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 ECG in the figure below was obtained from a patient with new-onset chest pain. He was hemodynamically stable at the time this tracing was obtained.
In addition to the artifact that is most pronounced in the lateral chest leads, there are four major findings. Admittedly, two findings are subtle. How many findings can you identify? The ventricular rhythm is fairly rapid and irregular. Clearly, atrial activity is present. Use of calipers would be the easiest way to confirm that P waves are regular. Since the QRS complex is narrow, the mechanism of the rhythm is supraventricular. If one steps back from the tracing, it should be apparent that there is a repetitive pattern (group beating) for QRS complexes, with alternating groups of one or two beats. That said, at this point in the interpretation process, one could defer further assessment of the rhythm until after studying the rest of the 12-lead ECG.
Finding 1. The most obvious finding on this ECG is the presence of ST-segment elevation in each inferior lead, with reciprocal ST-segment depression in lead aVL. In a patient with new-onset chest pain, this is diagnostic of acute inferior ST-segment elevation myocardial infarction (STEMI).
Finding 2. QRS amplitude is higher in the lateral chest leads. The presence of an R wave ≥ 18 mm in lead V6 satisfies voltage criteria for left ventricular hypertrophy (LVH). Although difficult to tell due to artifact, the ST-T waves in leads V5 and V6 suggest repolarization changes of LV “strain.”
Finding 3. In addition to acute inferior infarction, one might suspect there is acute posterior MI. Usually, acute posterior MI is recognized by anterior ST-depression. It is possible the upright ST-T waves seen so commonly in the anterior leads of patients with LVH attenuated the anterior ST depression and that this is the reason anterior ST-T waves are so uncharacteristically flat.
Finding 4. Second-degree AV block and Mobitz Type I (AV Wenckebach) with alternating 3:2 and 2:1 AV conduction. The most common cause of group beating in association with acute inferior STEMI is Mobitz I second-degree AV block. If one focuses within the groups of two beats (i.e., beats #1-2, 4-5, 6-7, 8-9, and 11-12), it should be apparent that the PR interval increases until a beat is dropped. Then, the cycle begins again.