Cardiac Conduction and EKG Waves Explained
Updated October 27, 2026
Every EKG tracing shows the heart’s electrical activity as it travels through the conduction system. Knowing that path, and which wave matches which event, is the foundation of ECG interpretation and the NHA CET exam. Here’s how it works, followed by free practice questions.
The conduction system
- SA node (in the right atrium): the heart’s natural pacemaker, usually firing 60 to 100 times a minute.
- Atria: the impulse spreads across both atria, which contract.
- AV node: delays the impulse briefly so the ventricles can fill. If the SA node fails, it can pace at about 40 to 60 beats a minute.
- Bundle of His, then the right and left bundle branches.
- Purkinje fibers: spread the impulse through the ventricles, which contract. The ventricles can pace on their own at about 20 to 40 beats a minute.
What each wave means
| Wave or interval | Represents | Normal |
|---|---|---|
| P wave | Atrial depolarization (the atria contracting) | Upright, one before each QRS |
| PR interval | Time from the start of atrial depolarization to the start of ventricular depolarization | 0.12 to 0.20 seconds |
| QRS complex | Ventricular depolarization (the ventricles contracting) | Less than 0.12 seconds |
| ST segment | The time between ventricular depolarization and repolarization | Usually flat, at the baseline |
| T wave | Ventricular repolarization (the ventricles resetting) | Usually upright |
Atrial repolarization happens during the QRS complex, so it isn’t seen as a separate wave.
Free practice questions
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Which structure is the heart's natural pacemaker?Answer: CThe sinoatrial (SA) node, in the right atrium, normally starts each heartbeat at 60 to 100 beats per minute.
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What does the P wave represent?Answer: AThe P wave shows the atria depolarizing, which leads to atrial contraction.
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What does the QRS complex represent?Answer: DThe QRS complex shows the electrical impulse spreading through the ventricles, which leads to ventricular contraction.
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Which wave represents ventricular repolarization?Answer: BThe T wave shows the ventricles repolarizing, or resetting electrically, before the next beat.
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If the SA node fails, what is the usual rate when the AV junction takes over as pacemaker?Answer: BThe AV junction's intrinsic rate is about 40 to 60 beats per minute. The ventricles' own rate is lower, about 20 to 40.
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Why does the AV node briefly delay the electrical impulse?Answer: AThe short delay at the AV node lets the atria finish emptying blood into the ventricles before the ventricles contract.
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