Paroxysmal AV block overview

# Overview
Atrioventricular block may be classified anatomically by the site of block, usually divided into atrioventricular nodal, intra-Hisian (within the His bundle itself), and infra-Hisian (below the His bundle). Paroxysmal AV block is defined as a delayed escape rhythm which repetitively blocks conduction from the atria to the ventricles, thereby causing syncope, conduction defects such as asystole and sudden cardiac death. It may or may not be associated with Phase 3 or Phase 4 conduction defects. It may be due to an increased vagal tone, innately low adenosine levels or an intrinsic conduction defect, all of which lead to different ECG presentations. Insufficient data is available regarding the exact etiology, diagnostic study of choice and treatment of paroxysmal AV blocks. It can be thought of more as a disease of exclusion. However,efforts must be made to have a standardized approach to such patients. The site of block may be clinically important and can be determined by invasive EPS when not apparent from the ECG and clinical circumstances. In general, atrioventricular block at the atrioventricular nodal level is associated with slower progression, a faster and more reliable atrioventricular junctional escape mechanism, and greater responsiveness to autonomic manipulation such as atropine, isoproterenol, and epinephrine administration. In contrast, atrioventricular block within or below the His bundle may progress rapidly and unexpectedly, is associated with a slower and more unpredictable ventricular escape mechanism, will not respond to atropine but will sometimes improve with catecholamines.

# Historical Perspective
One of the first reported cases of paroxysmal AV block was secondary to mitral valvulitis, indicating an intrinsic conduction defect. A similar block was later seen in the Bundle of His, wherein during a hypothesized zone of opportunity, a spontaneous depolarization of conducting fibres was seen. Idiopathic paroxysmal AV block may be diagnosed by a positive response to adenosine triphosphate.

# Classification
Based on the cause, paroxysmal AV block maybe classified into Intrinsic paroxysmal AV Block, Extrinsic Vagal paroxysmal AV block and Extrinsic Idiopathic paroxysmal AV Block.

# Pathophysiology
Intrinsic paroxysmal AV block (I-AVB) is an AV block secondary to an innate anatomical defect. Given the presence of such a defect it's prognosis, compared to extrinsic paroxysmal vagal AV block and extrinsic paroxysmal idiopathic  AV block is poor. It may have a bradycardia or tachycardia component associated with it and is characterized by atrial/ventricular premature beats prior to the period of asystole. Extrinsic vagal paroxysmal AV Block occurs secondary to an increase in vagal tone. ECG findings reflecting this include sinus rate slowing and increasing PP interval/ PR interval prior to the period of asystole. Individuals with low levels of adenosine are susceptible to sudden surges in adenosine levels which act on the AV node and cause episodes of presyncope or syncope. This would be seen on an ECG as a sudden increase in sinus rate with narrow QRS complexes just prior to the period of asystole.

# Causes
Intrinsic conduction/ structural defects particularly those located in this intra His Bundle (intrinsic paroxysmal AV block), low adenosine levels (idiopathic paroxysmal AV block) and increase vagal tone/vagal surge (extrinsic vagal paroxysmal AV block) are the major causes of paroxysmal AV block. However, several reversible causes need to be ruled out before coming to a diagnosis.

# Differentiating Paroxysmal AV Block from other Diseases
Considering that a number of conditions may present with a history of syncope and presyncope, paroxysmal AV block must treated as a diagnosis of exclusion. Vasaovagal syncope, situational syncope, carotid sinus hypersensitivity, seizures, structural heart defects such as aortic stenosis, hypertrophic cardiomyopathy and conduction defects such as atrial fibrillation, atrial flutter are a few conditions that need to be ruled out initially.

# Epidemiology and Demographics
Exact data reflecting the epidemiology of paroxysmal AV block is unavailable. However certain studies have shown an increased incidence in the elderly, no gender predisposition and an association with bundle branch blocks.

