Case Report:
We present a case of a 46-year-old female with a history of anxiety and hypothyroidism who was referred to our clinic for possible pacemaker implantation. Her baseline 12-lead ECG is shown in Figure 1. She wore an ambulatory ECG monitor for 2 weeks after complaints of palpitation during which time 156 episodes of second-degree Mobitz II AV block were observed (Figure 2). Lyme titer was negative. Transthoracic echocardiogram revealed normal LV size and function without significant valvular heart disease. Cardiac MRI showed normal biventricular size and function with no late gadolinium enhancement. She had good exercise capacity (9 METS) with no evidence of ischemia on treadmill exercise stress test. During stage I of the Bruce protocol, there was a single non-conducted P wave following the 10th QRS complex with fixed PR and PP intervals consistent with Mobitz II AV block (Figure 3). Sinus tachycardia with frequent likely junctional extrasystoles with a typical right bundle branch block (RBBB) configuration were also present. Closely following the blocked P wave was a small negative deflection consistent with a retrograde P-wave. The P-wave is narrow and inverted in the inferior leads consistent with retrograde septal activation as would be expected to occur in association with a junctional extrasystole. While this could have been an opportunistic premature atrial extrasystole, the more likely explanation is that a junctional extrasystole conducted retrogradely through the AV node to the atrium and blocked antegradely in the His Purkinje system (Figure 4). The timing of the extrasystole would have had to occur during inscription of the blocked antegrade P wave and engaged the AV node retrogradely causing the oncoming antegrade P wave to block in the AV node. Furthermore, Mobitz II block is typically secondary to conduction disease below the AV node and tends to get worse at a faster heart rate, which was not observed here. All findings were supportive of pseudo AV block secondary to His extrasystoles. She was started on Flecainide which was up titrated to 100 mg twice daily. A repeat ambulatory ECG recording at 5 months follow-up showed normal sinus rhythm with no AV block. A repeat treadmill exercise stress test revealed a peak heart of 153 bpm without evidence of ischemia or AV block.