First-degree atrioventricular block (AV block) is a disease of the
electrical conduction system of the heart
The cardiac conduction system (CCS) (also called the electrical conduction system of the heart) transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through the ...
in which electrical impulses conduct from the cardiac
atria 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) more slowly than normal. First degree AV block does not generally cause any symptoms, but may progress to more severe forms of heart block such as
second- and
third-degree atrioventricular 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 ...
. It is diagnosed using an
electrocardiogram, and is defined as 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 depolarizatio ...
greater than 200 milliseconds.
First degree AV block affects 0.65-1.1% of the population with 0.13 new cases per 1000 persons each year.
Causes
The most common causes of first-degree heart block are AV nodal disease, enhanced
vagal tone (for example in athletes),
myocarditis
Myocarditis, also known as inflammatory cardiomyopathy, is an acquired cardiomyopathy due to inflammation of the heart muscle. Symptoms can include shortness of breath, chest pain, decreased ability to exercise, and an irregular heartbeat. The ...
, acute
myocardial infarction
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 ...
(especially acute inferior MI),
electrolyte disturbances and medication. The medications that most commonly cause first-degree heart block are those that increase the refractory time of the AV node, thereby slowing AV conduction. These include
calcium channel blockers
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., ...
,
beta-blockers,
cardiac glycosides, and anything that increases cholinergic activity such as
cholinesterase inhibitor
Cholinesterase inhibitors (ChEIs), also known as anti-cholinesterase, are chemicals that prevent the breakdown of the neurotransmitter acetylcholine or butyrylcholine. This increases the amount of the acetylcholine or butyrylcholine in the syn ...
s.
Diagnosis
In normal individuals, the AV node slows the conduction of electrical impulses through the heart. This is manifest on a surface
electrocardiogram (ECG) as the PR interval. The normal PR interval is from 120
ms to 200 ms in length. This is measured from the initial deflection of the P wave to the beginning of the QRS complex.
In first-degree heart block, the AV node conducts the electrical activity more slowly. This is seen as a PR interval greater than 200 ms in length on the surface ECG. It is usually an incidental finding on a routine ECG.
First-degree heart block does not require any particular investigations except for electrolyte and drug screens, especially if an overdose is suspected.
Treatment
The management includes identifying and correcting electrolyte imbalances and withholding any offending medications. This condition does not require admission unless there is an associated
myocardial infarction
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 ...
. Even though it usually does not progress to higher forms of heart block, it may require outpatient follow-up and monitoring of the ECG, especially if there is a comorbid
bundle branch block
A bundle branch block is a defect in one the bundle branches in the electrical conduction system of the heart.
Anatomy and physiology
The heart's electrical activity begins in the sinoatrial node (the heart's natural pacemaker), which is situat ...
. If there is a need for treatment of an unrelated condition, care should be taken not to introduce any medication that may slow AV conduction. If this is not feasible, clinicians should be very cautious when introducing any drug that may slow conduction; and regular monitoring of the ECG is indicated.
Prognosis
Isolated first-degree heart block has no direct clinical consequences. There are no symptoms or signs associated with it. It was originally thought of as having a benign prognosis. In the
Framingham Heart Study
The Framingham Heart Study is a long-term, ongoing cardiovascular cohort study of residents of the city of Framingham, Massachusetts. The study began in 1948 with 5,209 adult subjects from Framingham, and is now on its third generation of parti ...
, however, the presence of a prolonged PR interval or first degree AV block doubled the risk of developing
atrial fibrillation, tripled the risk of requiring an
artificial 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 ei ...
, and was associated with a small increase in mortality. This risk was proportional to the degree of PR prolongation.
A subset of individuals with the triad of first-degree heart block,
right bundle branch block
A right bundle branch block (RBBB) is a heart block in the right bundle branch of the electrical conduction system.
During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bund ...
, and either
left anterior fascicular block or
left posterior fascicular block (known as
trifascicular block
Trifascicular block is a problem with the electrical conduction of the heart, specifically the three fascicles of the bundle branches that carry electrical signals from the atrioventricular node to the ventricles. The three fascicles are one i ...
) may be at an increased risk of progression to
complete 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 ...
.
See also
*
Atrioventricular block
Atrioventricular block (AV block) is a type of heart block 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 sinoatr ...
*
Second-degree atrioventricular 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 ...
*
Third-degree atrioventricular 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 ...
References
External links
{{DEFAULTSORT:First-Degree Atrioventricular Block
Cardiac arrhythmia