First experience of POLARx™ versus Arctic Front Advance™: An early technology comparison

Cryoballoon ablation is an established technique to achieve pulmonary vein isolation in patients with atrial fibrillation (AF). Recently, a new manufacturer of cryoballoon achieved regulatory CE marking (POLARx™; Boston Scientific). We describe our early experience of using this new market entrant of the technology and describe procedural aspects in comparison to the incumbent Medtronic Arctic Front Advance™.


| INTRODUCTION
Catheter ablation is the most effective treatment for atrial fibrillation (AF), particularly if paroxysmal, symptomatic, and refractory to antiarrhythmic drugs. 1 Pulmonary vein isolation (PVI) represents the cornerstone of this procedure. 2 Since its introduction in 2007, cryoballoon (CB) ablation has emerged as a valid alternative to the pointto-point technique with radiofrequency for achieving PVI 3 and this reproducible and rapid technique has become established as a major technique for index procedures in paroxysmal and persistent AF ablation. Indeed, several landmark studies have demonstrated the feasibility, safety, and efficacy of the CB ablation for both paroxysmal and persistent AF. [3][4][5][6][7] Compared with radiofrequency, the CB technique offers several advantages such as shorter procedure duration, shorter learning curve and a higher degree of reproducibility. 8 Recently, a new CB technology (POLARx™; Boston Scientific) has achieved regulatory CE marking and has been introduced into the market. 9 The aim of the present study is to describe our early experience with the POLARx cryoablation system and describe procedural aspects in comparison to the incumbent Medtronic Arctic Front Advance™.

| Study design
This was a nonrandomised prospective single-center study. We analyzed clinical procedures from the first consecutive 40 PVI procedures performed using the POLARx in the United Kingdom. The procedures were performed by four operators using the novel POLARx cryoablation system (Boston Scientific). These data were compared with the 40 previous consecutive cases undergoing ablation by the same operators using the Arctic Front Advance CB (Medtronic). We systematically collected procedural metrics including skin-to-skin time, time to PVI, left atrial dwell time, fluoroscopy time and dose, nadir and isolation balloon temperatures, as well as acute efficacy and safety outcomes. All patients provided written informed consent before the procedure. The study complied with the Declaration of Helsinki and was registered and endorsed by the Barts Health NHS Trust Clinical Effectiveness Unit (registration ID: 11496). Prospective approval for the use of the POLARx system was obtained from the Barts Heart Centre New Technologies Committee. Cases where additional ablation was performed, for example, radiofrequency ablation of cavo-tricuspid isthmus, were excluded.

| The POLARx cryoablation system
The POLARx cryoablation system consists of four dedicated components 9 : the ablation console (SMARTFREEZE™), consisting of a cryoablation gas exchange and regulation system, integrated computer, cryoablation touchscreen display, and connectors; the POLARSHEATH™, a 15.5F deflectable sheath; the POLARx 28-mm balloon catheter; a pulmonary vein catheter (POLARMAP™) with eight electrodes, 20-mm loop diameter and 3F shaft diameter.
The SMARTFREEZE console was supplied to our laboratory with a control pedal, allowing a physician operator to control therapy delivery. The design of the POLARx recommended workflow is nearly identical to that of the Arctic Front Advance system, with a test inflation while the balloon is submerged in saline before introduction to the POLARx sheath specified. A novel feature of the system is an accelerometer-based adhesive diaphragm movement sensor. This provides a relative measure of diaphragm movement during phrenic pacing as an on-screen graph of instantaneous movement plus a numeric value of movement during pacing.

