ECG ECG Waveform Measurements and Intervals 1 — Questions and Answers
Question 1: What is the normal duration of the PR interval in adults?
- 0.04–0.10 seconds
- 0.12–0.20 seconds (Correct answer)
- 0.20–0.40 seconds
- 0.40–0.60 seconds
Correct 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.
Question 2: The QRS complex normally has a duration of:
- Less than 0.04 seconds
- 0.06–0.10 seconds (Correct answer)
- 0.12–0.20 seconds
- Greater than 0.20 seconds
Correct 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.
Question 3: The QT interval represents:
- Atrial depolarization only
- Total ventricular depolarization and repolarization (Correct answer)
- AV nodal conduction time
- Sinoatrial conduction time
Correct 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).
Question 4: A corrected QT interval (QTc) is calculated to account for:
- Patient age
- Heart rate variation (Correct answer)
- Lead placement differences
- Blood pressure
Correct 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.
Question 5: What does a prolonged QTc interval (>500 ms) indicate clinically?
- First-degree AV block
- Increased risk of torsades de pointes ventricular tachycardia (Correct answer)
- Left ventricular hypertrophy
- Normal variant in athletes
Correct 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.
Question 6: The ST segment on the ECG represents:
- Atrial repolarization
- The period between ventricular depolarization and repolarization (plateau phase) (Correct answer)
- P-wave production
- His-Purkinje conduction
Correct 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.
ECG ECG Waveform Measurements and Intervals 1
What is the normal duration of the PR interval in adults?