ECG waves and intervals: what P, PR, QRS, ST and QT mean
Each ECG wave and interval maps to an electrical event in the heart. Read the gaps and you can tell where a conduction delay sits.
Chapters (10)
Relate each ECG wave and interval (P, PR, QRS, ST, QT) to the electrical event it represents, and recognise how a conduction delay changes them: a long PR points to the AV node, a wide QRS with a normal PR to a delay below it.
The short version
- P is atrial depolarization, QRS is ventricular depolarization, and T is ventricular repolarization.
- A long PR interval (over 200 ms) means a slow AV node: first-degree AV block.
- A wide QRS with a normal PR means the delay is below the AV node.
- The ST segment sits between ventricular depolarization and repolarization; when it lifts or sags, think ischemia first.
- The QT interval spans ventricular depolarization plus repolarization, and it shortens at fast rates and lengthens at slow ones.
An ECG strip with a gap: where is the delay?
A 58-year-old woman comes in for a check-up. On her ECG strip every P wave is followed by a QRS complex, but the gap between them is 280 ms. Normal is under 200 ms. So what is slow, and where?
This lesson on ECG waves and intervals matches each wave to the electrical event behind it, then shows how a conduction delay changes the intervals.[1][2]
The path of the signal behind the ECG
The signal begins in the sinoatrial node, spreads across the atria and reaches the atrioventricular node. There it pauses on purpose, then races down the His bundle, the bundle branches and the Purkinje fibers into the ventricles.[2][3]
The ECG does not show the nodes firing. It shows the electrical currents of the heart muscle, recorded at the skin.[2][3]
One beat: P wave, QRS complex and T wave
| Wave | Electrical event |
|---|---|
| P wave | Atrial depolarization[1][2] |
| QRS complex | Ventricular depolarization[1][2] |
| T wave | Ventricular repolarization[1][2] |
The QRS is tall because the ventricles hold so much more muscle than the atria. Atrial repolarization happens at the same moment as the QRS, so it is hidden inside it.[2]
The PR interval and AV node delay
The PR interval runs from the start of the P wave to the start of the QRS. It is mostly the time the signal spends crossing the AV node.[1][2]
Normal is 120–200 ms, which is three to five small squares of 40 ms each (standard paper speed, 25 mm/s).[1]
- PR interval = 280 ms
- 280 / 40 = 7 small squares
- 280 ms is over 200 ms, so AV conduction is delayed: first-degree AV block
Every P wave still conducts; it is just late.[1]
QRS duration: a delay below the AV node
A normal QRS lasts about 70–100 ms. That is short because the His–Purkinje wiring spreads the signal through both ventricles almost at once.[1][3]
If a bundle branch is blocked, the signal must crawl from muscle cell to muscle cell instead of using the fast wiring. The ventricles take longer to depolarize, so the QRS widens, while the PR interval stays normal.[1][3]
The ST segment and the QT interval
The ST segment is the flat stretch after the QRS and before the T wave, between ventricular depolarization and repolarization. The line sits on the baseline. When it lifts or sags, think ischemia first.[1][2]
The QT interval runs from the start of the QRS to the end of the T wave. It covers ventricular depolarization plus repolarization, roughly the whole of ventricular systole.[1][2]
It shortens at fast heart rates and lengthens at slow ones. A prolonged QT raises the risk of dangerous ventricular rhythms, such as torsades de pointes.[1]
Practice question
Original · USMLE styleA 68-year-old man has a routine pre-operative ECG. He has no symptoms. The tracing shows sinus rhythm, and every P wave is followed by a QRS complex. The PR interval is 160 ms and the QRS duration is 150 ms.
Common questions
What do the P wave, QRS complex and T wave represent?
The P wave is atrial depolarization, the QRS complex is ventricular depolarization, and the T wave is ventricular repolarization.
What is a normal PR interval?
120–200 ms, which is three to five small squares of 40 ms each. A PR interval over 200 ms means delayed AV conduction: first-degree AV block.
What does a wide QRS with a normal PR interval mean?
With every P wave conducted, a slow ventricular depolarization with intact AV node conduction places the delay below the AV node.
What does the QT interval measure?
It runs from the start of the QRS to the end of the T wave, covering ventricular depolarization plus repolarization. It shortens at fast heart rates and lengthens at slow ones.
Transcript
Read the full narration, by chapter710 words
0:00A patient with a gap between P and QRS
A 58-year-old woman comes in for a check-up, and her ECG strip looks strange. Every P wave is followed by a QRS complex, but the gap between them is 280 milliseconds. Normal is under 200. So what is slow, and where? Today: match each ECG wave and interval to the electrical event behind it, and see how a conduction delay changes them.
0:26The path of the signal: SA node to Purkinje fibers
Start with the path of the signal. It begins in the sinoatrial node, spreads across the atria, and reaches the atrioventricular node. There the signal pauses on purpose, then races down the His bundle, the bundle branches and the Per-kin-jee fibers into the ventricles. The ECG does not show the nodes firing. It shows the electrical currents of the muscle, picked up on the skin. The P wave is atrial depolarization.
