In this prospective cohort study, we compared the first 41 patients, who underwent PVI for the treatment of paroxysmal or persistent atrial fibrillation with the 2nd generation cryoballoon (Arctic Front Advance, Medtronic Inc, Minneapolis, MN, USA) in combination with the SMC2 to the last 101 patients treated with the 2nd generation cryoballoon and SMC1. Exclusion criteria were long-standing persistent AF, previous left atrial (LA) ablation, LA diameter > 55 mm, uncontrolled heart failure (NYHA class IV) and severe valvular disease. The study complies with the Declaration of Helsinki and was approved by our institutional review committee. All patients gave written informed consent to the procedure and to the participation in this observational study.

We sought to evaluate the impact of SMC2 on procedure duration, fluoroscopy time, TTI observation rate and complication rate in patients during cryoballoon PVI in comparison to SMC1.

Intracardiac thrombi were ruled out in every patient by transesophageal echocardiography prior to PVI. Additional preprocedural imaging was not applied. Patients on vitamin K antagonists were scheduled for the procedure at a target INR of 2.0–3.0, whereas patients on non-vitamin K antagonist oral anticoagulants (NOACs) were advised to hold medication 24 h prior to the procedure. NOACs were continued on the evening of the day of the procedure. Anticoagulation was continued for at least 2 months following the procedure in all patients. Pericardial effusion was routinely ruled out by echocardiography immediately after the procedure and prior to hospital discharge.

Cryoballoon ablation with SMC1 has been described in detail before [9, 14]. Briefly, the cryoballoon was introduced to the left atrium via a steerable transseptal sheath (Flexcath Advance, Medtronic, Minneapolis, MN, USA) after single transseptal puncture. In the SMC2 group the selection of the diameter of the spiral mapping catheter (20 or 25 mm) was at the operator’s discretion in the first 41 cases based on PV diameter estimated by PV angiography. In the last 60 cases in the SMC2 group the 25 mm spiral mapping catheter was used exclusively. After balloon inflation and placement at the PV ostia, the spiral mapping catheter was positioned in the PV at the closest achievable proximity to the ostium to enable real-time observation of PV potentials during PV isolation. PV occlusion was documented by injection of contrast medium. The TTI was defined as the time of the last recording of a PV potential before sustained isolation. In both study groups individualized TTI-dependent titration of cryoenergy was performed. In case of a TTI < 30 s the total freeze cycle duration was decreased to 120 s and no bonus freeze was applied. If TTI was between 30 and 60 s a single 180 s freeze cycle was applied. If TTI was > 60 s a 180 s freeze cycle followed by a 180 s bonus freeze was applied. Also, if no TTI could be documented, but isolation was achieved, a single 180 s freeze cycle was applied followed by a 180 s bonus application of cryoenergy. Isolation of all PVs was reassessed at the end of the procedure by documentation of entrance- and exit-block except at PVs with a TTI ≤ 60 s. Cryoenergy application was aborted if luminal esophageal temperature (LET) measured by transnasally inserted temperature probe (S-CATH; Circa Scientific Inc., Englewood, CO, USA) decreased below 20 °C [15].

Significance of differences of numeric values between the two groups was calculated by t test if normal distribution with equal variance was given. Normal distribution was determined by Shapiro–Wilk test and equal variance by Brown–Forsythe test. Numeric variables that were not normally distributed were analyzed by Mann–Whitney rank sum test. Categorical variables were analyzed by Chi-square test or Fisher’s exact test. A p value < 0.05 was considered significant. Statistical assessment was performed with Excel (Version 2016, Microsoft Inc., Redmond, WA, USA) or XLStat software (V 2016.02.28430, Addinsoft, New York, NY, USA).

We included 158 patients (mean age 65.1 ± 12.4 years, female 39%, paroxysmal AF 60%, mean CHA₂DS₂-VASc-Score 2.8 ± 1.6) undergoing cryoballoon PVI with the second generation cryoballoon. In 57 patients (36%) the SMC1 was applied (SMC1 group) and 101 patients (64%) underwent cryoballoon PVI with the SMC2 (SMC2 group). Baseline characteristics did not differ significantly between both study groups (Table 1). SMC1 was exclusively used with a diameter of 20 mm, SMC2 with a 20 mm diameter was applied in 12 patients, SMC2 with a 25 mm diameter was used in 89 patients (Table 2).

