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?