ECG DIAGNOSIS OF ACUTE MYOCARDIAL-INFARCTION IN THE PRESENCE OF LEFT-BUNDLE-BRANCH BLOCK IN PATIENTS UNDERGOING CONTINUOUS ECG MONITORING

It is common knowledge that the ECG diagnosis of completed myocardial infarction in the presence of left bundle-branch block (LBBB) is extre mely difficult and often impossible. More than 50 rules have been prop osed as criteria for interpreting Q-wave equivalents superimposed on t he QRS complex in the presence of LBBB. However, because of misinterpr etation of the available literature, physicians frequently recommend t hat patients with chest pain in the presence of LBBB receive thromboly tic therapy or urgent coronary arteriography on the basis of the assum ption that acute injury and ischemia cannot be interpreted in the pres ence of LBBB. Unfortunately, many physicians fail to realize that alth ough completed infarction is difficult to confirm in the presence of L BBB, ongoing ischemia and injury can be detected in the presence of LB BB and may be seen as often as they are in the presence of normal card iac conduction. A deflection of the J point (and ST segment) in the di rection of the major QRS complex or an elevation of the ST segment of more than 7 to 8 mm opposite the direction of the major QRS complex ha s been demonstrated to have a sensitivity of more than 50% in detectin g acute injury, with a specificity of more than 90%. During the first half of an ongoing prospective study of the use of continuous 12-lead ECG monitoring in the emergency department, we encountered five patien ts with final diagnoses of acute myocardial infarction in the presence of LBBB who demonstrated significant ECG changes while undergoing con tinuous ST-segment monitoring with frequent serial ECGs. The five diff erent locations of the infarcts in these five patients were posterior, posterolateral, inferior, anterior, and anterolateral. We present the se patients' cases to demonstrate the ECG characteristics of acute inj ury in the presence of LBBB.