Which dysrhythmia is consistent with slowed conduction through the right ventricle, often presenting with a wide QRS complex and an rsR' pattern in V1?

• A. Left anterior fascicular block
• B. Right bundle branch block
• C. Left posterior fascicular block
• D. Left bundle branch block

Answer: (B)

Slowed conduction through the right bundle branch delays activation of the right ventricle, so the ventricles depolarize out of sequence and the QRS becomes wide. In the right precordial lead V1, this delayed right-sided activation produces an rsR' pattern—a small initial R wave, a subsequent S drop, and then a tall R' wave. The combination of a wide QRS and the rsR' pattern in V1 is the classic signature of a right bundle branch block. Other blocks change the QRS width or axis and produce different patterns in V1 and lateral leads, rather than this distinctive rsR' appearance with a widened QRS.