[Demonstration of retrograde block in a patient with preexcitation syndrome by intraoperative electrophysiological studies (author's transl)].

In a patient with a WPW syndrome, the electrophysiological investigation revealed a left-sided bypass conducting only in antegrade (A-V) direction. During right ventricular stimulation, there was a complete retrograde (V-A) block. Therefore no reentry tachycardia could be initiated. The patient was operated upon because of an aortic valve lesion. During operation the bypass was localized at the free wall of the mitral annulus by epicardial mapping. Even during direct stimulation at the ventricular insertion of the bypass, no retrograde conduction to the left atrium could be demonstrated. After ablation of the bypass by surgery, intra- and postoperative electrophysiological studies showed a normal antegrade activation pattern of the ventricles. The case reported demonstrates that the "heterodromia" of an accessory bypass can markedly influence the clinical setting in the WPW syndrome.