Ablation characteristics associated with steam pops in ablation index-guided radiofrequency ablation of atrial fibrillation

Abstract

Graphical Abstract

Open in new tabDownload slide

Introduction

Radiofrequency ablation of atrial fibrillation (AF) has been established as an effective therapy for maintaining sinus rhythm in patients with symptomatic AF.^1,2 During radiofrequency (RF) ablation, myocardial tissue in contact with the catheter is damaged by resistive heating, while surrounding and deeper tissues are affected by conducted heat. The extent of the thermal damage depends on the heating duration and tissue temperature.³ Steam pops, a rare occurrence during RF ablation, typically arise when tissue temperature significantly increases. Insufficient steam diffusion and subsequent accumulation within the heated area can lead to increased steam pressure, potentially resulting in steam pops.⁴ This phenomenon may result in serious, life-threatening complications such as cardiac perforation, cardiac tamponade, or ventricular septal defect. It is crucial to identify the factors associated with steam pops to prevent these life-threatening outcomes. This study aims to investigate the ablation characteristics associated with the audible steam pops during ablation index-guided RF catheter ablation of AF.

Methods

Patients’ population

This study included 3263 patients who underwent de novo RF catheter ablation of AF at Beijing Anzhen Hospital, Capital Medical University, from July 2023 to January 2024. Written informed consent was obtained from all participants prior to the procedure. This study was approved by the institutional review board.

Study procedure

Left atrial thrombi were excluded by intracardiac echocardiography or transoesophageal echocardiography as an alternative option in patients with a high risk of thrombosis. All antiarrhythmic drugs (except amiodarone) were discontinued at least five half-lives before the procedure. Non-vitamin K antagonist oral anticoagulants or warfarin were used for anticoagulation, and anticoagulation was not interrupted during the perioperative period. Patients taking warfarin were required to maintain the international normalized ratio between 2.0 and 3.0, and there was no need to discontinue warfarin before the procedure. Propafenone was routinely administered for paroxysmal AF, while amiodarone was regularly applied for persistent AF for at least 2–3 months post-procedure.

The electrophysiological study and ablation procedure were performed under conscious sedation with fentanyl and midazolam. A coronary sinus (CS) electrode (Abbott Medical, Abbott Park, IL, USA) was positioned in the CS via left femoral vein access. The SL1 Swartz sheath (Fast-Cath^TM, Abbott Medical) was inserted through the right femoral vein and introduced into the left atrium after a single successful transseptal puncture. The activated coagulation time was maintained between 300 and 350 s through intravenous administration of heparin. After a successful atrial septal puncture, a high-density mapping catheter (PentaRay Catheter, Biosense Webster, Irvine, CA, USA) was used to reconstruct left atrial geometry guided by the three-dimensional electroanatomic mapping system (CARTO 3; Biosense Webster). A 3.5-mm cool saline-irrigated ablation catheter with 56 holes (THERMOCOOL SMARTTOUCH Surround Flow Catheter, Biosense Webster) in conjunction with the SMARTABLATE generator (Biosense Webster) was used for ablation under Carto system guidance. The tool of automatic lesion tagging (VisiTag®, Biosense Webster) was employed with the following settings: lesion size of 4 mm or 6 mm, 2.5 mm stability for 3 s. The automatic cut-off of spike impedance was set to < 30 Ω/0.5 s. Point-by-point ablation was performed in a power-control mode with a temperature limit of 43°C and a flow rate of 15 mL/min. An intertag distance ≤ 6 mm was targeted.⁵ Radiofrequency energy was set with power outputs of 25 W in the CS, 35–40W in the roofline and tricuspid isthmus (CTI), 35–45 W in the mitral isthmus (MI), and 40–50W in pulmonary veins (PVs). The ablation index is an algorithm that integrates catheter contact force, ablation power, and ablation duration. Ablation was guided by ablation index target values for each lesion as follows: 450–550 for anterior/roof segments of the PV, 350–400 for posterior segments of the PV, 450–500 for the roofline and the CTI, and 500–600 for the MI. The power and ablation index target values were adjusted at the physician’s discretion.

Patients with paroxysmal AF underwent bilateral circumferential PV antral ablation. Patients with persistent AF first underwent ethanol infusion in the vein of Marshall (EI-VOM), followed by bilateral PV isolation and linear ablation across the roofline, MI, and CTI. Ablation was also performed in the distal CS and complex fractionated atrial electrograms (CFAEs). If AF persisted after ablation, direct current cardioversion was performed to restore sinus rhythm.⁶ The EI-VOM procedure was performed as previously described.⁷ The endpoint of the circumferential PV antral ablation was the achievement of a complete entrance block of all PVs recorded by the high-density mapping catheter during sinus rhythm. The endpoint of linear ablation was bidirectional block of the ablation lines.

