Sudden unexpected improvement in the atrioventricular conduction. What is the mechanism?

Figures

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  Fig. 1. Twelve-lead electrocardiogram showing 2nd degree atrioventricular block. Sinus arrhythmia, P waves conducting with left bundle branch block and a constant PR interval of 170 msec, sudden blocked P waves and first P wave of sequence conducting with PR interval of 140 msec and incomplete right bundle branch block QRS morphology.

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  Fig. 2. Ladder diagram explaining the first possibility. The beat after the blocked P wave is an escape beat arising in the proximal left bundle branch block and is dissociated with the preceding P wave. A is atria; AVN is atrioventricular node; His-BB is His bundle and bundle branches; RB is right bundle branch (thin lines); LB is left bundle branch (thick lines) and V is ventricles. PPi (PP interval), PRi (PR interval), RRi (RR interval). Vertical arrows demonstrate beat with incomplete RBBB morphology.

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  Fig. 3. Ladder diagram explaining the second possibility. Note that the left bundle branch shows a 2nd degree type 1 conduction pattern and the right bundle branch a 2nd degree type 2 conduction pattern both synchronized to P waves such that both block in the 6th P wave. Abbreviations as in Fig. 2. Vertical arrows demonstrate beat with incomplete RBBB morphology.