Author + information
- Published online April 16, 2018.
- aTulane University School of Medicine, New Orleans, Louisiana
- bBioMIT, Electronic Engineering Department, Universitat Politècnica de València, Valencia, Spain
- cCardiac Arrhythmia Service, Hospital SOS Cardio–Florianópolis, Florianópolis, Brazil
- ↵∗Address for correspondence:
Dr. Andre d’Avila, Cardiac Arrhythmia Service, Hospital SOS Cardio, Rodovia SC 401, 121, 88030-000 Florianópolis, SC/Brazil.
The field of catheter ablation has evolved significantly over the last 2 decades. Technological advances directed at increasing efficacy such as irrigation and contact force have also affected the safety of ablation procedures. A clear understanding of the potential untoward outcomes of such catheter-based therapies is critical; this is true for all procedures but particularly important when the goal of the procedure is to restore quality of life. Steam pop (SP) refers to the audible sound produced by intramyocardial explosion when tissue temperature reaches 100°C, leading to the production of gas. Coagulation necrosis starts above 50°C, but as higher tissue temperatures are achieved, steam formation within the myocardium ensues. SP results from the intratissue gas formation that cannot diffuse smoothly through cardiac tissue away from the heated zone: if gas formation happens too fast leading to sufficient pressure buildup, SP is likely to occur (1,2).
SPs are relatively infrequent (0.1% to 1.5%) but represent a potentially severe complication of radiofrequency ablation (RFA), as it has been associated with embolic stroke, cardiac perforation, and ventricular septal defect. In this context, technology aimed at preventing this potentially life-threatening complication is clinically desirable (2).
Biophysical variables as predictors of SP have been the subject of study for 2 decades. One initial logical assumption was that high impedance at the beginning of RF application could predict SPs. Unfortunately, several investigators have failed to correlate initial impedance with SPs (3).
Alternatively, ablation catheters with the ability to visualize near-field echogenic signals are capable of identifying increased signal intensity immediately before SP (4). Chik et al. (5) reported on the use of acoustic signals generated during RF application that are detected with a hydrophone in an in vitro experimental model are also able to predict the occurrence of SP. In addition, Ikeda et al. (6) found a direct relationship between increasing CF and development of SP in a canine model. Moreover, a higher signal intensity by intracardiac echocardiography (ICE) can be detected immediately before SP.
As most of these tools are not available for clinical use, electrophysiologists have been intuitively monitoring impedance changes to either predict the occurrence of SPs or the risk of clot formation during RF delivery (fast impedance drop with subsequent impedance rise). The decision to interrupt RF application based on impedance changes is grounded on few important principles:
1. RF lesions are caused by resistive heating, which depends on the electrical resistivity of the tissue and is inversely related to its water content;
2. At the beginning of heating, tissue impedance drops due to increase mobility of ions, which are electrical charge carriers in biological tissues;
3. No thermal lesion occurs without impedance fall and the magnitude of the drop depends on tissue temperature and the amount of cardiac tissue that is heated up to a given temperature; note that impedance drop during bipolar applications is roughly twice as much as during unipolar delivery despite similar tissue temperatures. Thus, the higher the tissue temperature and the faster it increases, the earlier and quicker the impedance drops;
4. If temperature increases too fast (resulting in a fast and significant impedance drop), vaporization with the subsequent gas formation may occur in some areas of the ablated tissue;
5. As gas is almost an electrical insulation, its presence within the ablated tissue will result in an increase in the measured total impedance.
Although apparently reasonable to interrupt RF delivery when impedance is reducing to “too fast and too much,” a precise definition of cutoff values to guide that decision and effectively prevent SP has been lacking thus far (7). Hence, most of the assessment regarding RF delivery interruption has been subjective, “visual,” based on the impedance curve, rather than in a numeric value: probably a very sensitive criterion with very low specificity. Likewise, a fast change in the echogenicity of the ablated tissue may herald high risk of SP but such modifications have been difficult to quantify.
In this issue of JACC: Clinical Electrophysiology, Nguyen et al. (8) report on the outcomes of ex vivo and in vivo experiments designed to assess echocardiographic and biophysical predictors of SPs. In an ex vivo bovine myocardium model, open-irrigated RFA was applied at 50 W for 60 s; the investigators compared ICE for RFA with and without SPs. Using an in vivo porcine model, open-irrigated RFA was applied at 50 W for 60 s, and RFA parameters of SPs were reported. They also conducted a retrospective analysis of recorded SPs during clinical ablation procedures over a 1-year period. They found that for RFA SPs, there was 32% greater ICE tissue echogenicity than for RFA without SPs (p < 0.001). In addition, RFA SPs had more rapid increases of tissue echogenicity, particularly in the last 5 s before SP. Compared with RFA without SPs, RFA SPs had larger impedance reductions (33 ± 16.0 Ω vs. 23 ± 10.8 Ω). SPs were also associated with more rapid initial impedance reduction (1.4 Ω/s vs. 0.38 Ω/s for RFA without SPs). Clinical SPs during ablation procedures had a significantly faster impedance reduction during the first 5 s of ablation, compared with matched control ablations (15.7 ± 6.7 Ω vs. 8.1 ± 4.7 Ω). They adequately concluded that echocardiographic and biophysical parameters during open-irrigated RFA are associated with increased SP risks. They propose using a discrete cutoff of >9% reduction in baseline impedance within the first 5 s of RF application, to predict subsequent SP formation, that should result in an immediate reduction of the power delivered during the radiofrequency application. Interestingly, they also failed to identify initial impedance as a predictor of SPs even though the initial impedance was higher (130.0 ± 21.2 Ω vs. 111.0 ± 10.7 Ω; p = 0.002) in RFA resulting in SP. It is unclear why the initial impedance was so variable in their bench study because contact force was well controlled. It may be related to biological dispersion in tissue electrical conductivity.
There are few limitations here. The reported parameters resulting in SP in the experimental models may not be attainable during ablation in human tissue, especially considering that most conclusions derived from bovine ventricular myocardium and porcine thighs. Atrial fibrillation ablation is the most common ablated arrhythmia and these models are unalike to human atrial tissue. Nevertheless, the observations by Nguyen et al. have very important clinical implications and deserve immediate attention. As human eyes and reflex time may not be enough to detect these changes and act hasty to impede SP, new numeric criteria that can be automated and incorporated to RF generators and ICE devices can be very helpful hopefully resulting in safer cardiac ablation strategies.
Analogously to the assessment of efficacy of RF applications, prevention of SP requires continued monitoring of multiple parameters including tissue temperature, tissue echogenicity, and impedance reduction. It is clear that all team members in the electrophysiology laboratory, not only the primary operator, ought to be familiar with the previously mentioned variables to be able to assist in the prevention of complications. Similarly, while the use of ICE is not standard of care for all catheter ablations, published data and clinical experience demonstrate its tremendous utility to define anatomy, catheter position, and monitor for complications including ultrasonic evidence of gas formation. As we accrue more experience with additional technologies for catheter-based RFA, it seems clear that no single factor will be able to predict the formation of SP; a combination of carefully monitored parameters is probably the most efficient strategy to prevent the dreadful “STEAM POP” as proposed by Nguyen et al.
↵∗ 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.
The authors have reported that they have no relationships relevant to the contents of this paper to disclose.
All authors attest they are 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.
- 2018 American College of Cardiology Foundation
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