ECG Review

Chest Pain and Pauses in Lead II

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.

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Figure — ECG from a woman with chest pain. What is the rhythm?

Scenario: The ECG shown above was obtained from a 50-year-old woman with new-onset chest pain. What is the rhythm? What are the clues to this rhythm diagnosis?

Interpretation: As always, it is best to begin with interpretation of the rhythm. Therefore, we focus our attention first on the lead II rhythm strip at the bottom of the tracing. Note that there are "groups" of beats (i.e., beats 1 through 5; 6-8; 9-11; and 12-14). Each of these groups is separated by a short pause. There is a pattern to what we see — each pause is approximately equal in duration — and a definite P wave with a fixed (albeit slightly prolonged) PR interval is seen at the end of each pause (i.e., before beats 6, 9, and 12). These P waves at the end of each pause are clearly conducting.

Additional P waves are present. These are seen to notch the ST segments of beats 3, 4, 5, 7, 8, 11, and 14. Close inspection suggests an extra peak to the T waves of beats 1, 6, 9, and 10. Use of calipers facilitates verifying that an underlying regular atrial rhythm is actually present. Setting calipers to the P-P interval between beat 6 and the obviously hiding P wave within the ST segment of beat 5 allows us to walk out regular P waves throughout the entire lead II rhythm strip.

Leaving the rhythm for a moment, let us interpret the rest of the 12-lead ECG obtained from this 50-year-old woman with new-onset chest pain. The QRS complex is narrow. There is marked ST segment elevation in each of the inferior leads (II, III, aVF) and reciprocal ST depression in leads aVL, V1, and V2. The overall impression suggests acute inferior (and possibly also posterior) infarction.

Putting the entire picture together suggests that the rhythm disturbance is second degree AV block, Mobitz Type I (= AV Wenckebach). Mobitz I is often seen during the early hours of acute inferior infarction. Additional clues to this conduction disturbance include the pattern of group beating described above and first-degree AV block for conducting beats. Although difficult to discern because of the relatively rapid atrial rate (that partially hides most P waves within preceding ST segments), the PR interval does progressively lengthen within each group until a beat is dropped.

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