Third degree AV block overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Soroush Seifirad, M.D.[2] Cafer Zorkun, M.D., Ph.D. [3]Qasim Khurshid, M.B.B.S.
Overview
Complete heart block is a disorder of the cardiac conduction system where there is complete dissociation of the atrial and ventricular activity due to the absence of conduction through the atrioventricular node (AVN) or His-Purkinje system.
Historical Perspective
In 1894, Dr. Engelman was the first to describe in detail the phenomenon of AV interval lengthening. In 1899, Karel Frederik published a paper on irregular pulses describing impairment of AV conduction and blockage. 1906 Einthiven was the first to present a presentation of normal and abnormal electrocardiograms recorded with string galvanometer. Dr. Ashmar in 1925 studied and described in detail this blocked impulses and their impact on the conduction in the muscle of the heart. In 1952 Dr. Paul Zoll developed first temporary trans-cutaneous pacing.[1]
Classification
There is no established system for the classification of third degree AV block. But AV dissociation can be further classified into two subtypes as AV dissociation by default and AV dissociation by usurpation.
Pathophysiology
Normally SA node generates impulses that travel to the AV node and gets delayed there to assure that the contraction cycle in atria is complete before a contraction begins in the ventricles. From the AV node, the impulses pass through the His-Purkinje system to cause ventricular contraction. Pathological delay in the AV node is visualized on an electrocardiogram as a change in the P-R interval. These delays are known as an AV block. No impulses from the SA node get conducted to the ventricles, and this leads to a complete atrioventricular dissociation. The SA node continues to activate at a set rate, but the ventricles will activate through an escape rhythm that can be mediated by either the AV node, one of the fascicles, or by ventricular myocytes themselves. The heart rate will mostly be less than 45 to 50 beats/min, and most patients will be hemodynamically unstable.
Causes
The most common cause of a complete heart block is coronary ischemia, but there are many other etiologies. The progressive degeneration of the electrical conduction system of the heart due to aging can cause a third-degree heart block. Complete heart block can be preceded by first degree AV block, second degree AV block, or bifascicular block. Acute myocardial infarction may present as a third-degree heart block. Lupus in a pregnant mother can cause congenital heart block in newborns. Maternal antibodies can cross the placenta and lead to a complete heart block during gestation. Sometimes no cause can be identified.
Differentiating Third degree AV block from other Diseases
Third-degree heart block should not be confused with:
- High-grade AV block: Second-degree block with a very slow ventricular rate with occasional AV conduction.
- AV Dissociation: This term is used to indicate the occurrence of independent atrial and ventricular contractions caused by entities other than third-degree heart block
Epidemiology and Demographics
AV blocks are fairly common however, third-degree AV block is relatively rare. The incidence in the general population appears to be low, approximately 20 to 40 in 100,000 individuals in the United States. Given the etiology of the disease, the incidence among the apparently healthy and presumptively asymptomatic is even lower at approximately 1 in 100,000.[2][3][4]
Risk Factors
Risk factors for complete heart blocks can be congenital or acquired. Some risk factors include
- Acute mayocardial infacrtion or coronary atery disease
- old age
- Cardiomyopathy
- Sarcoidosis
- Hyperkalemia
- Lyme disease
- Severe hypothyroidism
Screening
There is insufficient evidence to recommend routine screening for third degree AV block. However, screening for congenital AV block is recommended
Natural History, Complications and Prognosis
Spontaneous recovery from third-degree heart block is not common. Untreated third-degree heart block is associated with high mortality, which appears to occur as a consequence of the complications of decreased perfusion as a consequence of bradycardia and decreased cardiac output. Common complications of third-degree AV block include sudden cardiac death due to asystole, syncope, and musculoskeletal injuries secondary to fall after syncope. The prognosis of the third-degree heart block is most likely dependent on the patient's underlying disease burden and severity of the clinical presentation on arrival. Patients treated with permanent pacemaker have an excellent prognosis.
Diagnosis
Diagnostic Study of Choice
A 12-lead Electrocardiography (ECG) is the gold standard test for the diagnosis of third degree AV block.
History and Symptoms
Patients with third degree AV block typically experience a lower overall measured heart rate (as low as 28 beats per minute during sleep), low blood pressure, and poor circulation. 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 sustained activity. Complete heart block associated with a slower pacemaker can result in dizziness, presyncope andsyncope.
Physical Examination
Initial triage of patients with complete heart block consists of determining symptoms, taking vital signs, and looking for evidence of hemodynamic instability. Patients with complete heart block may have serve bradycardia, S3 gallop, new murmurs, peripheral edema, and hepatomegaly. Patients may have signs of hypoperfusion, such as altered mental status, lethargy, and hypotension.
