THE EFFECTS OF HALOTHANE ON VENTRICULAR-TACHYCARDIA IN INTACT DOGS

Halothane has either proarrhythmic or antiarrhythmic effects in a variety of clinical circumstances. This investigation tested the hypothesis that halothane would display different effects on ventricular tachycardia (VT) produced by different electrophysiologic mechanisms in intact dogs. Four models of VT produced by abnormal automaticity, reentry, delayed-afterdepolarization-induced triggered activity, and early-afterdepolarization-induced triggered automaticity (groups 1-4, respectively) were studied. In groups 1 and 2, the left anterior descending coronary artery (LAD) was ligated. In group 1 (n = 5), 24 h after LAD ligation and infarction, all dogs demonstrated incessant VT with 94.7 +/- 2.3% of beats of ventricular origin. This ectopy presumably was due to abnormal automaticity. Halothane reduced the frequency of ventricular ectopy until at 2% halothane only 34.8 +/- 15% of beats were of ventricular origin. One week after LAD ligation, programmed stimulation produced nonstimulated extrasystoles of presumably reentrant origin in six dogs. In three, halothane 1% abolished extrasystoles while increasing the ventricular refractory period by 23 +/- 3.8% (P < 0.05). In the three other dogs, halothane had no effect (two dogs) or worsened the severity of VT (one dog), while the refractory period increased by 7.7% (P > 0.05). In group 3 dogs, ouabain was infused until VT secondary to triggered activity occurred. Halothane restored sinus rhythm in 4 of 5 dogs. Overall the percentage of sinus beats increased from 11.1 +/- 2.8 to 97.4 +/- 2.6% when halothane 2% was added during ouabain toxicity. Cesium chloride infusion increased the QT interval and produced complex VT in 5 dogs. In no experiment did halothane influence cesium chloride induced VT due to early afterdepolarizations or modify the QT prolongation produced by cesium (group 4). These experiments demonstrate that halothane has diverse effects on VT due to different cellular electrophysiologic mechanisms.