Wide QRS complex supraventricular tachycardia with negative precordial concordance: Electrocardiographic clues for Mahaim pathway with Ebstein anomaly.

Ebstein's anomaly is often accompanied by either Wolff-Parkinson-White syndrome or atriofascicular Mahaim. These bypass tracts give rise to antidromic atrioventricular (AV) re-entrant tachycardias, in which the bypass tract serves as the anterograde limb of the circuit and the AV node as the retrograde limb of the reentrant circuit. Since the antidromic AV reentrant tachycardia over a Mahaim fibre has a typically left bundle braunch block (LBBB) morphology, it is easy to make a misdiagnosis of supraventricular tachycardia with functional LBBB or even of ventricular tachycardia particularly in the presence of negative concordance. Some electrocardiographic clues might prevent misdiagnosis of ventricular tachycardia and inadvertent ICD implantation.