Third-degree heart block (also called complete heart block) is the most severe form of atrioventricular (AV) block, in which there is complete dissociation between the electrical activity of the atria and the ventricles. No impulses from the atria are conducted to the ventricles at all; the ventricles rely on their own slow escape rhythm from a subsidiary pacemaker, typically producing a rate of 20 to 40 beats per minute.
The slow ventricular rate severely reduces cardiac output, causing dizziness, fatigue, breathlessness, and episodes of sudden loss of consciousness (Stokes-Adams attacks). The ECG shows P waves (atrial activity) and QRS complexes (ventricular activity) at different rates with no fixed relationship between them (complete AV dissociation). Causes include ischaemic heart disease (particularly inferior myocardial infarction affecting the AV node’s blood supply), degenerative conduction system fibrosis (the most common cause in older adults), certain medications, and inflammatory conditions such as Lyme disease or sarcoidosis.
Third-degree heart block is a recognised cause of cardiac arrest through prolonged asystole or ventricular fibrillation, and is an established indication for permanent pacemaker implantation. The pacemaker ensures the ventricles beat at an adequate rate regardless of AV node function.
Temporary complete heart block may occur as a complication of inferior myocardial infarction, typically resolving within days to weeks as the surrounding myocardium recovers. Permanent pacemaker implantation is required when complete heart block is persistent or when the underlying cause is progressive and irreversible. For detailed information see also Heart Block and Stokes-Adams Syndrome.
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