| Catheter ablation
Procedures were performed under conscious sedation or general anesthesia. A standardized protocol for CB ablation is in use at Barts Heart Centre, aiming to minimize interoperator variance across the department. This was used for all cases, and is provided in detail as Heparin was, thereafter, titrated to achieve an ACT of 300-350 s. The standard freezing time was 180 s; however, this was left at operator discretion depending on time to isolation, temperature lowering speed and nadir. PVI was considered to have been achieved with demonstration of entrance block. Adenosine challenge test was not routinely performed. Phrenic nerve pacing was performed during freezing of the right pulmonary veins via a quadrapolar catheter, and diaphragmatic movement and compound motor action potentials (C-MAP) monitored for phrenic compromise. A Z-suture 10 or Proglide™ (Abbott) closure was used at the end of the case for achieving femoral hemostasis. All suitable procedures were performed as day case according to our protocol, 11 with patients discharged from hospital 4 h after their procedure.

| STATISTICAL ANALYSIS
The χ 2 test and Student's t test were used for comparison of categorical and normally distributed continuous variables, respectively.
The Wilcoxon signed-rank test was employed where data were not normally distributed. Results with p < 0.05 were regarded as significant. RStudio (version 1.2.5033) was used for descriptive and inferential statistical analysis.

| Population
Forty patients underwent ablation using the POLARx system (mean age 62.8 ± 11.7, 57.5% male), with 28 paroxysmal AF, and the mean left atrial diameter was 39.3 ± 4.7 mm. Baseline demographic characteristics, with a comparison to patients undergoing ablation using the Arctic Front Advance system, are shown in Table 1.

| POLARx versus Arctic Front
We compared procedures performed with the POLARx system with a preceding 40 consecutive PVI cases performed with the Arctic Front Advance CB (Table 2). Duration and fluoroscopy use were slightly higher for the POLARx cases, which also had lower indicated nadir temperatures than Arctic Front Advance cases. Furthermore, more ablations were performed with the POLARx system, specifically for the right pulmonary veins (p = .02). Times to isolation were similar overall; however, when broken down by individual veins, isolation of the right upper pulmonary vein with POLARx required more than double the time (36 vs. 91 s; p = .06). These results are shown in Table 3.

| DISCUSSION
We present our early real-world clinical experience of the novel Subjectively, we found the system platform easy to use and the minor variances in workflow required were simple to accommodate into our standard procedural workflow. The footpedal user interface did appear to risk decreasing catheter lab team interactions, and the absence of the operator calling out instructions to "inflate," "freeze," or "stop" had a surprisingly isolating effect; we found an effort had to be made to keep other members of our cath lab team informed as to the stage, and the risks of the stage, of the procedure. Although use of the footpedal may increase operator autonomy, it may be wise to continue some functions as "console operated" by a lab assistant.
The standout procedural difference from previous technologies apparent from our study is that of indicated balloon temperature.
We consistently observed temperatures of between 5°C and 10°C below what would be indicated by a physiological effect. Indeed, it was apparent that an achieved temperature of −40°C would not necessarily be sufficient to indicate a sufficient cryodose, an observation borne out by our finding that the median indicated temperature at isolation was −57°C with the POLARx system compared to −49°C with the Arctic Front. a full occlusion is mandated before commencing the freeze of the POLARx. In the right-sided veins, stability is usually lower and in our experience a minor adjustment in technique is required to achieve consistent occlusion with the POLARx system. Furthermore, the POLARx balloon appears marginally more delicate, aggressive maneuvers (pull-downs, hockey-sticks, catheter flexion) are not recommended by the manufacturer and the avoidance of these may be reflected in our results.

| LIMITATIONS
This represents a short report of our initial experience with this new technology. Comparisons have been performed with consecutive cases rather than by randomization, observed differences between the two technologies could, therefore, arise either by hidden bias or be due to learning curves experienced with the POLARx technology.
A randomized study powered for clinically meaningful endpoints would be required to definitively show superiority or equivalence between these technologies.

| CONCLUSION
The POLARx CB is effective for PVI. Measured isolation and nadir temperatures are lower compared with the predicate Arctic Front Advance catheter. The technology appears similar in acute efficacy and has a short learning curve, but detailed dosing studies will be required to prove equivalence.

DATA AVAILABILITY STATEMENT
Data are available on request due to privacy/ethical restrictions.