Despite the current clinical use of radiofrequency (RF) catheter ablation in infants, the acute and late effects of RF lesion production in immature myocardium remain unknown. This study was specifically designed to investigate the pathology of RF lesions in developing sheep myocardium.Methods and ResultsIn study 1, RF lesions were made on the epicardial left ventricular surface of the beating heart in 15 sheep, 5 approximately 4 weeks of age (11.0±1.0 kg) and 10 approximately 8 weeks of age (23.8±3.4 kg), to assess the effects of RF application duration (10 to 90 seconds) and electrode tip temperature (45° to 90°C) on lesion size in immature myocardium. Lesion width and depth increased asymptotically with RF duration, to 7.0±0.7 and 4.8±1.0 mm at 90 seconds, respectively. The time to reach one-half lesion size was 6.5 seconds for width and 12.0 seconds for depth. Lesion width increased nearly linearly with tip temperature above 50°C, but depth followed a sigmoid relation, with no increase above 80°C. In study 2, RF lesions were made in all four cardiac chambers under fluoroscopic guidance in 19 infant sheep (10.9±1.4 kg). Lesion sizes and histological characteristics were assessed acutely (acute, n=5), at 1.07±0.02 months (1 month, n=5), and at 8.5±0.5 months (late, n=9). Atrial and ventricular lesions but not atrioventricular groove lesions apparently increased in size during the follow-up period. Atrial lesions width increased from 5.3±0.5 to 8.7±0.7 mm at 1 month (164%) but did not increase further at late follow-up, while ventricular lesion width increased from 5.9±0.8 to 10.1±0.7 mm (171%) at late follow-up but was not significantly changed at 1 month. Histological evaluation revealed replacement of normal myocytes with fibrous and elastic tissue at 1 month and late follow-up in all locations but also demonstrated a poorly delineated border with multiple extensions of fibrous and elastic tissue into surrounding normal myocardium in late ventricular lesions.ConclusionsRF lesion formation in immature sheep myocardium is similar to that in adult myocardium acutely but is associated with late lesion enlargement and fibrous tissue invasion of normal myocardium. These findings may have implications for clinical RF ablation procedures in infants.