Author + information
- Published online October 16, 2017.
- Dhiraj Gupta, MD, DM∗ ()
- Institute of Cardiovascular Medicine and Science, Liverpool Heart and Chest Hospital, Liverpool, United Kingdom
- ↵∗Address for correspondence:
Dr. Dhiraj Gupta, Liverpool Heart and Chest Hospital, Institute of Cardiovascular Medicine and Science, Thomas Drive, Liverpool, Merseyside L14 3PE, United Kingdom.
- ablation index
- contact force
- force time integral
- surround flow catheter
- surround flow irrigation
For all the advances in electrophysiology, one aspect has remained remarkably constant: the use of radiofrequency (RF) energy to create a focal ablation lesion. The recipe for this should be simple enough in theory; one cooks the meat for some time using the basic ingredient of power while monitoring temperature and ensuring good contact with the heat source, and voila, the perfect lesion is created: a lesion that is not only safe and effective, but also durable over time. Alas, a simple recipe is no guarantee of a perfect result, as many an aspiring cook has found out. So here we are in 2017, with decades of research and clinical experience behind us, and the recipe for the perfect RF lesion continues to be elusive. Late recovery of RF lesions because of “under-treatment” is all too common, and the odd disaster because of “over-treatment” is still a sobering reality. As we continue our search for the perfect ablation strategy, confusion abounds: Which parameter(s) should we monitor and target while delivering an RF lesion: local signal attenuation; impedance drop (ID); contact force (CF); force time integral (FTI); ablation index (AI); or some combination of these? Should we deliver RF in a temperature-controlled mode or a power-controlled mode? And given the critical role of catheter tip irrigation in influencing lesion creation, how should we adapt all the above for catheters with different irrigation characteristics?
In this issue of JACC: Clinical Electrophysiology, Ullah et al. (1) describe their experiments evaluating the relationships between ID during ablation and CF-based indices of RF delivery such as FTI and AI. They compared these relationships between 2 ablation catheters with different irrigation characteristics, the SmartTouch (ST) and the SmartTouch Surround Flow (STSF) (both Biosense Webster, Diamond Bar, California). They found that the AI algorithm provided a more accurate estimation of ablation quality than FTI did, and ablations delivered by the STSF catheter had lower absolute ID with earlier plateaus as compared to those created by the ST. Although the nature of their work was not designed to assess clinical endpoints, their findings are nonetheless highly relevant to everyday clinical practice. This piece will attempt to put their work into context while discussing 2 issues: 1) What is the role of AI in assessing lesion size? 2) Where does the STSF catheter sit among the cornucopia of RF ablation tools available to us today?
Ablation Index as a Marker of Lesion Size
No randomized study to date has shown an improvement in clinical outcomes with the use of CF-sensing catheters. Though disappointing, these negative results have merely highlighted that contact force is only one variable influencing lesion creation, with the others being the duration of energy application and power delivery. Even ablation guided by FTI, which combines application time with CF in a linear fashion, was associated with more than one-third of the patients exhibiting 1 or more gaps (2). AI potentially overcomes the limitations of FTI by not only incorporating the important factor of power delivery into the equation, but also by combining these 3 factors in a weighted manner that has been shown to correlate to lesion depth in an animal model (3). The findings of Ullah et al. (1) support the hypothesis that AI is indeed a more complete ablation descriptor than FTI is. Their results agree with our own study where absence of late pulmonary vein (PV) reconnection (assessed on protocol-mandated repeat procedures) was more strongly correlated with the minimum AI achieved in a given segment than with the minimum FTI (4). Whereas the clinical efficacy of AI-guided ablation (VISTAX [Evaluation of Ablation Index and VISITAG]; NCT03062046), and the incidence of late PV reconnection with this approach (PRAISE [Pulmonary Vein Reconnection Following Ablation Index-guided Ablation: a Success Evaluation]; NCT02628730) is still under evaluation, it is highly likely that physicians will start using AI as a composite measure of lesion size in real time in preference to FTI or CF.
Surround Flow Irrigation: Efficacy Versus Safety
The “pepper-pot design” of the STSF catheter tip with 56 tiny holes produces homogeneous irrigation of the electrode, which results in a greater proportion of the delivered power contributing to the resistive heating of the tissue, rather than being wasted in the blood stream. A recent study using the STSF catheter for PV isolation suggests good efficacy and low rate of complications while requiring around 0.5 l less saline for irrigation per case (5). A note of caution is warranted here: improvements in PV isolation efficacy are not difficult to demonstrate when one benchmarks against the 60% to 70% historical single-procedure success rates. On the other hand, an increase or decrease in serious complications is much harder to document and requires a longer period of observation. The field of AF ablation is replete with examples of technologies whose efficacy was demonstrated in earlier reports, but whose potential for harm only came to light after the products were rolled out widely (6). In that context, to assess the pros and cons of STSF critically, we should compare the data for the SF and the Navistar ThermoCool (TC) catheters (Biosense Webster), as these are the direct predecessors of, and share the same irrigation platforms as, the STSF and the ST catheters, respectively.
Moreno et al. (7) showed in an animal model that with identical RF settings (30 W at power-controlled delivery setting for 30 s using 10 g force), lesions with the SF catheter were 25% larger than with the TC catheter (6). In experiments using infrared camera thermal evaluation, the SF catheter was shown to focus its maximum calorific effect on deeper tissue (8.1 ± 1.6 mm) as compared to the TC catheter (5.3 ± 3.2 mm) (7). Indeed, several groups reported a higher incidence of unpredictable steam pops and esophageal injury with SF, including fatal cases of atrio-esophageal fistula (7). A field safety notification issued in June 2013 highlighted a 4-fold higher rate of perforation with the SF, compared with the TC catheters (8). The hope now is that the addition of CF-sensing capability to the SF platform will translate to better safety. However, given the difference in lesion characteristics that Moreno et al. (7) described between SF and TC catheters, it would be unwise to assume that the FTI and AI targets that have been derived with the ST catheter (2,4) will apply to STSF too. Thus, many operators may be tempted to guide their STSF lesions using ID. Here, too, the findings of Ullah et al. (1) should prompt caution: they found that ablations delivered by the STSF catheter had not only lower ID per AI or FTI as compared to the ST catheter, but that the ID plateaued sooner too. This suggests a risk of overablation if we use the ID targets derived from our experience with ST to guide STSF ablation.
So where does this leave us regarding the STSF catheter? Because its unique irrigation characteristics result in poor correlation between tip and tissue temperatures, it is mandatory to use this catheter in the power control mode. Many operators also prudently lower the delivered powers. In the absence of validated ID, temperature, FTI, or AI targets with the STSF catheter, at least for now, operators will need to weigh up its potential for saving 0.5 l of irrigated fluid per case against the wealth of experience and research data available for ST. Undoubtedly, these data for STSF will emerge in the coming years. Meanwhile, I for one will be reserving the STSF catheter preferentially for those ventricular-based ablations where there is the need for creating larger lesions and fewer perils in doing so.
↵∗ Editorials published in JACC: Clinical Electrophysiology reflect the views of the authors and do not necessarily represent the views of JACC: Clinical Electrophysiology or the American College of Cardiology.
Dr. Gupta has received research funding and fellowship support from Biosense Webster.
The author attests that he is in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the JACC: Clinical Electrophysiology author instructions page.
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