Case presentation
We herein report a male in his 40s who was referred for catheter ablation due to drug refractory paroxysmal atrial fibrillation (PAF) and frequent atrial premature contractions (APCs). Informed consent was obtained from the patient for publication of this case report. An electrocardiogram recording of the APCs demonstrated a negative P-wave morphology in the inferior leads and a positive P-wave morphology in the I and aVL leads (Figure 1A) . Other examinations, including a cardiac echocardiogram, chest X-ray, and laboratory tests, showed results within the normal range. After successful pulmonary vein isolation using a third-generation cryoballoon system (Arctic Front Advance Cardiac Cryoablation Catheter, Medtronic), frequent mono-morphological APCs remained. We therefore mapped the APC focus with a multielectrode mapping catheter (PentaRay; Boisense Webster, Diamond Bar, CA, USA) on a high-density three-dimensional electroanatomical mapping (3D-EAM) system (CARTO3; Boisense Webster). The activation map showed a focal activation pattern from the inferior vena cava (IVC). The earliest activation site was located at the posteromedial part within the IVC (6.2 mm below the IVC ostium), and the local signal demonstrated a very low amplitude of 0.05 mV (Figure 1B ). In addition, the voltage map (>0.05 mV) demonstrated the short IVC sleeve from the ostium (9.3 mm) (Figure 1C) . Interestingly, sustained continuous IVC firings (bigeminy) were observed, which propagated to the right atrium (RA) only when the coupling interval was prolonged from 190 msec to 200 msec (Figure 2) . We next mapped the concealed bigeminy alone, which revealed the confined tiny excitable area within the IVC (0.1 cm2) with low-voltage fragmented potentials recorded at the earliest site (Figure 3A and 3B) . Catheter ablation was performed at the transit area between the IVC and RA to isolate the IVC. The APCs and concealed IVC firings disappeared after a total of 4 radiofrequency applications (30 W, 192 seconds), and dissociated activity was observed immediately after IVC isolation. After four months of follow-up, there was no AF recurrence without any anti-arrhythmic drug, and the number of APCs was within normal limits.