His bundle electrocardiography during bidirectional tachycardia.

His bundle electrocardiography in a patient with a digitalis-induced bidirectional tachycardia revealed absence of His spike preceding earliest onset of ventricular activation during the bidirectional tachycardia. However, His potential preceded ventricular activation of normal complexes at a constant H-V interval of 35 msec. The presence of fusion QRS complexes, capture QRS complexes, the analysis of cycle lengths, and response to carotid sinus massage all favor a ventricular origin for this tachycardia in the patient presented.