Radiofrequency ablation of a "concealed" Mahaim-type accessory pathway

We report a case of supraventricular tachycardia associated with left bundle branch block due to a slowly conducting right sided accessory pathway. Preexcitation of the ventricles could not be demonstrated during sinus rhythm or incremental atrial pacing but its presence was confirmed during antedromic tachycardia with critically timed atrial extrastimuli. The pathway was mapped during tachycardia and successfully ablated.