Bradycardia can result from impaired impulse generation or impaired conduction, and a distal pacemaker may take over when the sinus rate becomes unduly slow.
Mechanisms of bradycardia
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Bradycardia may arise from impaired impulse generation by the sinoatrial node.
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It may also result from impaired impulse propagation through the atrioventricular node or His–Purkinje conduction system.
Escape rhythms
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When the sinus rate becomes unduly slow, a more distal part of the conducting system may assume the pacemaker role, producing an escape rhythm.
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An escape rhythm arising in the atrioventricular node or His bundle is a junctional rhythm.
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An escape rhythm arising in the ventricles is an idioventricular rhythm.