ECG Waveform Measurements and Intervals Flashcards

What is the normal duration of the PR interval in adults?

Answer: 0.12–0.20 seconds

The normal PR interval is 0.12–0.20 seconds (3–5 small squares at 25 mm/sec), representing conduction time from the SA node through the AV node.

The QRS complex normally has a duration of:

Answer: 0.06–0.10 seconds

A normal QRS duration is 0.06–0.10 seconds; values ≥0.12 seconds indicate intraventricular conduction delay such as bundle branch block.

The QT interval represents:

Answer: Total ventricular depolarization and repolarization

The QT interval spans from the start of the QRS to the end of the T-wave, encompassing all of ventricular electrical activity (depolarization and repolarization).

A corrected QT interval (QTc) is calculated to account for:

Answer: Heart rate variation

QT interval lengthens at slower heart rates and shortens at faster rates; the QTc corrects for this, allowing comparison across different heart rates using Bazett's formula.

What does a prolonged QTc interval (>500 ms) indicate clinically?

Answer: Increased risk of torsades de pointes ventricular tachycardia

A QTc >500 ms significantly increases the risk of torsades de pointes, a potentially fatal polymorphic ventricular tachycardia.

The ST segment on the ECG represents:

Answer: The period between ventricular depolarization and repolarization (plateau phase)

The ST segment corresponds to phase 2 (plateau) of the ventricular action potential, during which no net current flows and all cells are in an equally depolarized state.