Which description best characterizes right bundle branch block on an ECG?

• A. QRS duration >120 ms with rsR' pattern in V1 and V2 and broad S waves in lateral leads
• B. QRS duration <100 ms with a dominant R wave in aVL
• C. Peaked T waves in V2 with ST elevation
• D. Short QT interval with delta waves

Answer: (A)

Right bundle branch block on an ECG is defined by a widened QRS complex, reflecting delayed activation of the right ventricle. In the right precordial leads V1 and V2 you look for an rsR' pattern—the classic “bunny ears” appearance with a small initial R, followed by a second R' peak. In the lateral leads (I, aVL, V5, V6) you’ll see broad, late S waves because the left ventricle finishes activating later than usual. So the combination of QRS duration over 120 ms, an rsR' pattern in V1–V2, and broad S waves in lateral leads best characterizes a right bundle branch block. The other patterns don’t fit because they lack the QRS widening and the distinctive V1–V2 rsR' morphology or show features more consistent with other conditions (e.g., hyperkalemia/ischeia, WPW).