The New Normal

Ventriculr arrhythmias (VAs) are an important cause of mortality, morbidity, and poor quality of life (1). The incidence of VA, as well as the use of catheter ablation for treatment of VA, significantly increased over the past 2 decades (2,3), albeit with suboptimal ablation success rates, averaging 40% to 60% 1-year single-procedure freedom from VA (4–6).

Challenges to improvement in these outcomes can be broadly divided into difficulties in understanding or localizing the tachycardia circuits and inadequate technology in the modification of the arrhythmia substrates once they are identified. The former has largely been addressed with improvement in algorithms of electroanatomic mapping systems, availability of multielectrode mapping catheters (7,8), and direct visualization of the substrate with various imaging modalities, such as cardiac magnetic resonance and intracardiac echocardiography (9). However, once the relevant substrate is identified, endovascular radiofrequency ablative technologies may remain limited in achieving transmural lesions due to deep substrates, midmyocardial locations, and illdefined anatomic barriers. Irrigated catheter ablation was designed to allow for greater power delivery when it is otherwise limited by high temperatures at the catheter tip–tissue interface, which can lead to char formation. Nevertheless, impediments to durable and effective ablation lesions remain.

A novel approach to dealing with deep intramural circuits is through the use of open irrigated electrodes cooled with irrigants of lower ionic and charge density than normal saline (NS). The goal with these irrigants, such as half-normal saline (HNS), is to lessen dispersion of current toward the surrounding environment by increasing its impedance, thus allowing it to focus on the tissue that the electrode is in contact with (Figure 1A) (10). Through this approach, larger and deeper lesions can be created with the same power settings, albeit with a risk of higher temperatures at the catheter tip–tissue interface (11).

FIGURE 1. Biophysics of Low Ionic Irrigation Ablation.

(A) Use of irrigants with lower ionic tonicity decreases the radiofrequency (RF) energy dispersion to the surrounding environment, with (B) catheter orientation-dependent higher peak temperatures in the catheter tip. Adapted with permission from Nguyen et al. (10,11). HNS = half-normal saline.

An appropriate concern with the clinical use of high-impedance fluids is an increased incidence of steam pops, because of the greater energy delivered directly to the tissue, which may also be affected by catheter orientation (11). Another concern is char formation on the electrodes with ablation, which is more common with nonirrigated catheters, but can be a risk whenever there are higher temperatures at the catheter tip–tissue interface.

In this respect, the study by Tschabrunn et al. (12) in this issue of JACC: Clinical Electrophysiology provides additional data using HNS irrigation with a contemporary ablation catheter, the ThermoCool SmartTouch Surround Flow (STSF) (Biosense Webster, Diamond Bar, California). Specifically, this 3.5-mm tip catheter has 56 fenestrations, with a lower rate of irrigation (15 ml/min) compared with the ThermoCool ST catheter (Biosense Webster) (30 ml/min). In a swine beating-heart ablation model, ablation lesions with HNS in the left ventricle were compared with lesions with NS using the ThermoCool STSF catheter at the same settings (40 W, 10-s titration, target contact force 8 to 14 g, lesions separated by 15 mm) and under intracardiac echocardiography guidance to monitor for steam pops.

Tschabrunn et al. (12) concluded that with HNS, the tissue temperatures were higher, which has also been previously shown in ex vivo models (Figure 1B). This did not translate into a difference between lesion depth or width between the 2 groups in pathological evaluation, but the change in unipolar and bipolar voltage amplitude was significantly greater with HNS on electroanatomic mapping. Steam pops occurred more commonly with HNS, as well as with lesions that were deeper and wider, regardless of the type of irrigant that was used. Frequency of catheter tip char was similar between the HNS and NS groups.

Tschabrunn et al. (12) should be congratulated on this interesting study to characterize the use of HNS for ablation using a low-flow irrigation catheter. Their randomized, blinded design in a beating-heart model is novel and exemplary for this kind of analysis to compare lesion characteristics with different irrigants. They extend on prior ex vivo findings and confirm the higher clinical steam-pop rates observed with ThermoCool STSF catheters in patients undergoing ventricular tachycardia ablation using HNS (13). Not unexpectedly, their findings raise caution that, similar to standard ablation, with greater power delivery, there are greater risks. Clinically, titration of power, whether by increasing the wattage or with the use of HNS, should be used judiciously based on monitoring of applied energy, impedance changes, and clinical judgment.

The high rate of steam pops that was observed in the study, if reproduced, is worrisome, not only for the HNS group, but also for the control group, averaging 22% of all ablation lesions using NS irrigation, despite fairly standard ablation settings for a relatively short duration (30 s). In the initial description of Tschabrunn et al. (12) in vivo beating porcine heart model (14), no steam pops were observed despite longer-duration (60 s) lesions with similar ablation settings using the ThermoCool ST catheter with an irrigation rate of 30 ml/min. Because the catheter type was the only major difference, and barring experimental confounders, the findings from this study raise caution not necessarily regarding the use of HNS, but rather regarding the ThermoCool STSF catheter with low-flow irrigation. In addition, despite the randomized, blinded nature of the study, the HNS group had a statistically higher baseline impedance, which may suggest a lower-flow environment during ablation for this group, thus compounding the steam-pop risk from a low-flow irrigation catheter ablation. Furthermore, it is not known how the catheter tip orientation differed between the 2 groups, as perpendicular versus parallel orientation can affect current density and alter steam-pop risks (11).

Another surprising result in this study is the absence of difference in lesion size between HNS and NS ablation. This may be due to premature termination of potentially the largest lesions because of the occurrence of steam pops, which occurred more frequently in the HNS group. The lower force-time integral in the HNS group, compared with the NS group, may also explain the lack of difference in lesion size. In addition, the significantly greater postablation endocardial voltage change with HNS did not translate to greater lesion size. This inconsistency is difficult to explain and may be due to the study’s limitation of measuring acute, rather than mature ablation lesions. Finally, it is unknown how the findings of this study would translate to ablation biophysics using HNS in myocardial scar, in which there may be greater impedance mismatch with the surrounding environment.

This study should also be considered in light of alternative approaches to improve ablation. Some strategies to target midmyocardial and deep circuits include bipolar ablation to focus the flow of electric current (15), intramural needle ablation catheter design to better engage the thick myocardium (16), infusion of ethanol directly into the substrate via coronary arteries or veins (17), and use of different energy sources such as stereotactic body radiation and pulsed field ablation (18,19). Despite the advent of these advanced techniques, they may not be readily available, and they should be prudently considered for exceptional circumstances while being tested rigorously prior to widespread adoption.

In summary, this study serves as a reminder that, despite a new normal, safety as always remains paramount. More research is needed to establish the efficacy and safety of using low-ionic, high-impedance fluids during external irrigated ablation.