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Notes: Intracoronary Ethanol Ablation. Introduction: Intracoronary ethanol ablation has been successfully used as arrhythmia therapy, hut the dose response of ventricular function and arrhythmogenesis to varying ethanol concentrations is undefined. Methods and Results: Twenty-six anesthetized pigs weighing 50 ± 11 kg underwent left and right heart catheterization. Ablation solutions composed of normal saline with ethanol in concentrations of 0%, 10%, 25%, 50%, 75%, and 100% were mixed with metrizamide, a nonionic contrast agent (3.75 g per 20 mL), then infused into branch or distal coronary arteries in each of the left anterior descending and left circumflex coronary artery distributions. Hemodynamic measurements, and coronary and left ventricular angiography were performed before and after ablation. Programmed electrical stimulation was performed preablation and at a chronic-study at 4 to 6 days. Excised hearts were examined pathologically. Fifty-two lesions were created in 26 animals, and 24 animals survived to the follow-up study. Minimal hemodynamic alterations were observed in response to ablation. As the ethanol concentration of the ablation solution was increased, the prevalence of spontaneous nonsustained and sustained ventricular tachyarrhythmias increased (P = 0.0002), the ablation vessels were more persistently occluded (P = 0.028), and the postablation global left ventricular ejection fraction showed greater impairment (P = 0.002). Identifiable myocardial lesions were identified in all study groups, including those receiving the 0% ethanol infusion. Lesion size increased significantly with increasing ethanol concentration (P = 0.0004) but there was significant variance within groups. In response to programmed electrical stimulation, ventricular fibrillation was a nonspecific finding before and after ablation. In contrast, monomorphic ventricular tachycardia was induced only at postablation testing, and four of five of these animals underwent infusions with ethanol concentration of ≥ 50% ethanol. Conclusion: Concentrations of ≥ 50% ethanol are most effective in creating large ventricular lesions in intracoronary ethanol ablation, but are associated with more impairment of left ventricular function, and have a greater likelihood of acute and early chronic arrhythmia aggravation.