This cardiac arrhythmia can be underlain by several causes, which are best divided into cardiac and non-cardiac causes.
Non-cardiac causes are usually secondary, and can involve 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.
There are generally two types of problems that 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").
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.
Patients with bradycardia have likely acquired it, as opposed to having it congenitally.
Bradycardia is more common in older patients.
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