Laboratory Findings
Primary lab work up of patients with third degree AV block might include but not limitted to the followings:
- CBC to look for anemia and infection
- Serum electrolytes
- Serum creatinine
- Digoxin levels
- HIV serology
Electrocardiogram
Transthoracic echocardiography may be helpful in the diagnosis of the underlying diseases that tend to third-degree AV block. Echocardiography might show shreds of evidence in favor of cardiomyopathies or valvular heart diseases. In particular case scenarios, transesophageal echocardiography is warranted and may help to diagnose etiologies such as valvular ring abscess. Furthermore, the left ventricular function can be determined using an echo and provide pieces of evidence in favor of the placement of a pacemaker or defibrillator.
X-Ray
There are no x-ray findings associated with third degree AV block.
CT scan
CT can not diagnose complete heart block but might be helpful in the diagnosis oft cardiac and chest abnormalities related to the underlying organic disease in those with third-degree AV block
MRI
Cardiac MRI is not required for diagnosis of complete heart block but can halp to diagnose underlying organic disease associated with heart block.
Other Imaging Findings
Nuclear imaging techniques might rarely used and may be helpful in the diagnosis of complications of third degree AV block or provide shreds of evidence in favor of the underlying disease in those with compete heart block
Other Diagnostic Studies
Other diagnostic studies for third-degree AV block include diagnostic electrophysiologic studies, which may demonstrate atrioventricular (AV) conduction abnormalities and help to determine the level of the block.Ambulatory monitoring is warranted in cases of possible transient heart block, or some other bradyarrhythmias that might be mistaken with third-degree AV block. Cardiac catheterization or stress testing is warranted if ischemic heart disease is suspected
Treatment
Medical Therapy
The management of third-degree AV block depends on the severity of signs, symptoms, and the underlying cause. In symptomatic patients and with hemodynamic distress, pharmacological therapy should be initiated immediately to increase heart rate and cardiac output. Most of the patients who do not respond to pharmacologic therapy require a temporary pacemaker. After stabilizing the patients, assessment and treatment of potentially reversible causes should be done. Some patients without reversible cause or unidentified etiology require a permanent pacemaker. A new third degree AV block is an emergency. Management is slightly different between unstable and stable patients
Surgery
Cardiac pacemakers are effective treatments for a variety of cardiac conduction abnormalities and can reestablish adequate circulation by generating appropriate heart rate and cardiac response. Two main factors guide the majority of decisions regarding permanent pacemaker insertion. First is the association of symptoms with arrhythmia, and second is the potential for progression of the rhythm disturbance
Primary Prevention
Patients with renal insufficiency, potassium electrolyte disturbances, and dehydration are predisposed to develop digoxin toxicity. Careful monitoring of electrolytes, drug levels, and renal function is essential in patients on chronic digoxin therapy. Patients on multiple nodal agents (e.g., beta-blockers and calcium channel blockers) are at an increased risk for the development of third-degree atrioventricular (AV) block (complete heart block). These patients should be carefully monitored for heart blocks.
Secondary Prevention
There is no secondary prevention.
References
- ↑ LANGENDORF R. Concealed A-V conduction; the effect of blocked impulses on the formation and conduction of subsequent impulses. Am Heart J. 1948;35(4):542-552. doi:10.1016/0002-8703(48)90641-3
- ↑ OSTRANDER LD Jr, BRANDT RL, KJELSBERG MO, EPSTEIN FH. ELECTROCARDIOGRAPHIC FINDINGS AMONG THE ADULT POPULATION OF A TOTAL NATURAL COMMUNITY, TECUMSEH, MICHIGAN. Circulation. 1965;31:888-898. doi:10.1161/01.cir.31.6.888
- ↑ Benjamin EJ, Blaha MJ, Chiuve SE, et al. Heart Disease and Stroke Statistics-2017 Update: A Report From the American Heart Association [published correction appears in Circulation. 2017 Mar 7;135(10 ):e646] [published correction appears in Circulation. 2017 Sep 5;136(10 ):e196]. Circulation. 2017;135(10):e146-e603. doi:10.1161/CIR.0000000000000485
- ↑ Movahed MR, Hashemzadeh M, Jamal MM. Increased prevalence of third-degree atrioventricular block in patients with type II diabetes mellitus. Chest. 2005;128(4):2611-2614. doi:10.1378/chest.128.4.2611