Bradycardia
Encyclopedia
Bradycardia in the context of adult medicine, is the 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 in explaining a heart rate which although not actually below 60 beats per minute still is considered too slow for the individual's current medical condition.
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
).
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.
and cardiac pacing.
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.
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; 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.
If symptoms occur, a determination of electrolytes may be helpful in determining the underlying cause.
. Doses less than 0.5 mg should not be used as this may further decrease the rate. If this is not effective intravenous inotrope
infusion (dopamine
, epinephrine
) or transcutaneous pacing
should be used. Transvenous pacing
may be required if the cause of the bradycardia is not rapidly reversible.
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....
of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. It may cause cardiac arrest
Cardiac arrest
Cardiac arrest, is the cessation of normal circulation of the blood due to failure of the heart to contract effectively...
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
Miguel Indurain
Miguel Ángel Indurain Larraya is a retired Spanish road racing cyclist. He won five consecutive Tour de Frances from 1991 and 1995, the first to do so, and the fourth athlete to win five times. He won the Giro d'Italia twice, becoming one of only seven people in history to achieve the Giro Tour...
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 in explaining a heart rate which although not actually below 60 beats per minute still is considered too slow for the individual's current medical condition.
Definition
Bradycardia in an adult is any heart rate less than 60 beats per minute, although symptoms usually manifest only for heart rates less than 50.Atrial
Atrial bradycardias come in three different types. The first is respiratory sinus arrhythmia.. This is usually found in young and healthy adults. Heart rate increases during inhalation and decreases during exhalation. This is thought to be caused by changes in the vagal tone during respiration. If the decrease during exhalation drops the heart rate below 60bpm on each breath, this type of bradycardia is usually deemed benign and a sign of good autonomic tone.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
Athletic heart syndrome
Athletic heart syndrome, also known as athlete's heart or athletic bradycardia, is a medical syndrome in which the human heart is enlarged due to significant amounts of exercise...
. Studies have found that 50 - 85 percent of conditioned athletes have benign
Benign
A benign tumor is a tumor that lacks the ability to metastasize. Common examples of benign tumors include moles and uterine fibroids.The term "benign" implies a mild and nonprogressive disease. Indeed, many kinds of benign tumors are harmless to human health...
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
Sick sinus syndrome
Sick sinus syndrome, also called sinus node dysfunction, is a group of abnormal heart rhythms presumably caused by a malfunction of the sinus node, the heart's primary pacemaker...
covers conditions that include severe sinus bradycardia, sinoatrial block, sinus arrest, and bradycardi-tachycardia syndrome (atrial fibillation, flutter, and paroxysmal supraventricular tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
).
Atrioventricular nodal
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 complexQRS complex
The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...
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
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
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
PR interval
In electrocardiography, the PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. It is usually 120 to 200 ms long. On the usual 25 mm/s ECG tracing, this corresponds to 3 to 5 small boxes. The PR interval reflects the time the electrical impulse takes to...
. 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
A ventricular bradycardia, also known as ventricular escape rhythm or idioventricular rhythm, is a heart rate of less than 50 beats a minute. This is 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 atropineAtropine
Atropine is a naturally occurring tropane alkaloid extracted from deadly nightshade , Jimson weed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects...
and cardiac pacing.
Infantile
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 (theophyllineTheophylline
Theophylline, also known as dimethylxanthine, is a methylxanthine drug used in therapy for respiratory diseases such as COPD and asthma under a variety of brand names. Because of its numerous side-effects, the drug is now rarely administered for clinical use. As a member of the xanthine family, it...
or caffeine
Caffeine
Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...
) 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
Recreational drug use
Recreational drug use is the use of a drug, usually psychoactive, with the intention of creating or enhancing recreational experience. Such use is controversial, however, often being considered to be also drug abuse, and it is often illegal...
use or abuse
Drug abuse
Substance abuse, also known as drug abuse, refers to a maladaptive pattern of use of a substance that is not considered dependent. The term "drug abuse" does not exclude dependency, but is otherwise used in a similar manner in nonmedical contexts...
; metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
or endocrine issues, especially in the thyroid
Hypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...
; an electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
imbalance; neurologic factors; autonomic reflexes; situational factors such as prolonged bed rest
Bed rest
Bed rest is a medical treatment involving a period of consistent recumbence in bed. It is used as a treatment for an illness or medical condition, especially when prescribed or chosen rather than resulting from severe prostration or imminent death...
; and autoimmunity.
Cardiac causes include acute or chronic ischemic heart disease, vascular heart disease
Heart disease
Heart disease, cardiac disease or cardiopathy is an umbrella term for a variety of diseases affecting the heart. , it is the leading cause of death in the United States, England, Canada and Wales, accounting for 25.4% of the total deaths in the United States.-Types:-Coronary heart disease:Coronary...
, 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
Sinoatrial node
The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...
(SA node), and disorders of the atrioventricular node
Atrioventricular node
The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...
(AV node).
With sinus node dysfunction (sometimes called sick sinus syndrome
Sick sinus syndrome
Sick sinus syndrome, also called sinus node dysfunction, is a group of abnormal heart rhythms presumably caused by a malfunction of the sinus node, the heart's primary pacemaker...
), there may be disordered automaticity
Automaticity
Automaticity is the ability to do things without occupying the mind with the low-level details required, allowing it to become an automatic response pattern or habit. It is usually the result of learning, repetition, and practice....
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
Artificial pacemaker
A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...
.
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
Bundle of His
The bundle of His, known as the AV bundle or atrioventricular bundle, is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node to the point of the apex of the fascicular branches...
. 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
The treatment of bradycardia is dependent on whether or not the person is stable or unstable. If oxygen saturations are low supplemental oxygen should be provided.Stable
Emergent treatment is not needed if the person is asymptomatic or minimally symptomatic.Unstable
If a person is unstable the initial recommended treatment is intravenous atropineAtropine
Atropine is a naturally occurring tropane alkaloid extracted from deadly nightshade , Jimson weed , mandrake and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects...
. Doses less than 0.5 mg should not be used as this may further decrease the rate. If this is not effective intravenous inotrope
Inotrope
An inotrope is an agent that alters the force or energy of muscular contractions. Negatively inotropic agents weaken the force of muscular contractions...
infusion (dopamine
Dopamine
Dopamine is a catecholamine neurotransmitter present in a wide variety of animals, including both vertebrates and invertebrates. In the brain, this substituted phenethylamine functions as a neurotransmitter, activating the five known types of dopamine receptors—D1, D2, D3, D4, and D5—and their...
, epinephrine
Epinephrine
Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...
) or transcutaneous pacing
Transcutaneous pacing
Transcutaneous pacing is a temporary means of pacing a patient's heart during a medical emergency. It is accomplished by delivering pulses of electric current through the patient's chest, which stimulates the heart to contract....
should be used. Transvenous pacing
Transvenous pacing
Transvenous cardiac pacing, also called endocardial pacing, is a potentially life saving intervention used primarily to correct profound bradycardia. It can be used to treat symptomatic bradycardias that do not respond to transcutaneous pacing or to drug therapy...
may be required if the cause of the bradycardia is not rapidly reversible.