A 23-year-old, otherwise healthy woman presented with recurrent skipped and rapid sustained heart beats. Her physical examination and echocardiogram were normal. The presenting ECG in Figure 1 shows sinus rhythm, a nonconducted sinus p wave, and premature complexes. Both left bundle-branch block (5th and 6th complexes) and right bundle-branch block (10th complex) morphologies are present. The 5th and 10th complexes demonstrate a constant coupling interval and there was no discernable p wave preceding either complex. Although the 5th and 6th complexes appear to be consecutive premature QRS complexes, the timing of the 6th complex is also consistent with an aberrantly conducted sinus beat. This ECG pattern recurred incessantly. The “unexpected” heart block and the premature complexes suggested a diagnosis of His bundle extrasystoles, though this hypothesis did not explain the sustained rapid heart rates. An electrophysiology study was performed. Figure 1. Presenting ECG. With recording catheters at the high right atrium, His bundle, and right ventricular apex, the intracardiac correlates of the presenting ECG manifestations were observed. Figure 2 shows an isolated His signal preceding the nonconducted sinus p wave and the previous sinus beat (arrows). This His activation could lead to the atrioventricular (AV) block either by retrograde concealment into the AV node fast pathway or by rendering the His-Purkinje system refractory to the next sinus beat. The apparent isolated His signal is closer to the 3rd sinus beat compared …