0:52One beat: P wave, QRS complex, T wave
The QRS complex is ventricular depolarization, and it is tall because the ventricles hold so much more muscle. Atrial repolarization happens at the same moment, so it is hidden inside the QRS. The T wave is ventricular repolarization. P is the atria depolarizing, QRS is the ventricles depolarizing, and T is the ventricles recovering.
1:21The PR interval and AV node delay
Now the gaps, because the gaps carry the diagnosis. The PR interval runs from the start of the P wave to the start of the QRS. It is mostly the time the signal spends crossing the AV node. Normal is 120 to 200 milliseconds. That is three to five small squares, 40 milliseconds each. Our patient's 280 milliseconds is seven small squares. A PR interval over 200 milliseconds means delayed AV conduction: first-degree AV block.
1:57QRS duration and delay below the AV node
Next, the QRS duration. A normal QRS lasts about 70 to 100 milliseconds. That is short, because the His-Per-kin-jee wiring spreads the signal through both ventricles almost at once. If a bundle branch is blocked, the signal must crawl from muscle cell to muscle cell instead of using the fast wiring. The ventricles take longer to depolarize, so the QRS widens, while the PR interval stays normal. So a long PR points to the AV node, and a wide QRS with a normal PR points below the AV node.
2:35The ST segment
Now the ST segment. It is the flat stretch after the QRS and before the T wave, between ventricular depolarization and repolarization. The line sits on the baseline. When it lifts or sags, think ischemia first.
2:52The QT interval
Last, the QT interval. It runs from the start of the QRS to the end of the T wave. That covers ventricular depolarization plus repolarization, roughly the whole of ventricular systole. It shortens at fast heart rates and lengthens at slow ones. A prolonged QT raises the risk of dangerous ventricular rhythms, such as torsades de pointes.
3:17Back to our patient
On her strip the QRS is narrow and only the PR is long, so the delay sits in the AV node. Every P wave still conducts; it is just late. Your turn.
3:29Practice question
A 68-year-old man has a routine pre-operative ECG and no symptoms. It shows sinus rhythm, and every P wave is followed by a QRS complex. The PR interval is 160 milliseconds, and the QRS duration is 150 milliseconds. Which of the following best explains the QRS finding? A, slowed atrial depolarization. B, slowed conduction through the AV node. C, slowed ventricular repolarization. D, slowed conduction in a bundle branch. E, slowed sinoatrial node discharge. Pause here and decide. A is out: slow atrial depolarization would lengthen the P wave, not the QRS. B is out: AV node delay lengthens the PR interval, and his is normal. C is out: ventricular repolarization is the T wave and the end of the QT, not the QRS. E is out: sinoatrial discharge sets the rate, not the width of the ventricular complex. So the answer is D: a normal PR with a wide QRS means the delay is below the AV node, in a bundle branch.
4:50Recap and next lesson
Three things to keep. One: P is atrial depolarization, QRS is ventricular depolarization, and T is ventricular repolarization. Two: a long PR means a slow AV node, and a wide QRS with a normal PR means a delay below it. Three: the ST segment sits between depolarization and repolarization, and the QT interval spans both. Next, we use these ECG patterns at the bedside: bradycardia and AV block, stable versus unstable.
Sources
Numbers in the text, e.g. [1], point here- LITFL. ECG Library: P wave; PR interval; QRS interval; ST segment; T wave; QT interval. litfl.com/p-wave-ecg-library/ · litfl.com/pr-interval-ecg-library/ · litfl.com/qrs-interval-ecg-library/ · litfl.com/st-segment-ecg-library/ · litfl.com/t-wave-ecg-library/ · litfl.com/qt-interval-ecg-library/
- OpenStax. Anatomy and Physiology 2e, 19.2 Cardiac Muscle and Electrical Activity. openstax.org/books/anatomy-and-physiology-2e/pages/19-2-cardiac-muscle-and-electrical-activity
- Klabunde RE. CV Physiology: Electrocardiogram (ECG) basics (A013). cvphysiology.com/Arrhythmias/A013
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. 3D heart: "Anatomical Human Heart" by Nexal 3D (BlenderKit, BlenderKit Royalty Free licence). The ECG strips, conduction diagram, brackets, question card and all handwriting are drawn in code (original). Previous: EP 05 · SA Node Pacemaker Potential: Funny Current and Autonomic Control of Heart Rate · Next: Bradycardia: Unstable vs Stable (MS-CK-EMER-011)
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-CARD-015.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
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- EP 02Heart murmurs and dynamic manoeuvres, from preload and afterload5:15
- EP 03HOCM vs aortic stenosis murmur: telling them apart at the bedside5:31
- EP 04Ventricular action potential: phases 0 to 4 and their ion currents5:15
- EP 05SA node pacemaker potential: phase 4 sets the heart rate5:17
- EP 06ECG waves and intervals: what P, PR, QRS, ST and QT mean5:29