This tracing was recorded from the same patient following the intravenous administration of adenosine. It now reveals sinus rhythm with a short PR interval (0.08 seconds) and a wide QRS complex indicative of ventricular pre-excitation. There is a delta wave that is positive in leads I, aVL and V3-V6 and the QRS complex resembles left bundle branch block. These ECG findings localize the bypass tract to the lateral wall of the right atrium and provide evidence that the narrow complex tachyardia shown on the previous page was an atrio-ventricular reciprocating tachycardia (AVRT) with the antegrade limb of the reentry circuit (i.e. A to V) within the AV node and the retrograde limb (V to A), the bypass tract.