bradycardia

Bradycardia (Greek βραδυκαρδία, bradykardía, "heart slowness"), as applied to adult medicine, is defined as a resting heart rate of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. ^[1][2]

Trained athletes or young healthy individuals may also have a slow resting heart rate (e.g. professional cyclist Miguel Indurain had a resting heart rate of 28 beats per minute). Resting bradycardia is often considered normal if the individual has no other symptoms such as fatigue, weakness, dizziness, lightheadedness, fainting, chest discomfort, palpitations or shortness of breath associated with it.

The term relative bradycardia is used to explain a heart rate that, while not technically below 60 beats per minute, is considered too slow for the individual's current medical condition.

Infantile bradycardia

For infants, bradycardia is defined as a heart rate of less than 100 beats per minute. (Normal is around 120-160 beats per minute.) Premature babies are more likely than full-term babies to have apnea and bradycardia spells; their cause is not clearly understood. Some researchers think the spells are related to centers inside the brain that regulate breathing and that may not be fully developed. Touching the baby gently or rocking the incubator slightly will almost always get the baby to start breathing again, which increases the heart rate. Medications (theophylline or caffeine) can be used to treat these spells in babies if necessary. NICU standard practice is to electronically monitor the heart and lungs for this reason.

Causes

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; 1^o AV block, 2^o type I AV block, 2^o type II AV block, 3^o 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.

Management

There are 2 main reasons for treating any cardiac arrhythmias. With bradycardia, the first is to address the associated symptoms, such as fatigue, limitations on how much a person can physically exert, fainting (syncope), dizziness or lightheadedness, or other vague and non-specific symptoms. The other reason to treat bradycardia is if the person's ultimate outcome (prognosis) will be changed or impacted by the bradycardia. Treatment depends on whether any symptoms are present, and what the underlying cause is.

Treatment

Drug treatment is not needed if the patient is asymptomatic.

In symptomatic patients, draw a Chem 12 and arterial blood gas to assess electrolytes and blood gases. An atropine IV may provide temporary improvement in symptomatic patients.

For symptomatic patients: Atropine 0.5–1 mg IV or ET q3-5min [maximum dose is 3 mg total] (0.04 mg/kg)

See also

• Reflex bradycardia
• Sinus bradycardia
• Tachycardia
• Hypotension

References