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By Ken Grauer, MD
Figure. 12-lead ECG obtained from a 57-year-old woman with palpitations. Note the three-beat run in lead V1.
Clinical Scenario: The electrocardiogram (ECG) in the Figure was obtained from a 57-year-old woman with palpitations. Is there a short run of ventricular tachycardia (VT) in lead V1? What else may be wrong with the tracing?
Interpretation: The rhythm is rapid and irregularly irregular. The absence of P waves defines this as atrial fibrillation with a rapid ventricular response. Three abnormal looking beats are seen consecutively in lead V1 (beginning with beat Y). Although at first glance one might be tempted to interpret this three-beat run as a salvo of VT, it is much more likely that these three beats are supraventricular in etiology and conducted with aberration. Each of these three beats manifests an rsR' morphology consistent with a right bundle-branch block pattern (RBBB). Note that the initial direction and magnitude of the small r wave is identical for the three anomalous and the three narrow beats that occur in this lead. This is consistent with the conduction defect that occurs with RBBB, in which the initial vector of left-to-right septal depolarization is unaffected by the conduction disturbance. In further support that these beats are aberrantly conducted is the finding of a reason for aberrant conduction: the coupling interval of the first anomalous complex (the distance between beat X and beat Y) is short (increased likelihood that there will be relative refractoriness of the ventricular conduction system). In addition, the preceding R-R interval (i.e., the R-R interval before beat X) is relatively long. Because the length of the refractory period is determined by the duration of the preceding R-R interval, the opportunity for aberrant conduction to occur is enhanced when early occurring beats (such as Y) are preceded by a relatively longer R-R interval. Finally, the complex labeled Z (in lead V6) manifests a QRS consistent with incomplete RBBB conduction (it is the only complex in lead V6 with an S wave), thereby providing additional evidence of RBBB-type conduction delay for selected beats on this tracing. Although not infallible, the combination of morphologic features described above (that are consistent with a bundle-branch block form of conduction delay) occurring in a setting that predisposes to aberrant conduction (i.e., rapid atrial fibrillation) strongly suggest that the three-beat run seen in lead V1 is not VT. Rate control of this patient’s rapid atrial fibrillation probably will result in resolution of anomalous complexes. Otherwise, there is low-voltage, non-specific ST-T wave abnormalities and a highly unusual negative complex in lead I. In view of the upright QRS complex in lead aVR, limb lead reversal would not negate our interpretation of the rhythm, nor of the high likelihood for aberrant conduction of the three-beat run that is seen in the precordial leads.
Dr. Grauer, Professor and Associate Director, Family Practice Residency Program, Department of Community Health and Family Medicine, College of Medicine, University of Florida, Gainesville, is on the Editorial Board of Emergency Medicine Alert.