Atrioventricular block overview

# Overview
An atrioventricular block (or AV block) is a type of heart block involving an impairment of the conduction between the atria and ventricles of the heart.  It usually involves the atrioventricular node, but it can involve other structures too.

# Historical Perspective
In the late 1960s, there was a major breakthrough in the understanding of electrical heart conduction. Identification of blocks in the atrioventricular conduction system was made possible.

# Classification
AV block is categorized by degree and site of conduction block. In first-degree AV block, all atrial impulses are conducted to the ventricle. However, there is a delay within the AV node, resulting in a prolonged PR interval on ECG (>200 msec or >5 small blocks). Second-degree AV block can be of one of the two types: Mobitz type I and Mobitz type II. Mobitz type I, or Wenckebach block , consists of progressive prolongation of PR interval, until loss of conduction]] to the ventricle occurs (missed beat). Mobitz type I block is rarely symptomatic and does not require treatment. On the other hand, Mobitz type 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. Complete heart block (third-degree heart block) is characterized by a [complete lack of conduction from the atria to the ventricles. The ventricular rates in complete heart blocks are slower than the atrial rate. A junctional escape rate is generally between 40 and 60 beats/min and shows narrow QRS complex on ECG, whereas a ventricular escape rate is slower with a wide QRS complex.

# Pathophysiology
Atrioventricular (AV) block is caused by one of the following mechanisms i.e. fibrosis or degeneration of the conduction system, ischemic heart disease, or medications.

# Causes
Atrioventricular block can be due to several causes. It could be idiopathic, hereditary, metabolic, or iatrogenic.

# Differentiating Atrioventricular block from other diseases
Atrioventricular (AV) blocks must be differentiated from other heart rhythm abnormalities. The temporal association of the P waves, nature of the QRS complexes, and irregularities of RR interval can help in distinguishing AV block from other arrhythmias.

# Epidemiology and Demographics
First-degree atrioventricular blocks is commonly observed among patients below 50 years old, and beyond the sixth decade of life. It has a prevalence rate of 1% to 5%.

# Risk Factors

# Natural History, Complications and Prognosis

# Diagnosis

## History and Symptoms

## Physical Examination

## Laboratory Findings
Laboratory studies are especially important to exclude reversible causes of atrioventricular block, for example electrolyte imbalances and medications.

## Electrocardiogram
The main diagnostic modality used in determining whether a person has heart block, is the electrocardiogram.

## Chest X-ray
Chest X-ray is not used for diagnosing heart blocks. However, the enlarged cardiac shadow suggesting dilatation and congestion of pulmonary vessels may suggest congestive cardiac failure. It is used along with electrocardiogram (ECG) studies and echocardiogram to diagnose co-morbidities.

## Electrocardiography
The main diagnostic modality used in determining whether a person has heart block, is the electrocardiogram. First degree heart block consists of a prolonged PR interval of more than >200msec. Second degree heart block consists of Mobitz type I and Mobitz type II heart block. Mobitz I or Wenckebach block will show a progressive prolongation of the PR interval, until a ventricular beat is missed. Mobitz II AV block consists of a constant PR interval with intermittent missed beats. Complete heart block or third degree heart block will be depicted by a [complete disassociation of atrial and ventricular beats.

## Other Diagnostic Studies
Electrophysiologic studies are used in decision-making with regards to the method and type of pacemaker therapy.

# Treatment

## Medical Therapy
Treatment goal is to remove the extrinsic causes and treat reversible intrinsic causes.  Drugs used in the treatment of AV nodal blocks include atropine, isoproterenol, theophylline, antibiotics treatment for lyme disease and  treatment of ischemia. A permanent pacemaker is indicated for symptomatic bradycardia due to advanced second- or third-degree heart blocks.

## Surgery
First-degree atrioventricular block and second-degree Mobitz type I atrioventricular block do not need any treatment. Constant monitoring and avoidance of medications that trigger PR prolongation should be done. 
Second-degree Mobitz type II atrioventricular block and third-degree atrioventricular block put patients at a greater risk of progressing into ventricular tachycardia, asystole, and even death. Because of this, an urgent admission in the hospital is warranted for cardiac monitoring, and pacemaker implantation.

## Primary Prevention

## Secondary Prevention
Patients who manifest with some signs and symptoms of atrioventricular block should seek medical consult as soon as possible.
Evaluation using an electrocardiogram (ECG) is needed to monitor the possibility of having atrioventricular block.

## Cost-Effectiveness of Therapy
Some patients who develop a high-degree atrioventricular blocks often require a placement of pacemakers. 
An evaluation of the cost-effectiveness of therapy is imperative to serve as a guide in decision-making.

## Atrioventricular Block Future or Investigational Therapies
New approaches are being considered to treat cardiac conduction diseases. Safety measures are needed to be monitored before testing it in vivo.