Local impedance, current, power—what should be considered in RF ablation?

The innovations in the field of electrophysiology (EP) led to great clinical and scientific advancements in the interventional treatment of cardiac arrhythmias. Two fundamental procedural steps—mapping and ablation—are crucial for overall success and favorable patient outcome. Today's mapping and imaging technologies offer unique insights into arrhythmia mechanisms and operators consider these cardiac mapping results carefully to create best applicable ablation strategies. One remaining limitation of classic radiofrequency (RF) ablation is that operators cannot visually control RF lesion creation. Great efforts are undertaken to overcome this significant limitation, which is an inherent characteristic of almost every interventional procedure: Operators need to rely on surrogate parameters to estimate the real RF ablation effect. Therefore RF power, duration, impedance, RF current, and contact force as well as lesion indices based on those parameters are controllable variables and have been proofed to correlate with resulting lesion size. The present study by Takigawa et al. provides further important insights into the biophysical characteristics of RF lesion creation. The authors analyzed the influence of blood pool impedance and RF catheter orientation on lesion size and steam pop incidence, using local impedance (LI) sensing technology integrated into their RF ablation catheter.1, 2 The effect of RF ablation is based on the local heating effect of alternating current. During RF application, the RF generator, cables, the ablation catheter, cardiac and body tissue, and the dispersive electrode form a closed electric circuit. At the level of the RF ablation electrode, there is a high density of electric current surrounding the metallically conducting electrode and adjacent cardiac tissue of higher electric resistance. The high current density within cardiac tissue around the RF ablation electrode causes local tissue heating (“Joule heating”), which is then conducted along a temperature gradient into deeper cardiac tissue layers (“conductive heating”) over time.3, 4 It is RF power, as selected on the RF generator, but also global electric circuit impedance which determine this current density. Once the tissue temperature exceeds an upper limit, there is an irreversible denaturation of cardiac tissue resulting in permanent ablation effects.5 Takigawa et al. elucidate the importance of electric current distribution between the RF electrode, cardiac tissue, and surrounding blood pool during RF ablation. In RF lesion creation, usually only a small amount of RF current is conducted into the myocardium effecting lesion growth, while the major amount of RF current dissipates into the blood pool. The ratio of current going into tissue versus into blood pool is determined by first the area of contact and second the electric resistance between RF electrode, cardiac tissue, and blood pool.2 A recent technological development are LI sensing catheters, which were also used by Takigawa et al. These catheters transmit electric current between the distal ablation electrode and a more proximal ring electrode, facilitating measurements of the LI around the catheter tip and its mini electrodes. The analysis of LI provides information about electric contact between the ablation electrode and adjacent tissue (no contact: low blood pool impedance; contact: higher tissue impedance) and lesion size (positive correlation between LI drop and RF lesion size). The interaction of LI, electric contact, and resulting lesion size is summarized in Figure 1.6 The operator can consider this LI information to estimate electric contact, anticipated lesion size, and real lesion growth during RF application. The findings of previous studies on half-saline irrigation are a complement to the recent findings by Takigawa et al. Using half-saline irrigation artificially creates high blood pool impedances around the ablation catheter tip, resulting in a higher amount of current conducting into cardiac tissue and less current dissipation in the blood pool. This will result in increased lesion size and, considering the data of the current study, a higher incidence of steam pops.7 RF ablation is a complex biophysical process which is crucial for the success of an EP procedure. Finally, RF lesion formation solely depends on RF current, emanating from the ablation electrode, and its local distribution. In clinical practice, we utilize various parameters to estimate and modulate this process. It is known that not only RF power, as selected on the RF generator, but also global impedance should be considered parameters as both, power and impedance, determine global electric current flow. The study by Takigawa et al. implies that LI plays a similarly decisive role as it provides unique insights into the local distribution of RF current. All techniques which operators usually employ to increase lesion formation—simply increasing power, attaching a second dispersive electrode, increasing blood pool impedance—finally have the very same effect of altering current flow directed into the cardiac tissue. As depicted in Figure 1, local RF current distribution may have an enormous influence on lesion size, resulting in totally different lesions despite identical settings of RF power, duration, and contact force were chosen. Future technologies may improve our understanding of local current distribution, increasing efficacy and safety of RF ablation.