ECG Review: Incomplete LBBB?

By Ken Grauer, MD

Figure. 12-lead ECG recorded from a 47-year-old man with dyspnea.
Is there incomplete LBBB?

Clinical Scenario: Interpret the 12-lead ECG in the Figure, obtained from a 47-year-old man who presented with dyspnea. Is there incomplete LBBB (left bundle branch block)?

Interpretation: The rhythm is sinus at a rate of 75 beats/minute. Although PR and QT intervals are normal, the QRS complex appears to be slightly widened. Because criteria for assessment of axis, ventricular chamber enlargement and ischemia/infarction all change in the presence of a ventricular conduction defect, it is best to assess for the cause of QRS widening before going further. QRS morphology in the 3 key leads (I, V1, and V6) is consistent with a LBBB (left bundle branch block) pattern. However, the borderline amount of QRS widening (to 0.11 second) falls short of the 0.12 second duration usually required to diagnose complete LBBB. Instead, QRS widening in this tracing could represent incomplete LBBB. If this were the case, QRS voltage criteria for LVH (left ventricular hypertrophy), as well as the ST-T wave abnormalities seen in the lateral leads should not necessarily be interpreted as a strain or ischemic pattern, since they may simply reflect repolarization changes secondary to the conduction defect. Alternatively, QRS widening in this tracing might instead reflect marked ventricular enlargement, since conduction time through a hypertrophied ventricle would be expected to increase. If this were the case, then the ST-T wave changes seen here in the lateral leads would be perfectly consistent with a left ventricular strain pattern. This is in fact the situation here, as this patient demonstrated dramatic thickening on echo. Consistent with this echocardiographic picture of a dilated cardiomyopathy are the ECG findings in the Figure of right atrial enlargement (tall, peaked P wave in lead III), left atrial enlargement (deep negative component to the P wave in lead V1), and dramatic increase in QRS amplitude (very deep S waves in leads V1, V2 and tall R waves in leads V5, V6). Diagnosis of incomplete LBBB and distinction of this entity from LVH as the cause of slight QRS widening is often extremely difficult. And at times the two conditions may coexist . . .

Dr. Grauer, Professor, Assistant Director, Family Practice Residency Program, University of Florida, is Associate Editor of Internal Medicine Alert.