Left anterior hemiblock obscuring the diagnosis of right bundle branch block.

Three cases in which left anterior hemiblock (LAH) obscured the diagnosis of right bundle branch block (RBBB) are reported. The RBBB was incomplete in one case and complete or of high degree in the other two. The LAH was intermittent in two cases. In the three cases, the LAH abolished not only the S waves in leads I and aVL, but also the terminal R wave in Vl. The possibility that RBBB, complete or incomplete, can be totally missed in the presence of LAH may imply a significant clinical danger, because some of the well known diagnostic and prognostic connotations of this association of conduction disturbances may then be overlooked. However, if the QRS interval is excessively wide or more than expected for pure LAH, the concealed or hidden RBBB can be suspected. It can be readily uncovered if additional unipolar high and right sided chest leads are recorded. The vectorcardiogram can also be helpful, if the possibility of concealed RBBB is kept in mind.