ALTERATIONS IN THE ARRHYTHMOGENIC DOSE OF EPINEPHRINE AFTER XYLAZINE OR MEDETOMIDINE ADMINISTRATION IN HALOTHANE-ANESTHETIZED DOGS

Eight dogs (12.5 to 21.5 kg) were assigned at random to each of 3 grou ps that were not given glycopyrrolate (HS, HX, HM) and to each of 3 gr oups that were given glycopyrrolate (HGS, HGX, HGM). Dogs were anesthe tized with halothane (1.31% end-tidal concentration), and ventilation was controlled (P-CO2 35 to 40 mm of Hg end-tidal concentration). Glyc opyrrolate was administered IV and IM at a dosage of 11 mu g/kg of bod y weight, each. Saline solution, xylazine (1.1 mg/kg, IM), or medetomi dine (15 mu g/ kg, IM) was administered 10 minutes after baseline arrh ychmogenic dose of epinephrine (ADE) determination. Redetermination of the ADE at the same infusion rate was stared 10 minutes after drug ad ministration. Arrhythmogenic dose was determined by constant infusion of epinephrine at rates of 1.0 and 2.5 mu g/kg/min. The ADE was define d as the total dose of epinephrine inducing at least 4 ectopic ventric ular depolarizations within 15 seconds during a 3-minute infusion or w ithin 1 minute after the end of the infusion. Total dose was calculate d as the product of infusion rate and time to arrhythmia. Statistical analysis of the differences between baseline ADE and posttreatment ADE for groups HS, Hx, and HM was performed by use of one-way ANOVA. Mean +/- SEM baseline ADE values for groups HS, Hx, and HM were 1.50 +/- 0 .11, 1.49 +/- 0.10, and 1.57 +/- 0.22 mu g/kg, respectively, and for g roups HGS, HGX, and HGM were 3.37 +/- 0.61, 3.10 +/- 0.75, and 3.04 +/ - 0.94 mu g/kg, respectively. Differences for groups HS, HX, and HM we re -0.02 +/- 0.15, -0.00 +/- 0.14, and -0.21 +/- 0.17 mu g/kg, respect ively, and for groups HGS, HGX, and HGM, were -0.59 +/- 0.26, -0.41 +/ - 0.15, and -0.58 +/- 0.20 mu g/kg, respectively. Differences among gr oups HS, Hx, and HM, or among groups HGS, HGX, and HGM were not signif icant. Me conclude that without and with cholinergic blockade in halot hane-anesthetized dogs: preanesthetic dosages of xylazine (1.1 mg/kg, IM) or medetomidine (15 mu g/kg, IM) do not enhance arrhythmogenicity, and at these dosages, there is no difference in the arrhythmogenic po tential of either alpha(2)-adrenoceptor agonist.