Atrioventricular block (AV block) is a type of
heart block
Heart block (HB) is a disorder in the heart's rhythm due to a fault in the natural pacemaker. This is caused by an obstruction – a block – in the electrical conduction system of the heart. Sometimes a disorder can be inherited. Despite the se ...
that occurs when the electrical signal traveling from the
atria, or the upper chambers of the heart, to
ventricles, or the lower chambers of the heart, is impaired. Normally, the
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
(SA node) produces an electrical signal to control the heart rate. The signal travels from the SA node to the ventricles through the
atrioventricular node
The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
(AV node). In an AV block, this electrical signal is either delayed or completely blocked. When the signal is completely blocked, the ventricles produce their own electrical signal to control the heart rate. The heart rate produced by the ventricles is much slower than that produced by the SA node.
Some AV blocks are benign, or normal, in certain people, such as in athletes or children. Other blocks are pathologic, or abnormal, and have several causes, including ischemia, infarction, fibrosis, and drugs.
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Classification
There are three types, or degrees, of AV block: (1) first-degree, (2) second-degree, and (3) third-degree, with third-degree being the most severe. An ECG is used to differentiate between the different types of AV blocks. However, one important consideration when diagnosing AV blocks from ECGs is the possibility of pseudo- AV blocks which are due to concealed junctional extrasystoles. It is important to diagnose AV-blocks precisely because unnecessary pacemaker placement in patients with pseudo-AV blocks can worsen symptoms and create complications.
First-degree atrioventricular block
First-degree AV block
First-degree atrioventricular block (AV block) is a disease of the electrical conduction system of the heart in which electrical impulses conduct from the cardiac atria to the ventricles through the atrioventricular node (AV node) more slowly tha ...
occurs when there is a delay, but not disruption, as the electrical signal moves between the atrium and the ventricles through the AV node.
On ECG, this is defined by a
PR interval
In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization) ...
greater than 200 msec. Additionally, there are no dropped, or skipped, beats.
Second-degree atrioventricular block
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Second-degree AV block
Second-degree atrioventricular block (AV block) is a disease of the electrical conduction system of the heart. It is a conduction block between the atria and ventricles. The presence of second-degree AV block is diagnosed when one or more (but n ...
occurs when the electrical signal between the atria and ventricles is even more impaired than in a first-degree AV block. In a second-degree AV block, the impairment results in a failure to conduct an impulse, which causes a skipped beat.
Mobitz I
Mobitz I is characterized by a progressive yet reversible block of the AV node. On ECG, this is defined by progressive prolongation of the PR interval, with a resulting dropped beat (the PR interval gets longer and longer until a beat is finally dropped, or skipped).
Some patients are asymptomatic; those who have symptoms respond to treatment effectively. There is a low risk of a Mobitz I AV block leading to heart attack and complete heart block.
Mobitz II
Mobitz II is caused by a sudden, unexpected failure of the His-Purkinje cells to conduct the electrical impulse. On ECG, the PR interval is unchanged from beat to beat, but there is a sudden failure to conduct the signal to the ventricles, and resulting in random skipped beat.
The risks and possible effects of Mobitz II are much more severe than Mobitz I in that it can lead to severe heart attack.
Third-degree atrioventricular block
Third-degree AV block occurs when the signal between the atria and ventricles is completely blocked, and there is no communication between the two. None of the signals from the upper chambers makes it to the lower chambers. On ECG, there is no relationship between P waves and QRS complexes, meaning the P waves and QRS complexes are not in a 1:1 ratio.
Third-degree AV block is the most severe of the AV blocks. Persons with third-degree AV block need emergency treatment including but not limited to a
pacemaker
An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart eith ...
.
Etiology
There are many causes of AV block, ranging from a
normal variant
Human variability, or human variation, is the range of possible values for any characteristic, physical or mental, of human beings
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipeda ...
among people to the result of a
heart attack
A myocardial infarction (MI), commonly known as a heart attack, occurs when blood flow decreases or stops to the coronary artery of the heart, causing damage to the heart muscle. The most common symptom is chest pain or discomfort which may tr ...
.
First-degree AV block and Mobitz I second-degree block are often thought to be just normal, benign, conditions in people, and do not often result from a severe underlying condition.
Mobitz II second-degree block and third-degree AV block are not normal variants and are associated with an underlying condition.
[ Common causes include ]ischemia
Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems wi ...
(lack of blood flow and oxygen to the heart muscle) or progressive fibrosis
Fibrosis, also known as fibrotic scarring, is a pathological wound healing in which connective tissue replaces normal parenchymal tissue to the extent that it goes unchecked, leading to considerable tissue remodelling and the formation of perma ...
