ECG basics: waves, intervals and leads !
- Measurement
- Exam priority: high
The ECG records the heart's electrical activity from the skin; each wave shows a part of the heart activating or recovering.
Updated 2026-11-01
Also called: Electrocardiogram, 12-lead ECG
See it in 3D: Sinoatrial (SA) node · Atrioventricular (AV) node · Bundle branches · Left ventricle
What it is
The electrocardiogram records, from skin electrodes, the sum of the voltage changes of the heart muscle. Each lead looks at the heart from one direction: a wave is upright when depolarization moves toward that lead. Reading it means checking rate, rhythm, intervals, axis and the shape of the waves.
Key facts
- Sinus rhythm: a P wave before every QRS, a QRS after every P, P upright in II and inverted in aVR.
- ST segment = the plateau, when all ventricular cells are depolarized; normally isoelectric.
- The U wave (after T) becomes visible in hypokalemia.
Electrocardiogram: a normal beat in lead II
HR 75 /minRR 0.80 sPR 0.16 sQRS 0.09 sQT 0.37 sQTc 0.41 s
Tap a wave (P, QRS, T, U), an interval (PR, ST, QT) or the RR.
- ECG trace (lead II)
- Small box: 0.04 s · 0.1 mV
- Big box: 0.20 s · 0.5 mV
- Selected part
Equations
- HR
- heart rate (per minute) at 25 mm/s
Equivalent to 1500 / small boxes; for irregular rhythms count QRS in 6 s and multiply by 10.
- QTc
- rate-corrected QT
- RR
- interval between beats, in seconds
Overcorrects at fast rates; other formulas exist.
Normal adult values
| Parameter | Value | Note |
|---|---|---|
| Heart rate | 60–100/min | |
| PR interval | 0.12–0.20 s | |
| QRS duration | < 0.12 s | |
| QTc | ≈ 0.36–0.44 s | upper limit a little higher in women |
| Frontal QRS axis | ≈ −30° to +90° |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- ST elevation in contiguous leads points to transmural injury in that territory (e.g. II, III, aVF = inferior wall).
- Atrial fibrillation: no P waves and an irregularly irregular rhythm; it raises stroke risk.
Test yourself
Think of the answer, then tap the question to see it.
Why is there no separate wave for atrial repolarization on a normal ECG?
It is small and happens at the same time as ventricular depolarization, so it is hidden within the QRS complex.
There are 4 large boxes between consecutive R waves at standard speed. What is the heart rate?
About 75/min (300 ÷ 4).
In lead II, why are the P wave and QRS normally upright?
Lead II looks from the lower left; atrial and ventricular depolarization travel overall toward the lower left, toward its positive electrode.
Which electrical event does the QT interval represent, and why must it be corrected for rate?
The total duration of ventricular depolarization plus repolarization; action potentials shorten at fast rates, so QT shortens and needs correcting before comparing.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The cardiac conduction system
- The cardiac electrical axis
- The ECG in ischemia and infarction
- The ECG in potassium and calcium disorders
- How arrhythmias arise
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 19.2 Cardiac Muscle and Electrical Activity (OpenStax (Rice University), 2022); Atrial Fibrillation (StatPearls (NCBI Bookshelf), 2025); Myocardial Infarction (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 11). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.