In the SMC1 group we identified 226 PVs and 397 PVs were identified in the SMC2 group. All PVs (623/623, 100%) were isolated successfully with the SMC1 or SMC2 placed in the lumen of the 2nd generation cryoballoon. Exchange of the mapping catheter to a stiff guidewire was necessary in one case in the SMC2 group. Overall TTI observation rate in the SMC1 group was 68.6% (155/226 PVs), whereas TTI observation rate for all PVs was 82.6% (328/397 PVs) in the SMC2 group (p = 0.001). We were able to determine the time-to-isolation in 46/55 LSPVs (83.6%) in the SMC1 group and in 83/94 LSPVs (88.3%) in the SMC2 group (p = 0.696), in 32/55 LIPVs (58.2%) in the SMC1 group versus 73/94 LIPVs (77.7%) in the SMC2 group (p = 0.002), in 42/57 RSPVs (73.6%) in the SMC1 group compared to 81/101 RSPVs (80.2%) in the SMC2 group (p = 0.344) and in 35/57 RIPVs (61.4%) in the SMC1 group versus 84/101 RIPVs (83.2%) in the SMC2 group (p = 0.004, Fig. 2). No TTI was registered in 2 LCPVs in the SMC1 group, while in 3 out of 7 LCPVs a TTI was registered in the SMC2 group (p = 0.500).

We further analyzed the TTI observation rates with regard to the diameter of spiral mapping catheter in the 69 patients treated with a 20 mm catheter (SMC1 and SMC2) versus the 89 patients treated with a 25 mm catheter (only SMC2). While TTI observation rate with the 20 mm catheter was 70.2% (191/272 PVs), TTI was significantly more often registered with the 25 mm diagnostic catheter (83.2%, 292/351 PVs, p < 0.001), indicating that catheter size and additional electrodes facilitate registration of PV potentials during PV isolation. Accordingly, TTI observation rate between SMC1 (68.6%) and 20 mm SMC2 (78.2%) was statistically not different (p = 0.258). Comparison of TTI observation rate between SMC2 with 20 mm and 25 mm demonstrated a numerically higher real-time observation rate using a larger mapping catheter (20 mm: 78.2% vs 25 mm: 83.2%). However, this difference is statistically not significant (p = 0.407).

However, incidence of prematurely aborted freezes caused by phrenic nerve palsy (PNP), low oesophageal temperature (LET) < 20 °C, cryoballoon dislodgment or ineffective pulmonary vein isolation was significantly higher when the 2nd generation spiral mapping catheter was used (SMC1: 4/314 freezes (1.3%) vs. SMC2: 32/655 freezes (4.9%; p = 0.009). Especially, ineffective pulmonary vein isolation leading to premature freeze abortion was observed more frequently in the SMC2 group compared to the control group [SMC1: 0/314 freezes (0%) vs. SMC2: 9/655 freezes (1.4%; p = 0.036)]. Similarly, freeze abortion rate caused by LET < 20 °C tended to be higher when SMC2 was used although this was statistically not different (p = 0.120, Table 2).

Remarkably, mean total freeze duration per patient was significantly longer in the SMC2 group (17.6 ± 5.6 min) compared to the SMC1 group (14.1 ± 4.5 min, p < 0.001). Similarly, mean number of freezes per patient was significantly higher in the SMC2 study group with 6.5 ± 1.9 compared to 5.5 ± 1.5 in the SMC1 group (p = 0.001).

The shaft of the SMC2 is much softer compared to SMC1, which leads to decreased maneuverability in patients with difficult PV anatomies. Specifically, the softer shaft precludes placement of the spiral tip containing the electrodes of SMC2 inside the PV in certain difficult anatomies, especially tortuous PVs (supplementary online movie 1 and 2). In these cases, backup can be increased by advancing the cryoballoon catheter over the soft shaft of the SMC2 catheter into the PV (supplementary online movie 3 and 4). Nevertheless, decreased maneuverability of SMC2 did not translate into significantly increased procedure duration or fluoroscopy time. Mean procedure duration was 72.0 ± 18.9 min in the SMC1 group and 74.4 ± 19.1 min in the SMC2 group (p = 0.432). Mean fluoroscopy time was 15.0 ± 5.7 min in the SMC1 group and 15.7 ± 7.3 min in the SMC2 group (p = 0.593). Procedural details and acute ablation results are shown in Table 2.

Transient phrenic nerve palsy occurred in 11 patients of the whole study group with 4 PNP (5.8%) in the SMC1 group and 7 PNP (6.9%) in the SMC2 group (p = 1.0). All PNP resolved completely by the end of the procedure. No periprocedural stroke or systemic embolism occurred. Groin hematoma (without blood transfusion) was observed in one patient in the SMC1 group, whereas pericardial tamponade occurred in one patient in the SMC2 group (Table 2).