In the first part of our study, we analyzed freezing characteristics of two different CB ablation systems (AFA-Pro and POLARx) for the treatment of AF and their possible implications on performance, acute ablation success, and safety, utilizing a propensity score-matched patient cohort. In a second part, we provide prospectively evaluated troponin measurements from the left atrium as quantifiable marker of myocardial tissue damage during the ablation procedure.

One of the notable observations from this study are the distinct differences in the freezing characteristics between the AFA-Pro and POLARx CBs. In line with previous studies [9, 11,12,13,14,15], POLARx achieved significant lower minimal temperatures in all PVs and exhibited a more rapid cooling rate compared to AFA-Pro, as evidenced by a more substantial mean temperature decrease observed within the 20 to 40-s interval. A greater AUC below 0 °C might also indicate a superior ability to sustain sub-zero temperatures. However, the pivotal question remains whether these improved freezing properties have practical meaningful implications on CB selection for PVI.

To date, several predictors regarding freezing characteristics have been identified for durable and efficient lesion formation during cryoablation. Nadir temperatures of − 53.5 °C [13] or even − 56 °C [16] for POLARx are independent predictors of acute and sustained (after a waiting period and adenosine testing) PVI. While thawing times of > 10 s to 0 °C were acceptable for the AFA [17], times of up to > 17 s are desirable for POLARx according to Iacopino and colleagues [16]. Accordingly, the minimum temperatures and thawing times measured in our work should indicate promising and effective isolation. Furthermore, an early attainment (< 60 s) of a temperature of at least − 40 °C predicts durable lesions during cryoablation with AFA [18, 19]. Although in our analysis both CBs reached − 40 °C in less than 1 min, the POLARx catheter reached this temperature significantly faster (median 31–33 s depending on PV). The need for adaptation of previously established predictors to accommodate the lower nadir temperatures and cooling profile of the POLARx CB suggests that the effective range of the catheter’s action spectrum has essentially been reduced to lower temperature levels.

The procedural characteristics presented, including the duration of the ablation procedure, amount of contrast medium used, and fluoroscopy time, were similar between the two CBs, indicating comparable procedural complexity and efficiency, but also feasibility. Although some studies reported longer procedure and fluoroscopy times, and higher contrast agent consumption with POLARx [9, 20], these differences may be attributed to a learning curve. Recent evidence suggests that as more experience is gained with POLARx, reports of comparable procedure characteristics to AFA-Pro are increasingly emerging [10, 11, 14].

Our results demonstrate that both AFA-Pro and POLARx achieved high rates of acute ablation success, with excellent isolation of PVs. The overall success rates for PVI, as well as the rates of first-pass isolations, were comparable between the two CB systems, indicating similar effectiveness in achieving the primary endpoint of AF ablation therapy. These findings are consistent with previous studies that have demonstrated the high acute success rates of AFA-Pro [7, 21, 22] and POLARx [8,9,10, 20, 23], but further studies are required to evaluate the long-term efficacy of POLARx.

A TTI < 60 s represents an additional powerful predictor of durable PVI [17, 19]. Our study did not identify any significant variations in the rate of TTI recordings or the actual TTI values between both CBs for all PVs. Of note, the temperatures reached at isolation were significantly lower with POLARx; however, the unaltered TTI suggests that these differences may not be clinically relevant.