Comparison of steam pops parameters

Steam pops are defined as sudden bursting sounds audible during the ablation, often accompanied by transient oscillations of the catheter.⁸ To describe the locations of steam pops, the lesion line encircling the ipsilateral PVs was divided into six segments: roof, bottom, antero-superior, antero-inferior, postero-superior, and postero-inferior (Figure 1).⁹ Radiofrequency was stopped immediately upon the detection of a steam pop by the physician, regardless of whether the predetermined ablation index value had been reached. The location of a steam pop occurrence, the orientation of catheter application, the characteristics of the sound, and the presence of catheter oscillations were prospectively recorded by the physician. Ablation parameters were also documented, including RF duration, power, contact force variations, temperature, initial impedance, impedance drop (Δimpedance), and ablation index values. The impedance drop was defined as the difference between baseline and minimum impedance observed during the RF application. Radiofrequency catheter ablation was resumed after ruling out pericardial effusion using intracardiac or transthoracic echocardiography. In cases of pericardial tamponade, emergency pericardiocentesis was performed immediately.

Figure 1

The distribution of steam pops. Red dots represent the sites of steam pops. PA, posterior–anterior view of the left atrium; LAA, left atrial appendage; LSPV, left superior pulmonary vein; LIPV, left inferior pulmonary vein; RSPV, right superior pulmonary vein; RIPV, right inferior pulmonary vein; MAI, mitral isthmus; CFAEs, complex fractional atrial electrograms; CS, coronary sinus; TA, tricuspid annulus; CTI, tricuspid isthmus; IVC, inferior vena cava.

Open in new tabDownload slide

Two ablation sites adjacent to the location of steam pops point in opposite directions were selected as the control group, and the ablation parameters of the control group were recorded.

Statistical analysis

All statistical analyses were performed with SPSS 26.0 and R 4.0.2. Continuous variables normally distributed are presented as mean ± standard deviation or as median (first quartile, third quartile). Continuous data were compared using Student’s t-test or Mann–Whitney U test according to the distribution of the data. Categorical variables are presented as frequencies and percentages. Multivariate binary logistic regression analysis was used to evaluate the procedural predictive factors of steam pops. The area under the curve (AUC), sensitivity, specificity, and cut-off values of predictive factors were calculated by receiver operating characteristic (ROC) curve analysis. A two-tailed P-value < 0.05 was considered statistically significant.

Results

Patient characteristics

A total of 81 steam pops occurred in 71 of the 3263 patients during the ablation. The baseline clinical characteristics of these 71 patients are presented in Table 1. The mean age was 61.0 years. The average duration of AF was 37.5 months, 22 (31.0%) patients had paroxysmal AF, 34 (47.9%) had hypertension, 7 (9.9%) had diabetes, 9 (12.7%) had coronary artery disease, 16 (22.5%) had heart failure, and 10 (14.1%) had a history of cerebral infarction.

Incidence and distribution of steam pops

A total of 3263 (100%) patients completed circumferential PV ablation, with all PVs successfully isolated. During the circumferential PV ablation, 47 (1.4%) steam pops were recorded, of which 37 (1.1%) occurred in the left PV and 10 (0.3%) in the right PV. Among the patients undergoing linear ablation, 1378 patients underwent MI ablation, 1749 patients underwent CTI ablation, and 1506 patients underwent ablation of the left atrial roofline. Complete block of the CTI and roofline was achieved in all the cases, while MI line block was not achieved in 236 patients (17.1%). Additionally, 440 patients underwent ablation of CFAEs, and 561 patients underwent CS ablation. Steam pops were observed at various ablation sites: 6/1378 (0.4%) at the MI, 16/1749 (0.9%) at the CTI, 7/1506 (0.5%) along the roofline, 3/440 (0.7%) at CFAEs ablation, and 2/561 (0.4%) during the CS ablation. Ethanol infusion in the vein of Marshall was performed on 722 patients. Following EI-VOM, steam pops occurred in 4/722 (0.6%) cases during MI ablation, and steam pops occurred in 2/656 (0.3%) cases during MI ablation without EI-VOM (P = 0.483). The anterior–superior segment of the left PV and the inferior vena cava side of the CTI were identified as the most prone locations for steam pops. The distribution of the steam pops is illustrated in Figure 1.

