2007a, 2008a). The incremental pacing protocols applied cycles of 100 regular pacing stimuli at successively decremented BCLs. Figure 2 illustrates results of the subsequent incremental pacing procedure comparing ECG (i) and intracellular traces (ii) in conditions of regular activity (Fig. 2A) and occurrences of premature atrial complexes (Fig. 2B), atrial fibrillation (Fig. 2C) and APD90 alternans (Fig. 2D) in an aged Pgc1β−/− heart. Here, PPARGC1B is linked to atrial fibrillation.