Cardiac conduction system and the physiological basis of the ECG
The path of the cardiac impulse, its speeds, blood supply and autonomic control, how each ECG wave maps to the action potential, and how electrolytes, conduction blocks and accessory pathways change the trace.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- The impulse starts in the SA node, crosses the atria, is delayed in the AV node, then spreads fast through the His bundle, bundle branches and Purkinje fibres.
- Each ECG wave is a summed electrical vector: P atrial depolarisation, PR mostly AV nodal delay, QRS ventricular depolarisation, ST the plateau, T ventricular repolarisation.
- The QT interval approximates the duration of the ventricular action potential, so drugs and electrolytes that change phases 2 and 3 change the QT.
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Sources
- Guyton and Hall Textbook of Medical Physiology (14th ed.)Elsevier (Hall and Hall, 2021), 2021
- Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine (12th ed.)Elsevier (Libby et al., 2022), 2022
- AHA/ACCF/HRS Recommendations for the standardization and interpretation of the electrocardiogram, part IV: the ST segment, T and U waves, and the QT intervalCirculation 2009;119:e241-e250 (Rautaharju et al.), 2009
- Last updated
- 14 November 2026
What changed
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