LATE ENLARGEMENT OF RADIOFREQUENCY LESIONS IN INFANT LAMBS - IMPLICATIONS FOR ABLATION PROCEDURES IN SMALL CHILDREN

Background Despite the current clinical use of radiofrequency (RF) cat heter ablation in infants, the acute and late effects of RF lesion pro duction in immature myocardium remain unknown. This study was specific ally designed to investigate the pathology of RF lesions in developing sheep myocardium. Methods and Results In study 1, RF lesions were mad e on the epicardial left ventricular surface of the beating heart in 1 5 sheep, 5 approximately 4 weeks of age (11.0+/-1.0 kg) and 10 approxi mately 8 weeks of age (23.8+/-3.4 kg), to assess the effects of RF app lication duration (10 to 90 seconds) and electrode tip temperature (45 degrees to 90 degrees C) on lesion size in immature myocardium. Lesio n 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 on e-half lesion size was 6.5 seconds for width and 12.0 seconds for dept h. Lesion width increased nearly linearly with tip temperature above 5 0 degrees C, but depth followed a sigmoid relation, with no increase a bove 80 degrees C. In study 2, RF lesions were made in all four cardia c chambers under fluoroscopic guidance in 19 infant sheep (10.9+/-1.4 kg). Lesion sizes and histological characteristics were assessed acute ly (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 atriovent ricular 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, w hile ventricular lesion width increased from 5.9+/-0.8 to 10.1+/-0.7 m m (171%) at late follow-up but was not significantly changed at 1 mont h. Histological evaluation revealed replacement of normal myocytes wit h fibrous and elastic tissue at 1 month and late follow-up in all loca tions but also demonstrated a poorly delineated border with multiple e xtensions of fibrous and elastic tissue into surrounding normal myocar dium in late ventricular lesions. Conclusions RF lesion formation in i mmature sheep myocardium is similar to that in adult myocardium acutel y but is associated with late lesion enlargement and fibrous tissue in vasion of normal myocardium. These findings may have implications for clinical RF ablation procedures in infants.