Abstract
We present the case of a 58-year-old patient admitted for wide QRS tachycardia (200 bpm). There was no delta wave on a 12-lead surface electrocardiogram during sinus rhythm. During the electrophysiological study, stimulation of the high right atrium and the proximal coronary sinus resulted in decremental anterograde conduction through the atrioventricular node, whereas pacing at the right ventricular apex led to retrograde conduction through the atrioventricular node. Conduction through an anterograde accessory pathway was revealed during pacing at the distal site of its atrial insertion. Some nonmanifest accessory pathways may be unmasked by differential pacing maneuvers.