Third degree heart block
Encyclopedia
Presentation
Third-degree AV block, also known as complete heart block, is a medical condition in which the impulse generated in the SA node in the atrium does not propagate to the ventriclesVentricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...
.
Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram
Electrocardiogram
Electrocardiography 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...
(ECG).
- The P waves with a regular P to P interval represents the first rhythm.
- The QRS complexes with a regular R to R interval represent the second rhythm. The PR interval will be variable, as the hallmark of complete heart block is no apparent relationship between P waves and QRS complexes.
One of the pathognomonic characteristic of this block is the absolute absence of the opportunity for atrial impulses to enter and capture the ventricles(unlike AV dissociation,with functional block)due to organic nature of block(eg, fibrosis, calcification, or infiltration of the node) . It means that, in the presence of complete heart block, fusion or capture beats will never be seen.
Patients with third-degree AV block typically experience bradycardia
Bradycardia
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...
(an abnormally low measured heart rate), hypotension
Hypotension
In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...
, and at times, hemodynamic instability. In some cases, exercising may be difficult, as the heart cannot react quickly enough to sudden changes in demand or sustain the higher heart rates required for prolonged activity.
Etiology
Many conditions can cause third-degree heart block, but the most common cause is coronary ischemiaCoronary ischemia
Coronary ischemia is a medical term for not having enough blood through the coronary arteries. Coronary ischemia is linked to heart disease as well as heart attacks.It is also known as cardiac ischemia.-Causes:...
. Progressive degeneration of the electrical conduction system of the heart can lead to third-degree heart block. This may be preceded by first-degree AV block, second-degree AV block, bundle branch block, or bifascicular block. In addition, acute myocardial infarction may present with third-degree AV block.
An inferior wall myocardial infarction may cause damage to the AV node, causing third-degree heart block. In this case, the damage is usually transitory, and the AV node may recover. Studies have shown that third-degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks[citation is needed]. The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm.
An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third-degree heart block. This is typically extensive, permanent damage to the conduction system, necessitating 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...
to be placed. The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm.
Third-degree heart block may also be congenital and has been linked to the presence of lupus
Lupus erythematosus
Lupus erythematosus is a category for a collection of diseases with similar underlying problems with immunity . Symptoms of these diseases can affect many different body systems, including joints, skin, kidneys, blood cells, heart, and lungs...
in the mother. It is thought that maternal antibodies may cross the placenta and attack the heart tissue during gestation. The cause of congenital third-degree heart block in many patients is unknown. Studies suggest that the prevalence of congenital third-degree heart block is between 1 in 15,000 and 1 in 22,000 live births.
Lastly, Lyme disease
Lyme disease
Lyme disease, or Lyme borreliosis, is an emerging infectious disease caused by at least three species of bacteria belonging to the genus Borrelia. Borrelia burgdorferi sensu stricto is the main cause of Lyme disease in the United States, whereas Borrelia afzelii and Borrelia garinii cause most...
can also result in 3rd-degree heart block.
Treatment
Third-degree AV block can be treated by use of a dual-chamber artificial pacemakerArtificial 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...
. This type of device typically listens for a pulse from the SA node and sends a pulse to the AV node at an appropriate interval, essentially completing the connection between the two nodes. Pacemakers in this role are usually programmed to enforce a minimum heart rate and to record instances of atrial flutter
Atrial flutter
Atrial flutter is an abnormal heart rhythm that occurs in the atria of the heart. When it first occurs, it is usually associated with a fast heart rate or tachycardia , and falls into the category of supra-ventricular tachycardias. While this rhythm occurs most often in individuals with...
and 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...
, two common secondary conditions that can accompany third-degree AV block.
Treatment may also include medicines to control blood pressure and atrial fibrillation, as well as lifestyle and dietary changes to reduce risk factors associated with heart attack
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...
and stroke
Stroke
A stroke, previously known medically as a cerebrovascular accident , is the rapidly developing loss of brain function due to disturbance in the blood supply to the brain. This can be due to ischemia caused by blockage , or a hemorrhage...
.
Treatment in emergency situations ultimately involves electrical pacing. However the American Heart Association states that giving a trial of atropine while waiting for the pacer to be set up is acceptable. Atropine
Atropine
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...
is effective for treating early Heart Blocks (1st Degree and 2nd Degree type 1) but generally thought to have no effect on 3rd Degree Blocks.
The 2005 Joint European Resuscitation and Resuscitation Council (UK) guidelines state that Atropine is the first line treatment especially if there were any adverse signs, namely: 1) heart rate <40 bpm, 2) systolic blood pressure < 100 mm Hg, 3) signs of heart failure, and 4) ventricular arrhythmias requiring suppression. If these fail to respond to atropine or there is a potential risk of asystole, transvenous pacing is indicated. The risk factors for asystole include 1) previous asystole, 2) complete heart block with wide complexes and 3) ventricular pause for > 3 seconds. Mobitz Type 2 AV block is another indication for pacing.
See also
- Cardiac pacemakerCardiac pacemakerright|thumb|350px|Image showing the cardiac pacemaker which is the SA nodeThe contraction of heart muscle in all animals with hearts is initiated by chemical impulses. The rate at which these impulses fire controls the heart rate...
- 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...
(ECG) - 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...
- First-degree AV block
- Second-degree AV block