Characteristic of steam pops

Among the 81 audible steam pops, 21 (25.9%) were characterized by a muffled sound, while 60 (74.1%) presented with a crisp sound. The volume of steam pops varied, with 17 (21.0%) being loud enough for both the physician and the assistant to hear, while the remaining 64 (79.0%) were audible only to the physician. The physician distinctly perceived catheter vibrations in 52 (64.2%) cases during the steam pops. In 36 (44.4%) cases, the catheter tip was perpendicular to the endocardium during ablation, while in another 45 (55.6%) cases, the catheter tip was parallel to the tissue.

Among the 81 steam pops, 15 (18.5%) occurred within the first 10 s during ablation, 41 (50.6%) occurred between 10 to 20 s, and the remaining 25 (30.9%) occurred beyond 20 s. These steam pops were labelled as a0–9, b0–9, c0–9, d0–9, e0–9, f0–9, g0–9, h0–9 and i0 in ascending order of ablation time. Notably, the longest ablation time was 40.6 s. Point a0 had the highest ablation pressure of 87 g. Point i0 reached the maximum ablation index value of 565. Point b5 had the highest impedance of 220 Ω, while point e9 had the lowest impedance of 83 Ω. Lastly, point a1 had the largest Δimpedance of 61 Ω. The ablation parameters for all steam pops are illustrated in Figure 2.

Figure 2

The ablation parameters of steam pops. The changes in ablation parameters for all steam pops. (A) RF duration. (B) RF power. (C), AI value. (D) Minimum temperature. (E) Maximum temperature. (F) Average temperature. (G) Minimum impedance. (H) Maximum impedance. (I) Impedance drop. (J) Minimum contact force. (K) Maximum contact force. (L) Average contact force. RF, radiofrequency; AI, ablation index.

Open in new tabDownload slide

One case (point g1) of pericardial tamponade occurred following a steam pop. The patient was a 77-year-old male undergoing RF ablation for persistent AF. A steam pop with a crisp sound was observed during the ablation of the MI after EI-VOM. The patient’s blood pressure dropped from 120/70 mmHg to 55/35 mmHg, and a moderate amount of pericardial effusion was detected on intracardiac echocardiography. Pericardiocentesis was performed immediately, retrieving 180 mL of haemorrhagic pericardial effusion. The patient received 5 g of intravenous idarucizumab to reverse the effects of dabigatran and 25 mg of intravenous protamine to neutralize heparin. The activated coagulation time decreased from 586 to 108 s. After 30 min of continuous intracardiac echocardiography monitoring, no further increase in pericardial effusion was noted, and the patient’s blood pressure stabilized at approximately 110/60 mmHg. Bilateral PV isolation was completed, though MI was not blocked. No pericardial effusion was detected on echocardiography before discharge. The patient maintained sinus rhythm, and cardiac ultrasound showed no significant abnormalities during the follow-up.

Comparison of ablation parameters

Five steam pops were identified as isolated ablation points without adjacent sites, while 152 adjacent non-steam pops sites were included as the control group. The comparison of ablation parameters between steam pops sites and adjacent non-steam pops sites is listed in Table 2. The ablation power, average temperature, maximum temperature, minimum temperature, average contact force, maximum contact force, minimum contact force, minimum impedance, maximum impedance, and ablation index value were similar between the steam pops sites and adjacent non-steam pops sites. However, the Δimpedance at steam pops sites was significantly higher than adjacent non-pop sites. The RF energy delivery duration was significantly shorter in the steam pops group, as RF application was immediately halted upon the occurrence of a steam pop.

Risk factors for steam pops occurrence

Logistic regression analysis identified the Δimpedance within the first 3, 5, and 10 s and the total Δimpedance as independent risk factors for steam pops during RF ablation of AF. The ablation power, temperature, contact force, maximum impedance and minimum impedance cannot predict the occurrence of a steam pop during ablation (Figure 3). Receiver operating characteristic curve analysis determined the optimal cut-off values for predicting steam pops: 9.5Ω for Δimpedance within the first 3 s [sensitivity 73%, specificity 57%; AUC = 0.70, 95% confidence interval (CI) 0.63–0.78], 10.5Ω for Δimpedance within the first 5 s (sensitivity 68%, specificity 61%; AUC = 0.71, 95% CI 0.64–0.79), 13.5Ω for Δimpedance within the first 10 s (sensitivity 72%, specificity 60%; AUC = 0.71, 95% CI 0.63–0.79), and 18.5Ω for the whole Δimpedance (sensitivity 89%, specificity 40%; AUC = 0.66, 95% CI 0.59–0.74, Figure 4).

Figure 3

Forest plot of logistic regression analysis of ablation parameters. RF, radiofrequency.

