ICE and WT measurement
ICE imaging in the LA was performed before starting PVI. The ICE catheter, equipped with a linear-phased array multifrequency (5.5-10 MHz) transducer, was connected to an Acuson P500 system (SIEMENS, Munich, Germany). The ICE catheter in the RA was withdrawn after the trans-septal puncture and then advanced into the LA via the 8.5Fr deflectable sheath (Agilis NxT, short size). Only the tip (transducer) of ICE catheter was carefully put out from the deflectable sheath and was placed in the center of the LA under fluoroscopic guidance: the safe distance (more than 10 mm) between the tip and the LA endocardium was always confirmed and maintained throughout manipulation of the ICE catheter (Figure 2). The ICE catheter was finely rotated without any deflection (keeping neutral position) around the PV-junctional area to obtain images perpendicular to the LA/PV surface. When different angulation of ultrasound beam was required for clear anatomical visualization, only the deflectable sheath was manipulated and deflected to change the position of the ICE transducer and the direction of the ultrasound beam. The snapshots of ICE imaging in each segment for PVI were obtained during the diastolic phase in eight segments: the anterior, roof, posterior and bottom of both PVs. The endocardial boundary (separating the blood pool from myocardium) and the epicardial boundary (separating the myocardial tissue from other surrounding tissue) were manually outlined. The distance between the epicardial and endocardial boundary was measured and defined as the WT. The WT was measured more than five times in each segment between 5 mm inside and 5 mm outside of the LA-PV junction; the highest and lowest measurements were discarded and the average of the ≥3 middle values was considered as the WT in a segment.
To validate the accuracy and reliability of the ICE-based WT measurements, the LA WTs on cardiac CT were also measured in the same patient. The cardiac CT-based measurement was performed by cardiologists who were blinded to the result of the ICE-based measurement; the WT values were compared between the ICE- and cardiac CT-based measurements.