Exercise-Induced Complete AV Block.

A 8-year-old male presented with exertional shortness of breath since the last 6 months. He denied any history of syncope, presyncope, palpitations or angina. The resting 12 lead ECG revealed complete left bundle branch block (LBBB), with PR interval 160 ms [Figure 1a]. The patient underwent an exercise stress test for evaluation of his symptoms. Intact 1:1 atrioventricular conduction was noted during stage 1 and stage 2 of the stress test, untill a sinus rate of 168 bpm [Figure 1b]. However, during stage 3, a sudden complete AV block was noted with a prolonged asystole [Figure 1c]. The patient complained of dizziness and became hypotensive. The test was immediately terminated and the patient put in a supine position. The episode of complete AV block was then followed by intermittent 2:1 AVB [Figures 2a and b]. Transient 3:1 and 4:1 AV block [Figures 3a and b] were also noted before resumption of normal sinus rhythm occurred after nearly 20 min. Since the patient was hemodynamically stable and symptoms had resolved almost immediately on termination of the exercise test, temporary pacing was not initiated. The patient successfully underwent a dual chamber permanent pacemaker implantation 24 hours later.

Figure 1a

Resting 12 lead ECG with complete left bundle brunch block (LBBB), normal frontal QRS axis and a PR interval 160 ms

Figure 1b

1:1 AV conduction until HR of 168 bpm (Stage 2 of stress test)

Figure 1c

Sudden complete AV block with a prolonged asystole (Stage 3 of stress test)

Figure 2a

Complete AV block followed by intermittent 2:1 AVB (P-waves marked with arrows). During the period of 2:1 AVB, the rhythm could easily have been interpreted as normal sinus rhythm

Figure 2b

However, careful analysis reveals every alternate P-wave (marked arrows) falling on T-waves which appeared peaked confirming presence of 2:1 AV Block

Figure 3a

Brief period of 3:1 AV Block

Figure 3b

Brief period of 4:1 AV Block

Exercise-induced high grade atrioventricular (AV) block in patients with intact 1:1 AV conduction at rest is a rare phenomenon. Complete AV block occurring during exercise is an even more uncommon entity.[12] It is important for clinicians to be aware of this clinical entity since most such patients have conduction block at the infra-nodal intra-Hisian or infra-Hisian level, necessitating a pacemaker implantation.[3]