Open in new tabDownload slide

Figure 4

ROC curve analysis. (A) ROC curve analysis for Δimpedance in the first 3 s to predict the occurrence of steam pops (AUC was 0.70, 95% CI 0.63–0.78). (B) ROC curve analysis for Δimpedance in the first 5 s to predict the occurrence of steam pops (AUC 0.71, 95% CI 0.64–0.79). (C) ROC curve analysis for Δimpedance in the first 10 s to predict the occurrence of steam pops (AUC 0.71, 95% CI 0.63–0.79). (D) ROC curve analysis for whole Δimpedance to predict the occurrence of steam pops (AUC 0.66, 95% CI 0.59–0.74). AUC, area under the curve.

Open in new tabDownload slide

Discussion

This study included 3263 patients and is currently the largest prospective observational investigation into the characteristics of steam pops during RF ablation of AF. The incidence of steam pops was 2.5% with a complication event of pericardial tamponade. Ethanol infusion in the vein of Marshall was not associated with a significant increase in the risk of steam pops during MI ablation (P = 0.483). The most common locations of steam pops were the anterior–superior segment of the left PV and the inferior vena cava side of the CTI. Among all parameters evaluated during RF ablation of AF, the Δimpedance within the first 3, 5, and 10 s and the whole Δimpedance were identified as independent risk factors for steam pops.

Over the past few decades, there has been significant progress in RF ablation of arrhythmias with higher rates of success and lower complications.^1,2,10 Pericardial tamponade is one of the major complications of RF ablation. We are concerned about steam pops due to their potential risk of leading to pericardial tamponade. The mechanism of steam pops involves the rapid accumulation of vapour between the electrode and the tissue, leading to the sudden occurrence of a steam pop.³ Intracardiac echocardiography observed a continuously increasing echogenic bubble at the catheter–tissue interface prior to the occurrence of a steam pop. This bubble suddenly expanded into a spherical shape, followed by a steam pop within a few seconds.¹¹ In a piglet heart model, intracardiac echocardiography revealed an endocardial void measuring 15 mm in diameter and 8 mm in depth following a steam pop.¹² Eight weeks after ablation, significant coagulation necrosis was still observed in the middle layer at the site of steam rupture.¹³

Theis et al. enrolled 226 patients with AF who underwent ablation with THERMOCOOL Surround Flow catheter. This catheter lacked contact force sensors, and the maximum ablation power was 30W. A total of 59 audible steam pops were recorded during the ablation.⁸ In contrast, contact force-sensing catheters and high-power ablation with a maximum power of 50 W were used in this study. Tian et al.⁹ reported the steam pops incidence of 1.57% (11/701), while Di et al.¹⁴ reported 2.9% (15/516), with both studies using THERMOCOOL SMARTTOUCH Surround Flow catheter. However, these studies only included steam pops during PV isolation and did not investigate steam pops during linear ablation. In contrast, we used the contact force-sensing catheter for high-power ablation guided by ablation index and integrated data from three-dimensional VisiTag. This approach is more representative of current practices in AF ablation.

Our results indicate that the left superior PV is the most susceptible location for steam pops, particularly at the anterior edge of the left superior PV. During the ablation of the left superior PV, the catheter tip is often positioned on the PV side of the ridge to enhance stability. This placement results in higher tissue coverage at the catheter tip, which increases the likelihood of steam pops. Positioning the catheter tip at the edge of the ridge may reduce this risk.^9,15 Research on steam pops in the CTI remains limited, with most available data being case reports. It was found that the part of CTI near the inferior vena cava was a frequent site for steam pops. This increased risk may be associated with the Eustachian ridge on the inferior vena cava side, which often requires an inverted U-shaped catheter position for stability. Luo et al.¹⁶ demonstrated in porcine hearts that myocardium with pouch structures exhibits higher impedance, making it more susceptible to steam pops. In conclusion, physicians should exercise caution when performing ablation in the left superior PV and the CTI near the inferior vena cava, as these areas carry a higher risk for steam pops.

Ethanol infusion in the vein of Marshall has been shown to be beneficial for AF ablation.¹⁷ However, it remains uncertain whether myocardial oedema and low-voltage scar areas after EI-VOM increase the risk of steam pops. In our study, six pop events were recorded during MI ablation, with four occurring after EI-VOM. There was no significant difference of the incidence of steam pops in the EI-VOM group compared with the non-EI-VOM group. Notably, the number of steam pops in the MI area was relatively low and no ablation was performed in low-voltage and scar areas. Further research with larger sample size is needed to delineate the relationship between EI-VOM and steam pops incidence.

