By Ken Grauer, MD
Figure. 12-lead ECG obtained from a 55-year-old woman with chest pain and lots of P waves.
Clinical Scenario: The 12-lead ECG shown in the Figure was obtained from a 55-year-old woman with new-onset chest pain. Many more P waves than QRS complexes are seen on the tracing (see dots under P waves in leads III, aVF, and V3). How would you interpret this ECG?
Interpretation: Although a single lead rhythm strip is lacking, the 12-lead ECG in the Figure can still be interpreted. A narrow-complex marked bradycardia is present that is fairly regular at a rate just over 30 beats/minute. As noted above, many more P waves than QRS complexes are present. The atrial rhythm (marked by the dots) is regular at a rate of between 90-95/minute. Despite the fact that many more P waves than QRS complexes are present, P waves appear to conduct, as evidenced by the presence of a fixed PR interval preceding each QRS complex. This finding rules out the possibility of 3° (complete) AV block, in which there is no relationship between P waves and QRS complexes (P waves "march through" the QRS complex when there is 3° AV block). The rhythm must therefore be some type of high-grade 2° AV block, in this case with 3:1 AV conduction (3 P waves are present for each QRS complex). Although high-grade AV block (in which many if not most P waves fail to conduct) is most often the result of Mobitz II 2° AV block, the lack of consecutively conducted P waves anywhere on this tracing precludes definitive diagnosis. It is important to appreciate that on occasion, the usually less severe Mobitz I (Wenckbeach) form of 2° AV block also may be "high grade," with failure of consecutively occurring P waves to conduct. In such situations, the characteristic picture of progressive PR interval lengthening prior to dropping a beat may not be seen.
Analysis of the remainder of the ECG in the Figure reveals marked right axis deviation (RAD) consistent with a left posterior hemiblock (LPHB) pattern, incomplete right bundle-branch block (IRBBB) evidenced by an rsr´ pattern in lead V1, early transition (a relatively tall R wave is present in lead V2), and worrisome ST segment depression in leads I, aVL, and V2-V6. An ECG obtained one hour earlier showed ST segment elevation in the inferior leads (which has now resolved). The overall picture in this 55-year-old woman with new-onset chest pain is most consistent with acute evolving infero-posterior infarction. Telemetry tracings over the previous hour revealed clear evidence of Mobitz I (Wenckbach) 2° AV block—which in conjunction with the findings of normal QRS duration and acute inferior infarction strongly suggest that the 2° AV block with 3:1 AV conduction seen here is probably also a manifestation of the Mobitz I (Wenckbach) form of 2° AV block. That said—the bifascicular conduction defect and acute infarction with marked bradycardia are clear indication for emergency pacemaker placement.
Dr. Grauer, Professor and Assistant Director, Family Practice Residency Program, University of Florida, Gainesville, is on the Editorial Board of Emergency Medicine Alert.
The 12-lead ECG shown in the figure was obtained from a 55-year-old woman with new-onset chest pain. Many more P waves than QRS complexes are seen on the tracing. How would you interpret this ECG?
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