Bradycardia

Bradycardia

190px Lead II showing sinus bradycardia
ICD-10	R00.1
ICD-9	427.81, 659.7, 785.9, 779.81
MeSH	D001919

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. It may cause cardiac arrest in some patients, because those with bradycardia may not be pumping enough oxygen to their heart. It sometimes results in fainting, shortness of breath, and if severe enough, death.

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 actually below 60 beats per minute, is considered too slow for the individual's current medical condition.

Classification

Atrial bradycardia

See also: Sinus bradycardia and Sick sinus syndrome

Atrial bradycardias come in three different types. The first is sinus bradycardia. This is usually found in young and healthy adults. The symptoms represent with the individual's respirations. Each inhalation corresponds with the heart rate decreasing. Expiration causes an increase in the heart's rate of contraction. This is thought to be caused by changes in the vagal tone during respiration.

Sinus bradycardia is a sinus rhythm of less than 60 bpm. It is a common condition found in both healthy individuals and those who are considered well conditioned athletes. Studies have found that 50 - 85 percent of conditioned athletes have benign sinus bradycardia, as compared to 23 percent of the general population studied. The reason for this is that their heart muscle has become conditioned to have a higher stroke volume and therefore requires fewer contractions to circulate the same volume of blood.

Sick sinus syndrome covers conditions that include severe sinus bradycardia, sinoatrial block, sinus arrest, and bradycardi-tachycardia syndrome (atrial fibillation, flutter, and paroxysmal supraventricular tachycardia).

Atrioventricular nodal bradycardia

See also: Junctional rhythm

An atrioventricular nodal bradycardia or AV junction rhythm is usually caused by the absence of the electrical impulse from the sinus node. This usually appears on an EKG with a normal QRS complex accompanied with an inverted P wave either before, during, or after the QRS complex.

An AV junctional escape is a delayed heartbeat originating from an ectopic focus somewhere in the AV junction. It occurs when the rate of depolarization of the SA node falls below the rate of the AV node. This dysrhythmia also may occur when the electrical impulses from the SA node fail to reach the AV node because of SA or AV block. This is a protective mechanism for the heart, to compensate for a SA node that is no longer handling the pacemaking activity, and is one of a series of backup sites that can take over pacemaker function when the SA node fails to do so. This would present with a longer PR interval. A junctional escape complex is a normal response that may result from excessive vagal tone on the SA node. Pathological causes include sinus bradycardia, sinus arrest, sinus exit block, or AV block.

Ventricular bradycardia

See also: Atrioventricular block

A ventricular bradycardia, also known as ventricular escape rhythm or idioventricular rhythm, is a heart rate of less than 50 beats a minute. This a safety mechanism that arises when there is lack of electrical impulse or stimuli from the atrium. Impulses originating from or below the His bundle, also known as ventricular, will produce a wide QRS complex with heart rates between 20 and 40 beats a minute. Those above the His bundle, also known as junctional, will typically range between 40 and 60 bpm with a narrow QRS complex. In a third degree heart block, approximately 61% take place at the bundle branch-Purkinje system, 21% at the AV node, and 15% at the His bundle. AV block maybe ruled out with an EKG indicating "a 1:1 relationship between P waves and QRS complexes." Ventricular bradycardias occurs with sinus bradycardia, sinus arrest, and AV block. Treatment often consist of the administration of atropine and cardiac pacing.

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"). Only second degree sinostrial 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; 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. 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.

Diagnosis

A diagnosis of bradycardia in adults is based on a heart rate less than 60. This is determined usually either via palpation or an ECG.

If symptoms occur, a determination of electrolytes may be helpful in determining the underlying cause.

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 affected by the bradycardia. Treatment depends on whether any symptoms are present, and what the underlying cause is.

Drug treatment is not needed if the patient is asymptomatic.

Atropine

An atropine IV may provide temporary improvement in symptomatic patients.

See also

• Bradypnea
• Reflex bradycardia
• Sinus bradycardia
• Tachycardia
• Hypotension
• Hypoventilation
• Vagus nerve

References

ca:Bradicàrdia uk:Брадикардія