The electrocardiographic features of complete and partial left anterior and left posterior hemiblock

The aim of this study is to present the criteria for the diagnosis of incomplete or partial block within the anterior and posterior divisions of the left bundle-branch (LBB).To disclose incomplete left anterior hemiblock (LAH) and incomplete left posterior hemiblock (LPH), clinical cases of pathologic and physiologic intermittent or transient block in the divisions of the LBB are analyzed.When dealing with the diagnosis of incomplete LAH, an ÂQRS shift in the same or in successive tracings in a patient, showing electrical axis at + 50°, + 40°, + 30°, and 0° covering the whole range up to − 45° or even more negative, makes the diagnosis of incomplete to complete block in the anterior division of the LBB. Conversely, when LPH is the case, a progressive change of the ÂQRS from a normal axis to the right, up to + 120° in the same or subsequent tracings in a short period, can only be explained by increasing the degrees of LPH. When a partial or incomplete LAH or LPH is present and the ÂQRS direction can be considered normal in clinical practice, it is difficult or even impossible to reach a diagnosis. That is, small degrees of block in the divisions of the LBB totally overlap normal variants.