By Ken Grauer, MD

Professor Emeritus in Family Medicine, College of Medicine, University of Florida

Dr. Grauer reports no financial relationships relevant to this field of study.

The 12-lead ECG in the figure below was obtained from a middle-aged woman with syncope and hypotension but no chest pain. No long lead rhythm strip is available. What might be causing her syncope?

The cardiac rhythm is complex and virtually impossible to completely interpret from this 12-lead ECG without benefit of a long lead rhythm strip. However, there is bradycardia (heart rate is clearly below 60 beats/minute in many places). There are nine beats on this tracing. All QRS complexes are narrow. P waves are present, and many P waves are not conducted. Therefore, some form of high-grade AV block appears to be present.

That said, complete AV block is unlikely since the QRS rhythm is irregular (i.e., most of the time with complete AV block, the escape rhythm is at least fairly regular). It is hard to say more without a long lead rhythm strip.

As to interpretation of the rest of this ECG, there is a normal frontal plane axis, and there is no chamber enlargement. A small q wave is present in lead III. R wave progression is normal, with a transition occurring between leads V3 and V4. The most remarkable changes are seen in ST-T waves. There is marked ST elevation in each of the inferior leads, which manifest a hyperacute appearance. This is associated with reciprocal ST depression in high-lateral leads I and aVF. In the chest leads, there is ST depression that is most marked in leads V2 and V3, with overly peaked T waves. There is subtle-but-real ST elevation in lead V1.

ECG findings suggest high-grade, second-degree AV block with acute infero-postero ST-elevation myocardial infarction (STEMI). This more than explains this patient’s syncope and hypotension. The “culprit” artery is almost certain to be the right coronary artery (RCA) because 80-90% of patients have a right dominant circulation, so acute occlusion of the left circumflex (LCx) artery is, overall, much less common than acute RCA occlusion. ST elevation is much more marked in lead III compared to lead II, which favors RCA rather than LCx occlusion. In the setting of acute infero-postero STEMI, ST elevation as seen here in lead V1 strongly suggests acute right ventricular involvement. It is the proximal RCA that virtually always provides blood supply to the right ventricle.

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