A 62-year-old man with hypertension presents with palpitations. His ECG shows a wide QRS complex (140 ms) with the pattern marked **B** in the diagram — broad, slurred R waves in leads I, aVL, V5, and V6, with notably absent septal Q waves. Which of the following best explains the absence of the septal Q wave in this patient's lateral leads?

Explanation

## Why option 1 is right In left bundle branch block, the left bundle of His is blocked, preventing normal rapid conduction down the left side of the interventricular septum. Septal depolarization must now occur via slow myocyte-to-myocyte spread from the right ventricle (which is activated first via the intact right bundle). This reverses the normal septal activation vector from left-to-right to right-to-left. Since the septal Q wave in normal ECG represents the initial left-to-right septal depolarization, this reversal eliminates it entirely. This is a hallmark of LBBB and distinguishes it from other causes of wide QRS complexes (Harrison 21e Ch 247). ## Why each distractor is wrong - **Option 2**: LAD occlusion causes anterior MI with ST elevation and Q waves, not LBBB. Septal necrosis would produce pathologic Q waves, not their absence. The clinical presentation and ECG pattern here are consistent with LBBB, not acute MI. - **Option 3**: LV hypertrophy from hypertension may coexist with LBBB but does not explain the absence of septal Q waves. Hypertrophy alters voltage and axis, not the fundamental septal activation sequence. - **Option 4**: WPW causes a delta wave and short PR interval with a different QRS morphology (slurred upstroke). It does not produce the broad slurred R waves in lateral leads characteristic of LBBB, nor does it eliminate septal Q waves via the same mechanism. **High-Yield:** LBBB eliminates septal Q waves because the septum is now activated right-to-left (instead of left-to-right), reversing the normal septal vector — this is pathognomonic for LBBB and helps distinguish it from other wide QRS rhythms. [cite: Harrison 21e Ch 247]