Background

Radiofrequency (RF) energy is a classic ablation technology in the electrophysiology lab and its use is based on many years of experience. Whereas RF power is a well-known parameter and considered in clinical RF application, data about the importance of impedance are scarce. Finally, RF lesion creation is caused by RF current, resulting from RF power and electric impedance (Ohm’s law). The aim of this study was to analyze the influence of electric impedance on lesion creation and clinical RF application.

Methods

In the first part of the study, RF current was calculated for different values of RF power and circuit impedance according to Ohm’s law. Therefore, RF power and circuit impedance were systematically varied from 30W to 70W and from 80Ω to 200Ω (n=4800 calculations). Furthermore, RF power values, which result in the same amount of RF current, were calculated for systematically varied circuit impedance values (80Ω to 200Ω).

In the second part of the study, RF ablation lesions were created in a porcine ex-vivo animal model. RF applications were delivered in cardiac muscle preparations with systematically varied values of RF power, electric impedance and RF duration using a contact force ablation catheter (n=80 lesions; 80Ohm, 120Ohm, 160Ohm, and 200Ohm; 20W, 30W, 40W, 50W). Lesions volumes were analyzed using a digital precision caliper.

In the third part of the study, clinical and procedural data of n=100 consecutive RF ablation procedures were collected, which included n=3613 RF applications. RF power, electric impedance and resulting RF current values were stored, exported from the lab systems and analyzed regarding the clinical setting (patient characteristics, mapping system).

Results

Figure 1 shows the relation between RF current, RF power and impedance (A) as well as RF power corrected for 100 Ohm impedance (B).

In ex-vivo ablation lesions, RF current showed highest correlation with lesion volumes (R²=0.98), RF power showed significantly less correlation with lesion volumes (R²=0.79), p>0.05. Lesion volumes were significantly different when applied with the same RF power but different electric impedances (p<0.01).

Figure 2 shows the relation between RF current and RF power in a clinical setting. In n=3613 RF applications, mean power was 39±7 W, mean impedance was 124±18 Ohm range 78 - 214 Ohm, and mean current was 614±78 mA range 105 - 866 mA. Electric impedance and current were not significantly different between male and female patients (p=0.91), but significantly different when 3D mapping systems were used (CARTO: 613±74 mA @ 126±17 Ohm, Rhythmia: 637±62 mA @ 121±11 Ohm, p<0.01). Electric impedance was influenced by neutral electrode positioning, BMI and technical equipment.

Conclusions

RF lesion creation is rather determined by RF current than by RF power. In a clinical setting, electric impedance has a significant influence on RF current delivery and shows a large variation between patients. For instance, RF applications of 20W at 80 Ohm, 30W at 120 Ohm, 40W at 160 Ohm, and 50W at 200 Ohm result in the same RF current (500mA) and ablation lesion. Thus, impedance should become a well-considered parameter in clinical RF application. The best parameter to guide clinical RF application is RF current and not RF power.