The patient has second-degree atrioventricular (AV) block alternating with slowly-conducted P waves (long PR) via an intermittent left accessory pathway. The existence of this pathway avoids potential prolonged asystole due to advanced atrioventricular block. After pacemaker implantation, paced rhythm alternated with conduction via the pathway, which disappeared over time (Figures 1–3).
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2016. Sociedad Española de Cardiología