ANESTHETIC AND HEMODYNAMIC INTERACTIONS OF DEXMEDETOMIDINE AND FENTANYL IN DOGS

Background: Anesthetic doses of dexmedetomidine (DMED), a highly selec tive alpha(2), agonist, are not well tolerated hemodynamically. The co mbination of an opioid with DMED might reduce the dosage requirements for each drug and thereby allow the same anesthetic depth to be achiev ed with lesser degrees of their individual side effects. Methods: Dogs were anesthetized with enflurane. One group (n = 5) received intraven ous doses of DMED from 0.1 to 10 mu g/kg. Two other groups of five dog s each received fentanyl 15 mu g/kg plus 0.05 mu g.kg(-1).min(-1) or f entanyl 45 mu g/kg plus 0.2 mu g.kg(-1).min(-1). Thereafter, they rece ived DMED doses of 0.03-3 mu g/kg. After the effects of the last DMED dose were measured, atipamezole 0.3 mg/kg was infused intravenously an d all measurements were repeated. Then, naloxone (1 mg/kg) was injecte d intravenously and a final set of measurements obtained. Anesthetic e ffects were assessed by determining enflurane minimum alveolar concent ration (MAC). Hemodynamics and plasma fentanyl concentrations were mea sured at each determination of MAC. Results: DMED and fentanyl individ ually produced dose-related reductions of enflurane MAC. During the lo wer rate infusion of fentanyl (plasma fentanyl concentration 1.0 +/- 0 .3 ng/ml), DMED reduced enflurane MAC more than could be attributed to a simple additive interaction. During the higher rate infusion of fen tanyl (plasma fentanyl concentration 4.4 +/- 0.7 ng/ml), DMED reduced enflurane MAC to greater degrees than were achievable by fentanyl alon e. D;MED caused a dose-dependent increase in arterial pressure concomi tantly with a decrease in cardiac output, and these changes were not m odified by fentanyl. The bradycardia following DMED was augmented by f entanyl. Conclusions: There was a positive interaction, additive or sy nergistic, between DMED and fentanyl with respect to their enflurane-s paring effects. The interaction allowed the same depth of anesthesia t o be achieved by lower doses of all three drugs, potentially limiting the intensity of their individual side effects. However, the presence of fentanyl increased the degree of bradycardia induced by DMED.