Despite the absence of the coronary Q wave, there appear to be instances when myocardial infarction can be diagnosed in the presence of left bundle branch block. The experimental evidence showed that with a transmural infarction or patchy infarction with extensive surface damage there occurs a marked reduction in the size of the R wave recorded in the direct lead from that region. In some instances, the reduction in the R wave may be reflected in the corresponding precordial lead. Changes in the magnitude of the R wave in the left precordial leads in left bundle branch block will frequently be detected only if control tracings are available for comparison or if multiple precordial leads are taken.

Acute myocardial injury results in elevation of the S-T segment despite the presence of left bundle branch block. Thus, the diagnosis of acute myocardial infarction can be made from the finding of significant S-T segment elevation following the large, broad, positive complex of left bundle branch block. Other changes in the coronary circulation may also be reflected by alterations in the RS-T segment despite the presence of left bundle branch block. In our experience, coronary insufficiency as well as a positive two-step exercise test has resulted in changes in the S-T segment.

It would appear that the popular opinion that coronary artery disease cannot be diagnosed in the presence of left bundle branch block should be modified. It is true that there is absence of the coronary Q wave in myocardial infarction, but the previously discussed changes should aid in making more conclusive the diagnosis of coronary artery disease in the presence of left bundle branch block.