Supraventricular Tachycardia Initiated by Couplets of Atrial Extrasystoles but Not by Single Premature Atrial Beats: What Is the Mechanism?

The reported electrocardiogram shows several atrial extrasystoles (AEs) sometimes occurring in couplets. The former beat of each couplet is nonconducted, whereas the latter triggers a supraventricular tachycardia with negative P waves in inferior leads and RP > PR. This suggests an atypical atrioventricular nodal reentrant tachycardia involving the fast pathway anterogradely and the slow pathway retrogradely. The tachycardia is never precipitated by single AEs. The blocked AE of each pair is pivotal in tachycardia initiation, allowing the subsequent impulse to conduct down the fast pathway. A concealed slow pathway penetration during the blocked AE is invoked as the key mechanism.