ECG Review: Chest Pain and Two Key ECG Findings
By Ken Grauer, MD, Professor Emeritus in Family Medicine, College of Medicine, University of Florida. Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.
Figure — 12-lead ECG from a 50-year-old woman with chest pain.
Scenario: The 12-lead ECG shown above was obtained from a 50-year-old woman with “chest pain.” What two major conclusions should be reached regarding ECG interpretation of this tracing?
Interpretation: The two conclusions to be reached are: 1) there is a large acute evolving ST-elevation myocardial infarction (STEMI) in progress, and 2) there is second-degree AV block.
It may be easiest to comment first on the changes of acute MI. The QRS complex is narrow and the rhythm is supraventricular. Q waves are present in each of the inferior leads (II, III, aVF). After a small (but definitely present) initial r wave in lead V1, there is loss-of-R-wave, with a probable QS in V2 and a definite QS (with notch in downslope of S wave) by lead V3. Abnormal Q waves persist in leads V4 through V6.
Regarding ST-T wave changes, there is marked ST elevation in each of the inferior leads. In addition, there is ST segment coving and elevation that appears to begin in lead V2 and persists through to lead V6. T wave inversion (sometimes marked) is seen in each of the leads with ST elevation.
Reciprocal changes are seen in leads I and aVL. These reciprocal changes entail not only ST depression in lead aVL, but also T wave peaking (which is the “mirror image” of the deep inferior symmetric T wave inversion). Thus, a large infero-antero-lateral infarction is acutely evolving.
Perhaps the most interesting part of this tracing is the extra “shoulder” seen on the tail end of the elevated ST segment in lead II (small black dot in Figure). Setting one’s calipers to a P-P interval between the peak of the P wave preceding each QRS in lead II and the peak of this extra “shoulder” confirms the presence of underlying atrial tachycardia at a rate of ~140/minute (albeit short vertical lines at the bottom of the figure show how subtle non-conducted P waves are in other leads on the tracing). Thus, the rhythm is second-degree AV block with 2:1 AV conduction. This is almost certainly Mobitz Type I (AV Wenckebach) because of the narrow QRS complex in the setting of acute inferior infarction. A key clue that an extra P wave might be “hiding” is the fixed but prolonged PR interval preceding each QRS on the tracing. New-onset Mobitz I in the setting of acute inferior MI is often preceded by first-degree AV block.
For more information about AV blocks, please visit: https://www.kg-ekgpress.com/av_block_pdf_file/.
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