incremental changes in isoflurane concentrations until gross pur poseful movement resulted when using a hemostat stimulus applied for I min to a rear dewclaw. Results: MAC(ISO) for controls was 2.19 +/- 0. 17% (mean +/- SEM) and changed minimally over time (-0.13 +/- 4.77%), MAC(ISO) decreased significantly (P less than or equal to 0.01) 24.5 /- 3.2%, 29.9 +/- 4.8%, and 45.9 +/- 5.5% with fentanyl dosages of 50, 100, and 200 mu g . kg(-1). h(-1), respectively, Corresponding plasma fentanyl concentrations were 14 +/- 1 ng/ml, 26 +/- 3 ng/ml, and 59 /- 5 ng/ml, respectively. A ceiling effect on reduction of MAC,, was n ot observed, Changes over time or between groups were not observed for arterial blood gas tensions, blood pressure, heart and respiratory ra te, or body temperature. Conclusions: These fentanyl dosages are large r than those commonly used in humans and other species, Anesthetic pro tocols using fentanyl for swine should be designed with the knowledge that a fentanyl infusion of 200 mu g . kg(-1). h(-1) contributes appro ximately a 50% MAC(ISO) equivalent.