Predicting and preventing steam pops remains a significant challenge, as they can only be anticipated through characteristic changes in ablation parameters. Previous studies have confirmed that an Δimpedance > 12Ω is a predictive factor for steam pops occurrence, while other research has suggested that an impedance rise > 15Ω or an Δimpedance percentage increase > 15% is associated with a higher risk of steam pops.^9,16,18 Additionally, another study indicates that a rapid decrease in impedance within the first 5 s of ablation is linked to steam pops formation. It has been recommended that an impedance reduction >9% within the first 5 s of RF application be used to predict subsequent steam pops.¹⁹ Previous studies have derived their findings from in vitro animal models or comparisons between patients with and without steam pops. Our study compares the ablation parameters between steam pop sites and adjacent non-steam pop sites. It confirms the optimal cut-off points of Δimpedance for predicting steam pops. These findings provide valuable data for the future development of RF devices aimed at reducing the incidence of steam pops by setting Δimpedance cut-off value. Beyond impedance changes, sharp increases in potential, temperature, and abrupt fluctuations in contact pressure recorded by the catheter are also predictive factors for steam pops.¹⁴ It has been established that areas with relatively thin myocardium are at greater risk for steam pops during ablation, and reducing RF energy to minimize Δimpedance may help mitigate this risk.²⁰ There is a significant difference in RF energy delivery duration between the steam pops and the non-steam pops sites in this study. Due to the early occurrence of steam pops, the ablation process often terminates before reaching the planned ablation index value, resulting in a shorter ablation duration.

The currently used open-irrigated ablation catheters with a 56-hole porous tip have effectively reduced the risk of coagulum formation and steam pops.²¹ However, the surround flow catheter cannot monitor intramural tissue temperature, and the catheter tip temperature remains stable due to saline irrigation, which limits its capacity to provide adequate feedback from the ablated tissue to prevent steam pops. A novel irrigation catheter, the QDOT MICRO™, has been developed to enable surface temperature-controlled ablation. In a porcine model, temperature-controlled ablation using the QDOT MICRO™ demonstrated a complex temperature feedback mechanism associated with a reduced incidence of steam pops.²² An RF ablation needle,²³ which integrates temperature and pressure sensors, allows for reliable monitoring of steam pops occurrence and sensor changes. This device holds promise for providing more comprehensive information on steam pops, potentially contributing to developing safer and more effective ablation techniques.

This study has several limitations. First, as a single-centre prospective study with consistent procedural techniques and catheter positioning methods, it may only partially capture the general characteristics of steam pops observed at other centres. However, data from 15 physicians were included, which helps reduce the impact of individual variations in ablation practices. Secondly, only THERMOCOOL SMARTTOUCH Surround Flow Catheters were used in this study. Therefore, it is unclear whether these findings can apply to other RF generators and ablation catheters. Thirdly, intracardiac echocardiography did not continuously monitor the ablation process, limiting the detection of silent or non-audible steam pops and potentially underestimating their true incidence. This also restricted the ability to describe the intracardiac echocardiography characteristics associated with steam pops. Fourth, the study could not compare the effects of different ablation powers on steam pop occurrence, as the power settings were consistent between the control and steam pop groups. Lastly, the study’s observational nature restricts the ability to establish causal relationships between ablation parameters and steam pop occurrences, underscoring the need for further experimental and clinical research to validate these associations.

Conclusions

During AF RF ablation, the incidence of steam pops was 2.5%. The anterior edge of the left superior PV and the inferior vena cava side of the tricuspid isthmus were identified as most susceptible to steam pops. ΔImpedance was an independent risk factor, the Δimpedance > 9.5 Ω within the first 3 s, the Δimpedance > 10.5 Ω within the first 5 s, the Δimpedance > 13.5 Ω within the first 10 s, and the whole Δimpedance > 18.5 Ω are predictors for steam pops occurrence. Establishing an appropriate Δimpedance cut-off value can be effective in preventing steam pops.

Clinical perspectives

This study represents the largest prospective investigation for steam pops on high-power AF ablation guided by ablation index, providing valuable insights into the characteristics of steam pop occurrence and associated ablation parameters. Notably, Δimpedance was identified as the only predictive factor for steam pops. In the future, devices can be designed based on Δimpedance to prevent steam pops.

Acknowledgements

The authors would like to sincerely thank Ming-Di Han, Yan Zhu, and the clinical specialist team for their invaluable contributions to the electrophysiological study and data collection during this study.

Funding

This study was supported by the National Natural Science Foundation of China (no. 82170310), ZHONGNANSHAN MEDICAL FOUNDATION OF GUANGDONG PROVINCE (ZNSA-2020017), and Beijing Xinlian Zhicheng Cardiovascular Health Public Welfare Foundation.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.

References

Author notes