Second degree AV block overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mohammed Salih, M.D. Cafer Zorkun, M.D., Ph.D. [2] Syed Musadiq Ali M.B.B.S.[3]
Overview
Second degree AV block is a disease of the electrical conduction system of the heart. It refers to aconduction block between the atria and ventricles. The presence of second degree AV block is diagnosed when one or more (but not all) of the atrial impulses fail to conduct to the ventricles due to impaired conduction.
Historical perspective
Second degree AV block was first described as a progressive delay between the atrial and ventricular contraction by Dr. Wenckebach in 1899. Dr. Mobitz then divided the second degree AV block into two subtypes. In 1905, Dr. Hay figured out the pause following a wave was due to failure of ventricular muscles to respond to a stimulus.
Classification
Second-degree AV block can be of one of the two types: Mobitz I and Mobitz II. Mobitz I, or Wenckebach block , consists of progressive prolongation of PR interval, until loss of conduction to the ventricle occurs (missed beat). Mobitz I block is rarely symptomatic and does not require treatment. On the other hand, Mobitz II AV block is characterized by a constant PR interval with intermittent missed beats. The missed beats can occur with varying frequency such as occasional to 3:1 or 2:1.
Pathophysiology
In Mobitz type I (Wenckebach) there is a progressive prolongation of the PR interval (AV conduction) until eventually an atrial impulse is completely blocked. When an atrial impulse is completely blocked there will be a P wave without a QRS complex. This pattern is often referred to as a “dropped beat.” Mobitz type I occurs because each depolarization results in the prolongation of the refractory period of the atrioventricular (AV) node. When an atrial impulse comes through the AV node during the relative refractory period, the impulse will be conducted more slowly, resulting in a prolongation of the PR interval. Eventually, an impulse comes when the AV node is in its absolute refractory period and will not be conducted. This will manifest on the ECG as a P wave that is not followed by a QRS complex. This non-conducted impulse allows time for the AV node to reset, and the cycle continues. This phenomenon leads to a grouped beating. In Mobitz type II there is a constant PR interval across the rhythm strip both before and after the non-conducted atrial beat. Each P wave is associated with a QRS complex until there is one atrial conduction or P wave that is not followed by a QRS. Mobitz type II is often a problem in the infra-nodal conduction system, and therefore, is associated with a widened QRS complex, bundle-branch block, or fascicular block. Be aware that if more than one P wave is not conducted this is no longer a Mobitz type II and is considered a high degree AV block.
Causes
Life threatening causes of second degree AV block include acute myocardial infarction, acute rheumatic fever, myocarditis, and severe hypothermia. Common causes include endocarditis, digoxin, dilated cardiomyopathy, diltiazem, and calcific aortic stenosis.
Differentiating second degree AV block from Other Diseases
Second degree AV block must be differentiated from different abnormal and irregular cardiac rhythm as atrial fibrillation, atrial flutter, atriventricular nodal reentrant tachycardia, multifocal atrial tachycardia, and paroxysmal supraventricular tachycardia.
Epidemiology and Demographics
There have not been large population-based studies on the prevalence of Mobitz type I or II atrioventricular blocks. At this time, there is no associated age, racial, or gender correlation. AV block is sometimes seen in athletes and in patients with congenital heart disorders. In the United States, the prevalence of second-degree AV block is believed to be 3 in 100,000 individual. Men and women are affected equally by second degree AV block.
Risk Factors
Common risk factors associated with second degree AV block include Intrinsic atrioventricular node disease, Myocarditis , Acute myocardial infarction, Prior cardiac surgery, Older age,Heart attack or coronary artery disease, Cardiomyopathy, Sarcoidosis, Lyme disease, High potassium levels, Severe hypothyroidism, Certain inherited neuromuscular diseases, Medicines that slow the heart rate , After open heart surgery.
Screening
Natural History, Complications, and Prognosis
Patients with Mobitz type II second degree AV block who are hemodynamically stable do not require urgent therapy with atropine or temporary cardiac pacing. However, Mobitz type II second degree AV block is by nature unstable and frequently progresses to third degree (complete) AV block, so patients should be continuously monitored with transcutaneous pacing pads in place in the event of clinical deterioration. While stable patients are being monitored, reversible causes of Mobitz type II second degree AV block such as myocardial ischemia, increased vagal tone, hypothyroidism, hyperkalemia, and drugs that depress conduction, should be excluded in patients prior to implantation of a permanent pacemaker. For patients with Mobitz type II second degree AV block who do not have a reversible etiology, we recommend implantation of a permanent pacemaker (Grade 1A). We implant a dual chamber DDD pacemaker whenever possible in an effort to maintain physiologic AV synchrony.
Diagnosis
Diagnostic Study of Choice
History and Symptoms
Patients with second degree AV block are asymptomatic. Some patients may present with symptoms of reduced cardiac output. Symptoms include dizziness, fatigue, presyncope or syncope, and light headedness.
Physical Examination
Patients with second degree AV block are usually asymptomatic. However, patients with previous chronic cardiac condition may appear in a distress. In symptomatic patients, common physical examination findings include bradycardia, hypotension, and syncope. Common physical examination in patients associated with heart failure include lung crackles, jugular venous distension, and peripheral edema.
Laboratory Findings
Patients with second degree AV block laboratory tests include checking the levels of serum electrolytes as calcium, magnesium and potassium. Myocarditis related lab tests as lyme titres, HIV tests, PCR for enteroviruses, and Chagas titres should be done also.
Electrocardiogram
On ECG, type I second degree AV is characterized by a progressive prolongation of the PR interval and progressive shortening of RR interval until a P wave is blocked. The RR interval containing the blocked P wave is shorter than the sum of 2 PP intervals. The increase in the PR interval is longest in the second conducted beat after the pause. Type II second degree AV block is charecterized by a constant PR interval. Most patients with type II second-degree AV block have associated bundle branch block.
X-ray
There are no x-ray findings associated with second degree AV block.
Echocardiography and Ultrasound
CT scan
MRI
Other Imaging Findings
There are no other imaging findings associated with second degree AV block.
Other Diagnostic Studies
There are no other diagnostic studies associated with second degree AV block.
Treatment
Medical Therapy
Treatment for a Mobitz type I (Wenckebach) is often not necessary. Occasionally type I blocks may result in bradycardia leading to hypotension. If hypotension and bradycardia occur, type I blocks respond well to atropine. If unresponsive to atropine, pacing (transcutaneous or transvenous) should be initiated for stabilization. If the patient is on any beta blockers, calcium channel blockers or digoxin, the dose of these medications should be reduced or the medication discontinued. All patients with Mobitz 1 block should be admitted and monitored. Treatment for a Mobitz type II involves initiating pacing as soon as this rhythm is identified. Type II blocks imply structural damage to the AV conduction system. This rhythm often deteriorates into complete heart block. These patients require transvenous pacing until a permanent pacemaker is placed. Unlike Mobitz type I (Wenckebach), patients that are bradycardic and hypotensive with a Mobitz type II rhythm often do not respond to atropine.