Heart block is a condition affecting the conduction system of the heart, in which electrical signals that coordinate heartbeats are delayed or completely blocked as they pass from the atria to the ventricles. In a healthy heart, electrical impulses travel from the sinoatrial (SA) node through the atria and then through the atrioventricular (AV) node and His-Purkinje system to activate the ventricles in a coordinated sequence. In heart block, this conduction pathway is disrupted at the level of the AV node or below.
Heart block is classified by severity into three degrees. First-degree heart block involves prolonged conduction time through the AV node (a prolonged PR interval on the ECG) but every atrial impulse still reaches the ventricles; it typically causes no symptoms and requires no treatment. Second-degree heart block involves intermittent failure of conduction, so some atrial impulses do not reach the ventricles. Third-degree (complete) heart block is the most severe form, in which no atrial impulses reach the ventricles at all; the ventricles rely on a very slow, independent escape rhythm at 20 to 40 bpm, causing significant symptoms.
Complete heart block produces a slow, ineffective heart rate that reduces cardiac output, causing dizziness, fatigue, breathlessness, and episodes of sudden loss of consciousness (Stokes-Adams attacks). Prolonged ventricular standstill before the escape rhythm establishes can cause cardiac arrest. Causes include ischaemic heart disease (particularly inferior myocardial infarction), degenerative fibrosis of the conduction system (the most common cause in older adults), medications such as beta-blockers and digoxin, and inflammatory conditions such as Lyme disease or sarcoidosis.
Symptomatic heart block, particularly second-degree type II or complete heart block, is an established indication for permanent pacemaker implantation. The pacemaker senses the absence of a natural ventricular beat and delivers a pacing stimulus to ensure the ventricles contract at an adequate rate, relieving symptoms and eliminating the risk of cardiac arrest from asystole.
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