Changes in Myocardial Perfusion During Chronic Multi Site Biventricular Versus Right Ventricular Apex Pacing in Patients With Normal Left Ventricular Function Undergoing His Bundle Ablation for Drug-refractory Atrial Fibrillation

Inclusion Criteria: * Normal to nearly normal LVF (EF \> 40%) * Drug-refractory, persistent or permanent atrial fibrillation * Referred for His ablation Exclusion Criteria: * Unwilling or unable to sign the informed consent * Life expectancy \< 1year from non-cardiac causes * Previous myocardial infarction * Previous coronary bypass surgery * Poor left ventricle function (EF\< 40%) from any cause * Symptomatic obstructive coronary artery disease * Poor ultra sound imaging quality.