Atrial rate is known to modulate ventricular response during atrial fibrillation (AF). The resulting pulse irregularity translates into widely varying inter-beat intervals (IBIs) extracted from recorded surface electrocardiogram (ECG) activity. In AF, the random nature of the IBIs makes them difficult to analyze using traditional methods, and little work has investigated the relationship between ventricular response and persistent AF (persAF) disease progression. In this paper, we propose nonlinear approaches for characterizing IBI dynamics in patients undergoing catheter ablation for the treatment of persAF.