Analysis of body surface area as a determinant of impedance during radiofrequency catheter ablation in adults and children.

Since most radiofrequency (RF) generators used for catheter ablation approximate a constant voltage output, applied power is inversely proportional to the impedance load of the system. Knowledge of the expected impedance load for a patient may facilitate selection of safer and more effective voltage output. Preliminary observations suggest that in adults, impedance is directly proportional to body surface area (BSA), thus prompting this study to determine whether this relation was maintained in smaller patients undergoing RF catheter ablation. Prospective analysis of impedance from 949 RF deliveries in 76 patients (BSA, 0.69-2.3 m2) revealed the mean impedance for all deliveries to be 103 +/- 8 ohms. Two-phase linear regression analysis revealed a significant, direct correlation between impedance and BSA in patients with a BSA > or = 1.5 m2 (P = .001); however, for patients with a BSA < 1.5 m2 there was no correlation. These results indicate that as patient size decreases below 1.5 m2, impedance is constant. Radiofrequency catheter ablation procedures in children may require selection of a voltage output similar to that used in adults in order to produce effective RF lesions.