Which interval spans from the start of ventricular depolarization to the end of ventricular repolarization?

• A. Q-T interval
• B. P-R interval
• C. S-T segment
• D. A-V interval

Answer: (A)

The QT interval reflects the total time of ventricular electrical activity, spanning from the start of ventricular depolarization to the end of ventricular repolarization. It begins at the first deflection of the Q wave (the start of the QRS complex) and ends at the end of the T wave. This single interval covers both the depolarization and repolarization phases of the ventricles, i.e., the entire ventricular electrical cycle. Clinically, it’s important because a prolonged QT can predispose to dangerous arrhythmias.

The other intervals don’t fit this span: the P-R interval measures the time from atrial depolarization to the start of ventricular depolarization, not including repolarization. The S-T segment is the flat portion between depolarization and repolarization and does not cover the whole cycle. An A-V interval isn’t a standard ECG term for this purpose; AV conduction time corresponds to the PR interval.