[Unmasking an exclusively retrograde accessory pathway by catecholamines].

A 52-year-old man with frequent episodes of narrow QRS complex tachycardias with rates of 150/min was admitted for electrophysiological evaluation and treatment. P waves could be seen in the ST-segment of the surface ECG during tachycardia. Atrial stimulation during electrophysiological testing was not able to induce tachycardia. During atrial stimulation, there was no evidence of conduction via an accessory pathway or of dual AV node conduction properties. Ventricular stimulation showed complete ventriculoatrial block. After intravenous administration of the catecholamine orciprenaline, single atrial extrastimuli induced an AV macro-reentrant tachycardia with a rate of 165/min. VA conduction showed the earliest retrograde atrial activation in the left anterolateral area. Thus, there was an accessory pathway which only conducted in ventriculoatrial direction and only during adrenergic stimulation. After successful radiofrequency catheter ablation, complete ventriculoatrial block was recorded even after repeat administration of orciprenaline during ventricular stimulation. This case confirms the need to administer catecholamines in every undiagnosed tachycardia during electrophysiological testing to reveal the mechanism of the tachycardia.