Factors influencing impedance during radiofrequency ablation in humans.

Impedance during radiofrequency (RF) catheter ablation procedures is dependent on a variety of parameters related to the catheter, cabling, reference patch, body size, and temperature. To examine the influence of body size, impedance was measured during clinical ablation procedures in 93 patients (Group I) with a wide range of body sizes. In 14 other patients (Group II), impedance was measured during variations in catheter tip size (5, 6 and 7 Fr), reference patch size (120 and 60 cm2), patch location (chest vs. thigh), and catheter tip tissue contact. The average impedance was also compared to average tip temperature in Group II patients. Impedance decreased with increasing catheter tip size, reference patch size and proximity of the patch to the heart. However, the effects of body geometry were complex. For example, using a chest patch, impedance increased with body surface area, but using a thigh patch it decreased, suggesting that lung volume may increase impedance, but body width may actually decrease it. An increase in tip tissue contact, relative to blood contact, increased the impedance, suggesting that impedance may be a useful measure of tip tissue contact. Finally impedance decreased with increasing tip temperature, suggesting that impedance may be useful as a real time measure of tissue and blood heating. The results are interpreted in terms of an electrical analog which suggests further that despite the lower total power when the same voltage is applied to a higher impedance, less voltage should be applied to achieve the same tissue effect when the measured impedance is higher.