First degree heart block
Encyclopedia
First-degree AV block, or PR prolongation, is a disease of the electrical conduction system
of the heart
in which the PR interval is lengthened beyond 0.20 seconds.
In first-degree AV block, the impulse conducting from atria to ventricles through the AV node is delayed and travels slower than normal. It has a prevalence
in the normal (young adult) population of 0.65-1.1% and the incidence
is 0.13 per 1000 persons.
, acute myocardial infarction
(especially acute inferior MI), electrolyte disturbances and medication. The drugs 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, beta-blockers, cardiac glycosides, and anything that increases cholinergic activity such as cholinesterase inhibitors. Drugs that increase calcium concentration, such as Digitalis decrease AVN conduction time.
In first-degree heart block, the diseased 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.
. 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
. 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.
, however, the presence of a prolonged PR interval or first degree AV block doubled the risk of developing atrial fibrillation
(irregular heart beat), tripled the risk of requiring an artificial pacemaker
, 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
, and either left anterior fascicular block
or left posterior fascicular block
(known as trifascicular block) may be at an increased risk of progression to complete heart block.
Electrical conduction system of the heart
The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...
of the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
in which the PR interval is lengthened beyond 0.20 seconds.
In first-degree AV block, the impulse conducting from atria to ventricles through the AV node is delayed and travels slower than normal. It has a prevalence
Prevalence
In epidemiology, the prevalence of a health-related state in a statistical population is defined as the total number of cases of the risk factor in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population...
in the normal (young adult) population of 0.65-1.1% and the incidence
Incidence (epidemiology)
Incidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.Incidence proportion is the...
is 0.13 per 1000 persons.
Causes
The most common causes of first-degree heart block are an AV nodal disease, enhanced vagal tone (for example in athletes), myocarditisMyocarditis
Myocarditis is inflammation of heart muscle . It resembles a heart attack but coronary arteries are not blocked.Myocarditis is most often due to infection by common viruses, such as parvovirus B19, less commonly non-viral pathogens such as Borrelia burgdorferi or Trypanosoma cruzi, or as a...
, acute myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
(especially acute inferior MI), electrolyte disturbances and medication. The drugs 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, beta-blockers, cardiac glycosides, and anything that increases cholinergic activity such as cholinesterase inhibitors. Drugs that increase calcium concentration, such as Digitalis decrease AVN conduction time.
Diagnosis
In normal individuals, the AV node slows the conduction of electrical impulse through the heart. This is manifest on a surface 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 diseased 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 infarctionMyocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
. 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
Bundle branch block
A bundle branch block refers to a defect of the heart's electrical conduction system.-Anatomy and physiology:The heart's electrical activity begins in the sinoatrial node , which is situated on the upper right atrium. The impulse travels next through the left and right atria and summates at the...
. 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 StudyFramingham Heart Study
The Framingham Heart Study is a long-term, ongoing cardiovascular study on residents of the town of Framingham, Massachusetts. The study began in 1948 with 5,209 adult subjects from Framingham, and is now on its third generation of participants...
, however, the presence of a prolonged PR interval or first degree AV block doubled the risk of developing atrial fibrillation
Atrial fibrillation
Atrial fibrillation is the most common cardiac arrhythmia . It is a common cause of irregular heart beat, identified clinically by taking a pulse. Chaotic electrical activity in the two upper chambers of the heart result in the muscle fibrillating , instead of achieving coordinated contraction...
(irregular heart beat), tripled the risk of requiring an artificial 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...
, 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
Right bundle branch block
A right bundle branch block is a defect in the heart's electrical conduction system.During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bundle branch. The left ventricle however, is still normally activated by the left bundle...
, and either left anterior fascicular block
Left anterior fascicular block
Left anterior fascicular block is a cardiac condition, distinguished from left bundle branch block.It is caused by only the anterior half of the bundle of His being defective...
or left posterior fascicular block
Left posterior fascicular block
A left posterior fascicular block is a condition where the left posterior fascicle, i.e. the backside part of the left cardiac bundle, does not conduct the electrical impulses from the atrioventricular node...
(known as trifascicular block) may be at an increased risk of progression to complete heart block.
See also
- Atrioventricular nodeAtrioventricular nodeThe 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) - Electrical conduction system of the heartElectrical conduction system of the heartThe normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...
- ElectrocardiogramElectrocardiogramElectrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...
(EKG) - Atrioventricular blockAtrioventricular blockAn atrioventricular block involves the impairment of the conduction between the atria and ventricles of the heart.The causes of pathological AV block are varied and include ischaemia, infarction, fibrosis or drugs. Certain AV blocks can also be found as normal variants, such as in athletes or...
- Trifascicular block
- Second-degree AV block
- Third-degree AV block