Normal QRS duration of atrial tachycardia?

• A. 0.12 seconds
• B. 0.10 or less
• C. 0.15 seconds
• D. 0.20 seconds

Answer: (B)

QRS duration shows how long the ventricles take to depolarize. In atrial tachycardia, the atria fire rapidly, but the ventricles usually conduct through the normal His-Purkinje system, so the QRS complex stays narrow. A normal QRS duration is about 0.08–0.10 seconds, with 0.10 seconds or less often used as the cut-off. That’s why the best choice is 0.10 seconds or less. If the QRS were wider (around 0.12 seconds or more), it would suggest slowed or aberrant ventricular conduction, such as a bundle branch block or a ventricular-origin rhythm, which isn’t typical for a straightforward atrial tachycardia.