(excessive scaring) of the heart.[ It is also possible that a high degree block can result after cardiac surgery during which the surgeon was in close proximity to the electrical conduction system and accidentally injured it. Reversible causes of Mobitz II and third-degree heart block include untreated ]Lyme disease
Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium, which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migran ...
, hypothyroidism
Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as po ...
, hyperkalemia
Hyperkalemia is an elevated level of potassium (K+) in the blood. Normal potassium levels are between 3.5 and 5.0mmol/L (3.5 and 5.0mEq/L) with levels above 5.5mmol/L defined as hyperkalemia. Typically hyperkalemia does not cause symptoms. Occasi ...
(high levels of potassium), and drug toxicity. Drugs that slow the conduction of the electrical signal through AV node, such as beta-blockers
Beta blockers, also spelled β-blockers, are a class of medications that are predominantly used to manage abnormal heart rhythms, and to protect the heart from a second heart attack after a first heart attack (secondary prevention). They are al ...
, digoxin
Digoxin (better known as Digitalis), sold under the brand name Lanoxin among others, is a medication used to treat various heart conditions. Most frequently it is used for atrial fibrillation, atrial flutter, and heart failure. Digoxin is on ...
, calcium channel blocker
Calcium channel blockers (CCB), calcium channel antagonists or calcium antagonists are a group of medications that disrupt the movement of calcium () through calcium channels. Calcium channel blockers are used as antihypertensive drugs, i.e., as ...
s, and amiodarone
Amiodarone is an antiarrhythmic medication used to treat and prevent a number of types of cardiac dysrhythmias. This includes ventricular tachycardia (VT), ventricular fibrillation (VF), and wide complex tachycardia, as well as atrial fibrilla ...
, can cause heart block if they are taken in excessive amounts, or the levels in the blood get too high.
Anatomy
The synchronized contraction of the heart occurs through a well-coordinated electrical signal pathway. The initial electrical signal originates from the SA node located in the upper portion of the right atrium
Atrium may refer to:
Anatomy
* Atrium (heart), an anatomical structure of the heart
* Atrium, the genital structure next to the genital aperture in the reproductive system of gastropods
* Atrium of the ventricular system of the brain
* Pulmona ...
. The electrical signal then travels through both the right and left atrium and causes the two atria to contract at the same time. This simultaneous contraction results in the P wave
A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
seen in an ECG
Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the hear ...
tracing.
The electrical signal then travels to the AV node
The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
located on the lower portion of the interatrial septum
The interatrial septum is the wall of tissue that separates the right and left atria of the heart.
Structure
The interatrial septum is a that lies between the left atrium and right atrium of the human heart. The interatrial septum lies at angl ...
. At the AV node there is a delay in the electrical signal, which allows the atria to contract and blood to flow from the atria to the ventricles. This delay accounts for the ECG period between the P wave and the QRS complex
The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the ri ...
, and creates the PR interval
In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization) ...
.
From the AV nodes, the electrical signal travels through Bundle of His
The bundle of His (BH) or His bundle (HB) ( "hiss"Medical Terminology for Health Professions, Spiral bound Version'. Cengage Learning; 2016. . pp. 129–.) is a collection of heart muscle cells specialized for electrical conduction. As part of t ...
and divides into the right bundle and left bundle, which are located within the interventricular septum
The interventricular septum (IVS, or ventricular septum, or during development septum inferius) is the stout wall separating the ventricles, the lower chambers of the heart, from one another.
The ventricular septum is directed obliquely backwar ...
. Finally, the electrical signal travels into the Purkinje fibers
The Purkinje fibers (; often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specia ...
. The division of the signal into a right and left bundle and then into the Purkinje fibers allows for a simultaneous depolarization and contraction of the right and left ventricles. The contraction of the ventricles results in the QRS complex seen on an ECG tracing.
After contraction, the ventricles must repolarize, or reset themselves, in order to allow for a second depolarization and contraction. The repolarization creates the T wave
In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the ''absolute refractory period''. The last half of the T wave ...
in the ECG tracing.
Diagnosis
An electrocardiogram, or ECG, is used to differentiate between the different types of AV block. In AV block, there is a disruption between the signal traveling from the atria to the ventricles. This results in abnormalities in the PR interval
In electrocardiography, the PR interval is the period, measured in milliseconds, that extends from the beginning of the P wave (the onset of atrial depolarization) until the beginning of the QRS complex (the onset of ventricular depolarization) ...
