The never‐ending search for safe and effective RF cardiac tissue ablation

With the introduction of trans-vascular cardiac RF ablation for the treatment of cardiac arrhythmias in the 1980s,1 the search for real-time indicators of targeted tissue ablation safety and efficacy has been ongoing. Dr. David Haines was the first to study the use of temperature sensors embedded in the ablation electrode and the titration of the RF power to avoid steam pops and char formation.2 However, the introduction of irrigation to prevent char formation and increase ablation lesion depth,3 the ability to assess tissue temperature at the catheter tissue interface became compromised, even at low irrigation rates. Nakagawa et al.4 were the first to test a composite measure of the weighted RF power and contact force integrated over the ablation time. This ablation index is intended to provide clinicians with a measure of safety and efficacy. Given the limited assessment of ablative tissue temperature with irrigated catheters, temperature is not included in any ablation index formulas. Limited thermal conductance is expected with short RF applications, high or very high power over short durations (HPSD or vHPSD) of 50 and 90 watts is increasingly used during pulmonary vein isolation. While RF delivered with HPSD may have favorable resistive versus conductive properties in tissues with shallow anatomic depth, its application is not immune to collateral injuries. With predominantly rapid resistive heating, there is little to no time for the operator to terminate the RF application to spare incidental harm. Any extracardiac damage occurs rapidly after RF is initiated, and as such, lethal power delivery to these structures must be avoided in the same way as with lower power and more prolonged ablation. In the current issue of JCE Drs. otsuka et al.5 examined the differences in in-vivo tissue temperature dynamics and damage to collateral structures. They administered ablation lesions in 29 pigs using either 50 W guided by an ablation index of 400 or lesion size index of 5 using the STSF and QDOT or TactiCath catheters, respectively, as well as 90 w lesions using the QDOT catheter's Q+ mode. The investigators implanted thermocouples on the epicardial surface at the superior vena cava along the phrenic nerve, at the right PV, near the esophagus, and the esophagus along with radiopaque markers attached to the tip of each thermocouple to aid in fluoroscopic localization. The RF lesions were delivered endocardially, targeting the closest proximity to extracardiac thermocouples. They defined a lethal 'maximum tissue temperature (Tmax) and catheter tip-to-thermocouple distance at all sites, Tmax and time to reach a lethal tissue temperature ≥50°C, time to Tmax (t-Tmax), and time from t-Tmax to baseline temperature (t-Tbase). Following ablation, atrial tissues were histologically assessed for lesion transmurality, and the SVC, PV, IVC, phrenic nerve, and esophageal segments underwent histological evaluation for any RF damage. Despite varied catheter designs between the TactiCath, STSF, and QDOT catheter, lesions with any of the catheters and ablation parameters resulted in transmurality due to the shallow depth of the veno-atrial junction tissues involved. Not surprisingly, the rate of temperature rise to reach lethal levels was the fastest (4.3 ± 2.5 s) with the application of 90 W for 4 s. Using 50 W short duration guided by AI of 400 or LSI of 5, 50°C was reached within 6.4 ± 1.9 s using the TacthCath, but the differences versus the STSF and the Qmode are small but significant. The difference in collateral tissue thermodynamic impact of the various catheter and power delivery combinations was primarily driven by the irrigation scheme, leading to varied resistive heating components to RF delivery. The TactiCath and Q+ modes delivered the most rapid Tmax at the closest distances, resulting from a more significant resistive heating effect. Likewise, the STSF and Qdot catheters in 50 W power delivery showed more conductive heating, with the Q mode resulting in the largest relative conductive effect with lower impedance drops and smaller lesion sizes. Regardless of predominant thermal delivery, all catheters were capable of delivering rapid, lethal energy to collateral tissue. Extracardiac injuries associated with RF atrial fibrillation ablation do not always present with clinical symptoms. It is difficult to assess these injuries accurately, and it is estimated that the incidence of esophageal injury is 5−40%.6, 7 At the same time, Esophageal perforation and fistula formation incidence occur <.25%.7, 8 With the increasing awareness of the devastating consequences of esophageal perforation and phrenic nerve paralysis, operators have learned to monitor both esophageal temperature and phrenic function and reduce the RF power or AI when RF lesions are placed in proximity to these organs. The use of both 50 and 90 watts of power increases the potential of distal injuries, and the 4 s application time is aimed to limit the heat conductance to deeper distant tissues. By understanding the nature of power delivery, operators have modulated how they ablate in thin tissues within proximity of high-risk collateral structures. This study provides further insight into the in vivo application of RF energy and the role resistive-conductive modulation plays in delivering unwanted extracardiac lethal heating. The authors readily acknowledge some of the limitations of direct extrapolation of this pig model to human subjects. The need to move the pig esophagus to relate to cardiac anatomy stands as one example of this limitation, but given the nature of this study, this is the only real in-vivo option for assessing lethal collateral lesion delivery. While advances in catheter design and power delivery combined with increased awareness among operators have given more tools to avoid collateral damage, these results are a warning that AI, LSI, HPSD, and vHPSD RF applications using TactiCath, STSF, and QDOT MICRO do not provide immunity from significant extracardiac injury.