Direct Wenckebach periods in the left bundle branch during bradycardia-dependent left bundle branch block in a patient with sick sinus syndrome.

Direct Wenckebach periods in the left bundle branch with periods of 4:3 and 5:4 conduction alternating with 2:1 left bundle branch block (LBBB) were observed in a patient who was admitted because of congestive heart failure. There was no previous documented LBBB and serial electrocardiographic (ECG) mainfestations afterwards were indicative of sick sinus syndrome. This is probably the first report to describe direct Wenckebach periods in the left bundle branch during bradycardia-dependent (phase-4) LBBB in a patient with sick sinus syndrome.