It is common knowledge that the ECG diagnosis of completed myocardial infarction in the presence of left bundle-branch block (LBBB) is extremely difficult and often impossible. More than 50 rules have been proposed as criteria for interpreting Q-wave equivalents superimposed on the QRS complex in the presence of LBBB. However, because of misinterpretation of the available literature, physicians frequently recommend that patients with chest pain in the presence of LBBB receive thrombolytic therapy or urgent coronary arteriography on the basis of the assumption that acute injury and ischemia cannot be interpreted in the presence of LBBB. Unfortunately, many physicians fail to realize that although completed infarction is difficult to confirm in the presence of LBBB, ongoing ischemia and injury can be detected in the presence of LBBB and may be seen as often as they are in the presence of normal cardiac conduction. A deflection of the J point (and ST segment) in the direction 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 has been demonstrated to have a sensitivity of more than 50% in detecting 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 patients with final diagnoses of acute myocardial infarction in the presence of LBBB who demonstrated significant ECG changes while undergoing continuous ST-segment monitoring with frequent serial ECGs. The five different locations of the infarcts in these five patients were posterior, posterolateral, inferior, anterior, and anterolateral. We present these patients' cases to demonstrate the ECG characteristics of acute injury in the presence of LBBB.

[Fesmire FM: ECG diagnosis of acute myocardial infarction in the presence of left bundle-branch block in patients undergoing continuous ECG monitoring. Ann Emerg Med July 1995;26:69-82.]