Determination of the minimum alveolar concentration of isoflurane that blunts adrenergic responses in sheep and evaluation of the effects of fentanyl

Barletta, Michele; Kleine, Stephanie A.; Hofmeister, Erik H.; Thoresen, Merrilee; Peroni, John F.; Knych, Heather K.; Scharf, Alexandra M.; Quandt, Jane E.

Determination of the minimum alveolar concentration of isoflurane that blunts adrenergic responses in sheep and evaluation of the effects of fentanyl

2016

OBJECTIVE To determine the minimum alveolar concentration that blunts adrenergic responses (MAC(BAR)) for isoflurane and evaluate effects of fentanyl on isoflurane MAC(BAR) in sheep. ANIMALS 13 healthy adult Dorset-cross adult ewes. PROCEDURES In a crossover design, each ewe was anesthetized 2 times for determination of isoflurane MAC(BAR). Anesthesia was induced with propofol administered IV. Sheep initially received fentanyl (5 μg/kg, IV, followed by a constant rate infusion of 5 μg/kg/h) or an equivalent volume of saline (0.9% NaCl) solution (control treatment). After a washout period of at least 8 days, the other treatment was administered. For MAC(BAR) determination, a mechanical nociceptive stimulus (ie, sponge forceps) was applied at the coronary band for 1 minute. The MAC(BAR) values of the 2 treatments were compared by means of a paired t test. During MAC(BAR) determination, blood samples were collected for measurement of plasma fentanyl concentration. RESULTS Mean ± SD isoflurane MAC(BAR) of the fentanyl and control treatments was 1.70 ± 0.28% and 1.79 ± 0.35%, respectively; no significant difference was found between the 2 treatments. Plasma concentration of fentanyl reached a median steady-state concentration of 1.69 ng/mL (interquartile range [25th to 75th percentile], 1.47 to 1.79 ng/mL), which was maintained throughout the study. CONCLUSIONS AND CLINICAL RELEVANCE Administration of fentanyl at 5 μg/kg, IV, followed by a constant rate infusion of the drug at 5 μg/kg/h did not decrease isoflurane MAC(BAR). Further studies to determine the effect of higher doses of fentanyl on inhalation anesthetic agents and their potential adverse effects are warranted.

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