Causes
This cardiac arrhythmia can be underlain by several causes, which are best divided into cardiac and noncardiac causes. Noncardiac causes are usually secondary, and can involve recreational drug use or abuse; metabolic or endocrine issues, especially in the thyroid; an electrolyte imbalance; neurologic factors; autonomic reflexes; situational factors such as prolonged bed rest; and autoimmunity. Cardiac causes include acute or chronic ischemic heart disease, vascular heart disease, valvular heart disease, or degenerative primary electrical disease. Ultimately, the causes act by three mechanisms: depressed automaticity of the heart, conduction block, or escape pacemakers and rhythms.
Generally, two types of problems result in bradycardias: disorders of the sinoatrial node (SA node), and disorders of the atrioventricular node (AV node).
With sinus node dysfunction (sometimes called sick sinus syndrome), there may be disordered automaticity or impaired conduction of the impulse from the sinus node into the surrounding atrial tissue (an "exit block"). Only second degree sinoatrial blocks can be detected by use of a 12-lead EKG. It is difficult and sometimes impossible to assign a mechanism to any particular bradycardia, but the underlying mechanism is not clinically relevant to treatment, which is the same in both cases of sick sinus syndrome: a permanent pacemaker.
Atrioventricular conduction disturbances (aka: AV block; 1o AV block, 2o type I AV block, 2o type II AV block, 3o AV block) may result from impaired conduction in the AV node, or anywhere below it, such as in the Bundle of His. The clinical relevance pertaining to AV blocks is greater than that of sinoatrial blocks.
Patients with bradycardia have likely acquired it, as opposed to having it congenitally. Bradycardia is more common in older patients.
Beta-blocker medicines also can slow the heart rate and decrease how forcefully the heart contracts. Beta blockers may slow the heart rate to a dangerous level if prescribed together with calcium channel blocker type medications.
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