CASE PRESENTATION
A 61-year-old man was referred to our institution for ablation of an atrial tachycardia (AT) occurring after previous ablation of atrial fibrillation (AF). The earlier procedure consisted of pulmonary vein isolation and cavotricuspid isthmus (CTI) ablation along with elimination of complex fractionated atrial electrograms (CFAEs) located at the roof and lateral wall of the left atrium, within the coronary sinus (CS), and at both sides of the interatrial septum. Baseline surface electrocardiogram showed sinus rhythm and first-degree atrioventricular (AV) block with a PR interval of 360 ms (Figure 1A). A decapolar catheter was positioned in the CS, and a quadripolar catheter in the left atrial appendage (LAA). Intracardiac recording revealed that: (1) LAA potentials preceded CS potentials, which exhibited a distal-to-proximal activation sequence (Figure 1B); (2) all pulmonary veins were still isolated; (3) CTI was still blocked, and (3) a line of block was observed at the left atrial roof. The clinical AT (Figures 1C and 1D) with the cycle length of 300 ms was induced by programmed atrial stimulation. High density mapping and post-pacing intervals were consistent with a localized reentry utilizing the vein of Marshall (VOM) epicardially and the mitral isthmus endocardially (Figure 1E). Ethanol was infused to the VOM as previously described1 and resulted in immediate AT termination. Bidirectional lateral mitral isthmus block was then confirmed by differential pacing maneuver. However, a transitory and complete AV block with a junctional escape rhythm spontaneously occurred (Figure 2A). For the duration of the complete AV block, LAA potentials consistently corresponded to P waves, and ventricular potentials constantly preceded CS potentials, which had no correlation with P waves (Figure 2B). These findings suggested that the LAA and the CS were activated by the sinus rhythm and the junctional rhythm, respectively. After the recovery of the AV conduction, the CS exhibited a proximal-to-distal activation sequence (Figure 2C). What is the mechanism of AV block with CS potential dissociation after the creation of a block line at the lateral mitral isthmus?