A 45-year-old man is admitted to the CCU with crushing anterior chest pain:

• Sinus rhythm
• Left bundle branch block with acute septal myocardial infarction

When a q wave is present in lead I in a patient with left bundle branch block, the possibility of an underlying septal infarction should be considered. In this tracing, the R wave is unusually tall in V1 in the presence of LBBB, and the R wave voltage decreases from V1 to V4. This also suggests an underlying septal infarction. The following are signs to look out for of acute infarction in left bundle branch block:

a) ST elevation >1 mm that is concordant with the QRS complex (probability of MI = 88%)
b) ST elevation >5 mm that is discordant with the QRS complex (a+b = MI probability 93%)

The following ECG was recorded four hours before the first ECG, prior to the development of left bundle branch block:    

• Sinus tachycardia
• Acute anteroseptal myocardial infarction