The ECG records the electrical activity that makes the heart beat. Each wave matches one step of that activity.
P wave
The atria depolarize. A normal P wave before every QRS means the beat started in the sinus node.
PR interval
The time from the atria to the ventricles, including the delay in the AV node. Normal is 0.12 to 0.20 seconds.
QRS complex
The ventricles depolarize. Normal width is under 0.12 seconds. Wide QRS suggests a bundle branch block or a beat from the ventricle.
ST segment and T wave
The ventricles recover (repolarize). ST elevation or depression can signal heart muscle injury or ischemia.
QT interval
The whole ventricular cycle. A long QT, often from medicines or low potassium or magnesium, raises the risk of dangerous rhythms.
| Part | What it shows | Normal |
|---|---|---|
| P wave | Atria depolarize | Before every QRS |
| PR interval | Atria to ventricles | 0.12 to 0.20 s |
| QRS | Ventricles depolarize | Under 0.12 s |
| QT (corrected) | Whole ventricular cycle | About under 0.45 s in men, 0.46 s in women |
At the bedside
Read every strip in the same order: rate, rhythm, P waves, PR, QRS, then ST and T. Then look at the patient.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).