[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Cardiovascular](https://durgical.com/en/physiology/#s-cardio) › ECG basics: waves, intervals and leads [Physiology](https://durgical.com/en/physiology/) · [Cardiovascular](https://durgical.com/en/physiology/#s-cardio) # 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](https://durgical.com/en/anatomy/sinuatrial-node/) · [Atrioventricular (AV) node](https://durgical.com/en/anatomy/atrioventricular-node/) · [Bundle branches](https://durgical.com/en/anatomy/right-bundle-branch/) · [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) ## Key points - P = atrial depolarization; QRS = ventricular depolarization (atrial repolarization hidden inside); T = ventricular repolarization. - PR 0.12–0.20 s; QRS - 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 /min **RR** 0.80 s **PR** 0.16 s **QRS** 0.09 s **QT** 0.37 s **QTc** 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 Tap each wave and interval; change the heart rate and watch the TP segment shrink first. ## Equations **Heart rate from the ECG**HR = 300 / large boxes between R waves **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. **Bazett correction**QTc = QT / √RR **QT c** 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 | - 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. ## Memory aid Read every ECG in the same order, like a pilot's checklist: Rate, Rhythm, Intervals, Axis, Waves. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/ecg-basics-waves-intervals-and-leads/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Sinoatrial (SA) node](https://durgical.com/en/anatomy/sinuatrial-node/) - [Atrioventricular (AV) node](https://durgical.com/en/anatomy/atrioventricular-node/) - [Bundle branches](https://durgical.com/en/anatomy/right-bundle-branch/) - [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) ## Related topics - [The cardiac conduction system](https://durgical.com/en/physiology/the-cardiac-conduction-system/) - [The cardiac electrical axis](https://durgical.com/en/physiology/the-cardiac-electrical-axis/) - [The ECG in ischemia and infarction](https://durgical.com/en/physiology/the-ecg-in-ischemia-and-infarction/) - [The ECG in potassium and calcium disorders](https://durgical.com/en/physiology/the-ecg-in-potassium-and-calcium-disorders/) - [How arrhythmias arise](https://durgical.com/en/physiology/how-arrhythmias-arise/) ## Related conditions - [Myocardial Infarction (Heart Attack)](https://durgical.com/en/conditions/myocardial-infarction/) - [Atrioventricular (heart) block](https://durgical.com/en/conditions/atrioventricular-block/) - [Atrial fibrillation](https://durgical.com/en/conditions/atrial-fibrillation/) - [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/) [← All of physiology](https://durgical.com/en/physiology/) 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](https://openstax.org/books/anatomy-and-physiology-2e/pages/19-2-cardiac-muscle-and-electrical-activity) (OpenStax (Rice University), 2022); [Atrial Fibrillation](https://www.ncbi.nlm.nih.gov/books/NBK526072/) (StatPearls (NCBI Bookshelf), 2025); [Myocardial Infarction](https://www.ncbi.nlm.nih.gov/books/NBK537076/) (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). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/