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Electrocardiographic diagnosis of myocardial infarction in the presence of bundle branch block or a paced rhythm

Ary L Goldberger, MD
Section Editors
Bernard J Gersh, MB, ChB, DPhil, FRCP, MACC
Peter J Zimetbaum, MD
Deputy Editor
Gordon M Saperia, MD, FACC


The diagnosis of myocardial infarction (MI) is typically suspected from the history of chest pain and may be confirmed by the electrocardiogram (ECG) and elevations in serum troponins and CK-MB. (See "Criteria for the diagnosis of acute myocardial infarction" and "Electrocardiogram in the diagnosis of myocardial ischemia and infarction".)

The ECG diagnosis of an acute MI in patients with right or left bundle branch block (RBBB or LBBB) or a paced rhythm will be reviewed here. Other issues related to RBBB and LBBB are discussed separately. (See "Right bundle branch block" and "Left bundle branch block".)


The electrocardiogram (ECG) diagnosis of myocardial infarction (MI) is more difficult when the baseline ECG shows a bundle branch block pattern that may precede or be a complication of the infarct or when the patient has a ventricular paced rhythm [1-5]. The frequency of bundle branch block was assessed in a review of almost 300,000 infarctions from the National Registry of Myocardial Infarction 2 investigators [6]. Right bundle branch block was present in approximately 6 percent and left bundle branch block in 7 percent of infarctions. Patients with bundle branch block were significantly less likely than those without bundle branch block to receive appropriate therapy with aspirin or beta blockers, had more comorbid disease, and had a significant increase in in-hospital mortality.


The effect of right bundle branch block (RBBB) must be considered in both Q wave (ST elevation) and non-Q wave (non-ST elevation) infarctions.

Q wave MI — Right bundle branch block does not usually interfere with the diagnosis of a Q wave myocardial infarction (MI). MI most often involves the left ventricle and therefore affects the initial phase of ventricular depolarization, sometimes producing abnormal Q waves. In contrast, right bundle branch block (RBBB) primarily affects the terminal phase of ventricular depolarization, producing a wide R' wave in the right chest leads and a wide S wave in the left chest leads (waveform 1A-B). These changes are due to delayed depolarization of the right ventricle, while depolarization of the left ventricle is not affected. (See "Right bundle branch block".)

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Literature review current through: Nov 2017. | This topic last updated: Sep 05, 2017.
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