Demonstration of two anomalous bypasses in concealed Wolff-Parkinson-White syndrome.

Two patients with a history of paroxysmal supraventricular tachycardia but no evidence of the Wolff-Parkinson-White syndrome in the conventional electrocardiograms were studied utilizing atrial and ventricular extrastimuli. The evidence obtained in these patients was consistent with a concealed Wolff-Parkinson-White syndrome. In one patient, ventricular extrastimuli revealed three different responses in the curve of ventriculoatrial conduction, depending upon the interval of stimulus. In the other patient, intracardiac recordings, including the left atrial potential, showed that paroxysmal supraventricular tachycardia was induced by an impulse via an anomalous bypass which diverged from the main bypass or stemmed independently from the left ventricle and was maintained by impulses via the main bypass which lay between the left ventricle and the left atrium. The mechanisms for initiation of the paroxysmal supraventricular tachycardia were discussed.