Author’s Reply—Phase-4 RBBB in Lyme carditis?

We are honored by Professor Belhassen’s interest in our report¹ of Lyme carditis manifesting with high-grade atrioventricular block and intermittent late-coupled beats of right bundle branch–like morphology. We proposed that these beats were conducted with aberrancy owing to phase 4 block in the right bundle. In retrospect, perhaps we should have set out several alternatives, for as Professor Mauricio Rosenbaum used to say, “Every complex arrhythmia has at least 3 different explanations!”^2,3

Professor Belhassen hypothesizes instead that these beats are from a ventricular escape rhythm. We acknowledge the merits of this explanation, and appreciate particularly his astute observation of a very subtle fusion beat (third beat in Figure 2C). Additionally, we observe the wide beats lack typical right bundle branch block pattern as might be expected with a discrete proximal injury in an otherwise normal heart.

With regard to the observation that there is no other disturbance of His-Purkinje conduction, we note the right axis deviation seen on narrow-complex beats at presentation and throughout the acute period, but absent at outpatient follow-up (not shown). This may represent transient left posterior fascicular block⁴ (seen best in Figure 3B; precordial lead placement may have been imperfect in Figure 2 owing to external pacing pads).

We lack provocative maneuvers or intracardiac recordings to provide definitive resolution to this question. However, in either case, there is reason to question the integrity of the His-Purkinje system; as Professor Belhassen notes, an adolescent with atrioventricular nodal block should have a junctional escape.