THE CONTEXT
Normal cardiac activation begins in the sinoatrial node, spreads through the atria and reaches the ventricles through the atrioventricular node and His–Purkinje system.
Sinoatrial initiation and atrial spread
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The sinoatrial (SA) node lies at the junction of the superior vena cava and right atrium.
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Its specialised atrial cells depolarise at a rate influenced by the autonomic nervous system and circulating catecholamines.
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During sinus rhythm, the depolarisation wave spreads through sheets of atrial myocytes.
Atrioventricular conduction and delay
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The annulus fibrosus forms an electrical barrier between the atria and ventricles, preventing conduction except through the atrioventricular (AV) node.
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The AV node is a midline structure extending from the right side of the interatrial septum and penetrating the annulus fibrosus anteriorly.
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Slow conduction through the AV node provides the necessary delay between atrial and ventricular contraction.
Ventricular activation and repolarisation
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The bundle of His extends from the AV node into the interventricular septum. The right and left bundle branches pass along the septum into their respective ventricles.
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The left bundle branch has anterior and posterior fascicles. Smaller Purkinje fibres ramify through the ventricular myocardium.
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Rapid conduction through the His–Purkinje system allows near-simultaneous depolarisation of the ventricular myocardium.
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Ventricular repolarisation spreads from epicardium to endocardium.