, as well as the relationship between P waves
A P wave (primary wave or pressure wave) is one of the two main types of elastic Body wave (seismology), body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an eart ...
and QRS complex
The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the ri ...
es on the ECG tracing. If the patient is symptomatic from their suspected AV block, it is important that an ECG is also obtained while having symptoms. Physicians may also order a continuous ECG (i.e. Holter monitor
In medicine, a Holter monitor (often simply Holter) is a type of ambulatory electrocardiography device, a portable device for cardiac monitoring (the monitoring of the electrical activity of the cardiovascular system) for at least 24 hours.
T ...
or implanted cardiac monitor) to monitor the patient for symptoms and conduction abnormalities over a longer period of time, as AV blocks can be intermittent.
Because some types of AV block can be associated with underlying structural heart disease, patients may also undergo echocardiogram
An echocardiography, echocardiogram, cardiac echo or simply an echo, is an ultrasound of the heart.
It is a type of medical imaging of the heart, using standard ultrasound or Doppler ultrasound.
Echocardiography has become routinely used in th ...
to look at the heart and assess the function.
Laboratory diagnosis for AV blocks include electrolyte, drug level and cardiac enzyme level tests. Based upon clinical suspicion, the physician may do lab tests to assess for reversible causes of AV block, such as hypothyroidism
Hypothyroidism (also called ''underactive thyroid'', ''low thyroid'' or ''hypothyreosis'') is a disorder of the endocrine system in which the thyroid gland does not produce enough thyroid hormone. It can cause a number of symptoms, such as po ...
, rheumatologic disorders, and infections (such as Lyme disease
Lyme disease, also known as Lyme borreliosis, is a vector-borne disease caused by the ''Borrelia'' bacterium, which is spread by ticks in the genus ''Ixodes''. The most common sign of infection is an expanding red rash, known as erythema migran ...
).
Management
Management is dependent upon the severity, or degree, of the blockage, the consistency of symptoms, as well as the cause of the AV block.[
Patients with ]first-degree AV block
First-degree atrioventricular block (AV block) is a disease of the electrical conduction system of the heart in which electrical impulses conduct from the cardiac atria to the ventricles through the atrioventricular node (AV node) more slowly tha ...
do not have any resulting severe or life-threatening symptoms, such as symptomatic bradycardia
Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, a ...
or hypotension
Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dias ...
, and, thus, do not require treatment.
Similarly, patients with second-degree Mobitz I AV block rarely develop life-threatening symptoms, and patients who are asymptomatic do not require treatment. However, in some cases, patients with Mobitz I block can develop life-threatening symptoms that require intervention. These patients often respond well to atropine
Atropine is a tropane alkaloid and anticholinergic medication used to treat certain types of nerve agent and pesticide poisonings as well as some types of slow heart rate, and to decrease saliva production during surgery. It is typically given i ...
, but may require temporary transcutaneous pacing
Transcutaneous pacing (TCP), also called external pacing, is a temporary means of pacing a patient's heart during a medical emergency. It should not be confused with defibrillation (used in more serious cases, in ventricular fibrillation and other ...
or transvenous pacing
Transvenous cardiac pacing (TVP), 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 pacin ...
until they are no longer symptomatic.
Patients with second-degree Mobitz II and third-degree heart block
Third-degree atrioventricular block (AV block) is a medical condition in which the electrical impulse generated in the sinoatrial node (SA node) in the atrium of the heart can not propagate to the ventricles.
Because the impulse is blocked, an ...
are much more likely to have symptomatic bradycardia
Bradycardia (also sinus bradycardia) is a slow resting heart rate, commonly under 60 beats per minute (BPM) as determined by an electrocardiogram. It is considered to be a normal heart rate during sleep, in young and healthy or elderly adults, a ...
and hemodynamic instability, such as hypotension
Hypotension is low blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood. Blood pressure is indicated by two numbers, the systolic blood pressure (the top number) and the dias ...
. Additionally, there is an increased risk of patients with Mobitz II heart block developing third-degree heart block. Therefore, these patients often require temporary pacing with transcutaneous or transvenous pacing
Transvenous cardiac pacing (TVP), 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 pacin ...
wires, and many will ultimately require a permanent implanted pacemaker
An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart eith ...
.
If the heart block is found to be caused by a reversible condition, such as Lyme disease, the underlying condition should first be treated. Often, this will lead to resolution of the heart block and the associated symptoms.
References
External links
*
{{DEFAULTSORT:Atrioventricular Block
Cardiac arrhythmia