Consequences of atrial tachycardia-induced remodeling depend on the preexisting atrial substrate

Background-All animal studies of atrial tachycardia (AT) remodeling to date have been performed in normal hearts, but clinical atrial fibrillation (AF) often occurs in the setting of heart disease. This study evaluated the effects of a pathological AF substrate on AT-induced remodeling. Methods and Results-Fourteen control dogs, 12 AT-only dogs (400 bpm for 1 week), 14 congestive heart failure (CHF) dogs (CHF only, ventricular tachypacing, 220 to 240 bpm for 5 weeks), and 13 CHF+AT dogs (ventricular tachypacing-induced CHF, 1 week of AT superimposed on the last week of ventricular tachypacing) were studied for evaluation of AT effects in normal hearts (AT-only versus control dogs) and CHF hearts (CHF+AT versus CHF-only dogs). In normal hearts, AT strongly decreased the effective refractory period (ER-P) and abolished ERP rate adaptation, whereas conduction velocity was unaltered. In CHF dogs, AT reduced ERP to a significantly lesser extent, did not alter ERP rate adaptation, and reduced conduction velocity. AT alone increased atrial vulnerability to extrastimuli and prolonged AF. In the presence of CHF, AT had no clear effect on atrial vulnerability but increased the prevalence of prolonged AF. Conclusions-The electrophysiological effects of AT are different in hearts with a CHF-induced pathological substrate for AF than in normal hearts. These findings have potentially important implications for understanding how AF occurring in diseased hearts begets AF.