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Figure 1 (Top left) Relative changes in right atrium (RA) size measured during the first four weeks of total atrioventricular (AV) block. The average RA diameter was measured during five consecutive ventricular cycles from the distance between the tips of two screw-in leads (ultrasound crystals). Immediately after His bundle ablation (dotted line), the atrial diameter decreased owing to the lower ventricular rate. The size of the atria then progressively increased, resulting in 13.2 ± 3.0% dilation after four weeks of AV-block. **p < 0.01. (Top right) Time course of the increase in duration of atrial fibrillation (AF) paroxysms. In four of seven animals, the AF episodes lasted longer than 10 min after four weeks of idioventricular rhythm. (Lower panels) Photomicrographs from sections of the left atrium. A modified azan stain was used to determine cell size, and Sirius red was used to visualize collagen. After four weeks of AV block, the atrial myocytes were clearly enlarged and the amount of collagen was not increased.