Steroids prevent late extension of radiofrequency lesions in the thigh muscle of infant rats: implications for pediatric ablation.

INTRODUCTION: Marked late enlargement of radiofrequency (RF) lesions may occur in immature myocardium, suggesting that late proarrhythmic effects may occur in infants and small children undergoing RF ablation. Because late lesion extension may be involved in this phenomenon, we evaluated the impact of corticosteroids on the healing of RF lesions created in the thigh muscle of 29 infant Wistar rats (30 days; 55 g).
METHODS: Lesion dimensions and histological characteristics were assessed acutely (n = 11), and at 30 days in controls (n = 11, 183 g) and rats (n = 7, 173 g) receiving hydrocortisone after ablation and betametasone for 29 days. Acute (n = 16) and chronic (30 days; n = 5) lesions were also evaluated in adult Wistar rats (300 g).
RESULTS: Acutely, lesions in adults and infants were well demarcated from the surrounding tissue. In adults, chronic lesions did not increase in size and were well demarcated histologically. Controls and treated infant rats did not differ with respect to the gross appearance of chronic lesions. Late lesions doubled in size (20 mm in diameter) and were poorly demarcated from the surrounding tissue, exhibiting multiple collagen strands extending from the lesion into normal muscular tissue. In the treatment group, healing was markedly delayed and the extent of collagen proliferation was significantly less than controls.
CONCLUSION: RF lesions created in the thigh muscle of infant rats reveal late enlargement and invasion of normal muscle by intense collagen proliferation. Steroids seem to limit late extension of RF lesions. These findings may have implications for RF ablation procedures in pediatric populations.