Electrophysiology study (EPS) tracings of surface and intracardiac electrograms showing degeneration of orthodromic atrioventricular reciprocating tachycardia (AVRT) into atrial tachycardia

Surface ECG leads I, AVF, and VI and intracardiac electrograms are simultaneously recorded. The first three QRS complexes represent an orthodromic atrioventricular reentrant tachycardia (AVRT). The QRS complexes are narrow and there is retrograde activation of the atrium (P) which, based upon timing with the vertical line after the second QRS complex, begins at the distal coronary sinus (DCS) which is the site of earliest atrial activation and is consistent with a left lateral accessory pathway. After the third complex, the vertical line indicates atrial activation at the proximal coronary sinus (PCS) which is premature and has a different morphology (A'). This atrial premature beat initiates a rapid atrial tachycardia (*) and atrial activity now precedes the QRS complex. The atrial tachycardia has a wide complex because of antegrade conduction via the accessory pathway.