Bundle branch block is defined by which QRS criterion?

Bundle branch block is defined by which QRS criterion?

• A. QRS duration less than 0.10 seconds
• B. QRS duration 0.12 seconds or larger
• C. Absent QRS
• D. Prolonged QT interval

Answer: (B)

A wide QRS complex signals a bundle branch block because ventricular depolarization is delayed when the conduction pathway through one of the bundle branches is blocked. The standard criterion is a QRS duration of 0.12 seconds (120 milliseconds) or longer, which shows that ventricular activation is taking longer than normal. Normal QRS duration is up to about 0.10 seconds, so anything at or above 0.12 s indicates intraventricular conduction delay consistent with a block. The other options don’t fit: a QRS shorter than 0.10 s is normal, an absent QRS would imply no ventricular activity or an electrode issue, and a prolonged QT interval relates to repolarization timing, not the width of the QRS. In practice, measure from the start of the Q wave to the end of the S wave; three small squares equal 0.12 seconds.

A wide QRS complex signals a bundle branch block because ventricular depolarization is delayed when the conduction pathway through one of the bundle branches is blocked. The standard criterion is a QRS duration of 0.12 seconds (120 milliseconds) or longer, which shows that ventricular activation is taking longer than normal. Normal QRS duration is up to about 0.10 seconds, so anything at or above 0.12 s indicates intraventricular conduction delay consistent with a block. The other options don’t fit: a QRS shorter than 0.10 s is normal, an absent QRS would imply no ventricular activity or an electrode issue, and a prolonged QT interval relates to repolarization timing, not the width of the QRS. In practice, measure from the start of the Q wave to the end of the S wave; three small squares equal 0.12 seconds.