Changes in the pattern of old inferior wall myocardial infarction produced by acute left bundle branch block and hemiblock.

Varying degrees of functional conduction disturbances occurred in two patients with old IWMI during aberration of spontaneous, or induced, premature atrial contractions. Ventricular complexes transitional in form between those having normal (control) intraventricular conduction and those showing the most advanced grades of LBBB, LAH, LPH were interpreted as "incomplete" forms of the latter. Increasing degrees of LBBB first obscured, and finally masqueraded, the residual QRS changes of IWMI. On the other hand, IWMI could be suspected in beats showing LAH and RBBB since the left axis shifts produced by LAH were associated with Q waves in leads 2 and 3. However, the diagnosis of IWMI was impossible when LAH appeared without RBBB. Three possible explanations were offered for this paradoxic phenomenon, namely that: a) it was the RBBB not the LAH which permitted the diagnosis of IWMI when both processes coexisted; b) a minor degree of LBBB was present; and c) the site (s) at which the impulse entered into the ventricles varied with different degrees and combinations of functional blocks. Finally, LPH was characterized by an increase in the height of the R waves in leads 2 and 3 without a concomitant change in the size of the q waves. Hence, LPH made the diagnosis of IWMI more difficult