Infra-His blocked premature atrial contractions simulating 2:1 sinoatrial block in a patient with an atrio-His bypass tract.

Blocked atrial premature contractions simulated 2:1 sinoatrial block because those were superimposed on the T wave of the preceding sinus beats and were not visible on the surface electrocardiogram of our patient. Additionally, His bundle recordings and premature atrial stimulation demonstrated the presence of an atrio-His anomalous pathway. The premature atrial contractions traveled anterograde via the anomalous pathway and were blocked distally to the His bundle. The term, pseudosinoatrial block, is used to describe the arrhythmia because there was no evidence of an intrinsic abnormality of sinus node function and sinoatrial conduction.