An unusual ECG pattern: left posterior fascicular block obscuring a right ventricular conduction defect?

A 23-year-old man with a history of palpitations and a single syncopal attack is described. Results of the physical examination were negative. The ECG showed a marked right axis deviation, a QRS duration of 0.13 sec. and an rS pattern from V1 to V6. In the high right precordial leads an rsR' pattern was recorded. The VCG (Frank system) showed a rightward dislocation of QRS vectors, a clockwise inscription of the QRS loop in the frontal and transverse planes and a slowing of the rightward and posterior terminal vectors. The X-ray film and the echocardiogram were negative. Neither intra-cardiac shunts nor pressure abnormalities were detected at the heart catheterization. The basal His bundle electrogram showed an H-V prolongation. Atrial pacing caused a progressive increase of the QRS duration and the appearance of an R configuration in the V1 lead, without axis shifting. We propose that the electrocardiographic pattern was suggestive of a left posterior fascicular block masking in the standard right precordial leads an associated right ventricular conduction defect.