[Inconstant left bundle-branch block with a dependent wenckebach period in childhood]

Paroxysmal supraventricular tachycardia with left bundle branch block was found in a six-year-old girl. After prophylactic treatment with digitalis and with beta-adrenergic blocking agents, intermittent left bundle branch blocks appeared frequently. Thereby one could see repeatedly the pattern of a dependent Wenckebach phenomenon not yet described in childhood. Dependent Wenckebach phenomenon means the gradual development of a complete bundle branch block depending on the increasing atrial frequency drugin respiratory sinusarrhythmia. Immediately after complete bundle branch block, when the atrial frequency decreases significantly, the conductiondisturbance again normalized. Contrary to this, preexisting left bundle branch block disappeared when the frequency was increased by Orciprenalininfusions and be exercise. An evident explanation for this contrary behaviour cannot be given.