The observed differences in freezing characteristics of POLARx, as compared to AFA-Pro, have become a focal point in contemporary electrophysiological research. Notably, several comparative studies [10, 11, 13,14,15, 23] have reported no significant discrepancies in key parameters such as minimal esophageal temperature, TTI, and procedural complications, specifically concerning phrenic nerve palsy. These findings have intensified the debate regarding whether the distinct freezing characteristics of POLARx are an actual physiological phenomenon or an artifact of measurement. Initial hypotheses proposed various explanations for these observations. Moser et al. [20] highlighted potential factors such as the lower internal balloon pressure of POLARx, differences in balloon expansion, and the more flexible design of the POLARSHEATH. Concurrently, Creta et al. [10] postulated that the unique material properties of POLARx might facilitate the formation of more antral oriented lesions. Significant insights were provided by Knecht et al., who dissected both ablation catheters and revealed noteworthy differences. Their findings included an altered positioning and injection orientation of the nitrous oxide injection coil in the POLARx, coupled with a reduced distance between the thermocouple and the gas outflow. Additionally, the nitrous oxide flow rate during the freezing process was higher in POLARx (7800 sccm) compared to AFA-Pro (7200 sccm), as detailed in the study by Guckel et al. [24]. Further elucidating this topic, Hayashi et al.’s work [25] in a porcine model provided pivotal data: by directly measuring myocardial tissue temperature, they demonstrated significantly lower temperatures during cryoablation with POLARx (− 58.4 °C ± 5.9 °C) as opposed to AFA-Pro (− 41.5 °C ± 4.9 °C, p < 0.001). This finding in a biological setting offers a crucial perspective on whether the technical distinctions between the two catheters translate to actual differences in tissue temperature during ablation.

In terms of safety, both CB systems demonstrated a high level of procedural safety with low complication rates that are in line with previous studies [9, 12, 23]. Although POLARx developed significant lower nadir temperatures, our analysis revealed no difference in minimal esophagus temperature or rate of phrenic nerve injuries. In our study, use of the DMS in combination with the POLARx CB did not reduce phrenic nerve palsy, and thus, there is currently no evidence in the literature supporting the clear benefit of the DMS. Further investigations are required to assess its potential advantages.

In recent years, the field of electrophysiology has witnessed substantial progress, particularly in the realms of ablation techniques and energy sources. Despite these advancements, a significant gap persists in our ability to directly evaluate the efficacy, durability, and overall success of lesions created during PVI. In this context, troponin, a biomarker typically released following cardiomyocyte injury, emerges as a potential candidate for assessing the acute impact of ablation procedures and quantifying thermal tissue damage. Previous investigations have primarily focused on troponin measurements following radiofrequency (RF) ablation, examining correlations with cumulative RF energy, duration of energy application, lesion size, and lesion type (linear vs. focal) [26,27,28]. Subsequent studies have compared troponin release across different energy sources, notably RF and CB ablation, with markedly diverse findings. Some researchers reported higher troponin levels post-RF ablation [29, 30], while others observed elevated levels following CB ablation [31] or found no significant differences [32]. These studies’ comparability is hindered by variations in the ablation techniques employed, the specific troponin assays used, and the timing of blood sample collection. Our study uniquely compares troponin levels between two catheters employing the same ablation technique in patients with AF. We aimed to quantify and compare myocardial injury following AFA-Pro and POLARx ablation. We observed median troponin levels of 63 ng/L for AFA-Pro and 118 ng/L for POLARx. Prior studies reported varying high-sensitive troponin levels: 850 ng/L after 4 h [33], 806–840 ng/L after 18–24 h [32], and up to approximately 1500 ng/L after 1 day [34]. The lower troponin values in our study could be attributed to the immediate post-ablation blood sample collection, aligning with the short-term collection reported by other authors (151 pg/mL by Lermoine et al. [35], 168 pg/mL by Scherschel et al. [36]). Furthermore, our study’s unique approach of drawing blood directly from the cryoballoon sheath within the left atrium contrasts with the peripheral blood sampling in previous studies, possibly contributing to the observed troponin value differences. Interestingly, we noted a trend towards higher post-ablation troponin levels in the POLARx group, along with a numerically higher troponin delta. These results might indicate a greater degree of myocardial injury attributed to the altered freezing characteristics of the POLARx CB, and statistical significance might have been missed due to the low number of patients. Despite these observations, no impact on clinical endpoints, such as feasibility, performance, efficacy, and safety, was apparent. The relationship between troponin release, long-term outcomes, and procedural safety remains a subject of debate and warrants further investigation. In our study, no increase in procedural complications was observed in patients with elevated troponin levels, although the small patient population for troponin determinations limited the study’s power in this aspect.