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.