# Risk Factors
There are no established risk factors for paroxysmal AV Block.

# Screening
There is insufficient evidence to recommend routine screening for paroxysmal AV Block.

# Natural History,Complications and Prognosis
Natural history most commonly includes recurrent unexplained syncope and presyncope. Complications such as sudden cardian death or indefinite periods of asystole may arise. Prognosis of intrinsic paroxysmal AV block is more dire than extrinsic idiopathic paroxysmal AV block or extrinsic vagal paroxysmal AV block.

# Diagnosis

## History and Symptoms
An initial evaluation strategy of taking a detailed history, physical examination, risk stratification, ECG recording and BP measurement should help decide what investigations should be ordered (based on whether the syncope is cardiac related, reflex/neutrally mediated, secondary to cerebrovascular disease or due to orthostatic hypotension). The majority of patients with paroxysmal AV Block present with presyncope, syncope, with or without a prodrome or are asymptomatic.

## Laboratory Findings
Adenosine Plasma levels, adenosine triphosphate stimulation tests and lab values related to potential reversible causes of AV block such as a thyroid profile, electrolyte values,etc are important laboratory investigations.

## Electrocardiogram
Electrocardiography is an important initial diagnostic test in diagnosing paroxysmal AV Block. Excercise ECG testing and ambulatory ECG monitoring may be employed.Intrinsic paroxysmal AV block is characterized by atrial premature beats/ventricular premature beats prior to and during the period of asystole. Extrinsic vagal paroxysmal AV block is characterized by sinus rate slowing, increasing PP interval/PR interval prior to the period or asystole. Extrinsic idiopathic paroxysmal AV block is characterized by narrowing of QRS complexes and sinus rate increase prior to the period of asystole.

## X-Ray
There are no x-ray findings associated with paroxysmal AV block.

## MRI
MRI can be helpful in diagnosing infiltrative processes, including sarcoidosis, hemochromatosis, and amyloidosis.

## CT Scan
CT offers superior information regarding calcification of cardiac structures and has some advantages in evaluating coronary artery anatomy when epicardial coronary atherosclerotic disease is suspected.

## Echocardiography
Echocardiography has a highler yield where diagnosing syncope and presyncope is concerned, in patients with structural heart disease.

## Other Imaging Findings
Cardiac nuclear imaging techniques can be useful to detect and/or discriminate amongst infiltrative cardiomyopathies.

## Other Diagnostic Studies
An Implantable Cardiac Monitor is almost exclusively used in the diagnosis of bradycardia related disorders such as high-grade atrioventricular block, sinus node dysfunction and neurocardiogenic syncope (with predominant cardio-inhibitory component). This prolonged monitoring (up to 3 years) can help correlate bradycardia conduction disorders with symptoms. An EPS is an invasive catheter based procedure that is employed to detect and anatomically locate conduction disorders. An increased HH interval is seen in intrinsic paroxysmal AV Block. Certain maneuvers cause an increase in vagal surge and may precipitate symptoms in extrinsic vagal paroxysmal AV block. These include carotid sinus massage and tilt table testing.

# Treatment

## Medical Therapy
In patients with acute onset AV block,reversible causes such as drug toxicity,thyroid dysfunction, Lyme disease, etc should be taken into consideration. A decision should then be made regarding usage of ] therapy or other treatment modalities such as temporary pacing. Theophylline is an adenosine antagonist that may be used in the diagnosis of extrinsic vagal paroxysmal AV Block.

## Interventions
Several studies have demonstrated the efficacy of cardiac pacing in paroxysmal AV block. Temporary pacing should be used for the minimum duration necessary to prevent hemodynamic compromise and asystole. The presence or absence of symptoms and the correlation of those symptoms with a conduction defect is an important determinant of cardiac pacing. An improvement in conduction suggests that the level of the block is at the level of the AV node. Counterpressure maneuvers may be helpful in preventing vagally mediated syncope.

## Surgery
Surgical intervention is not recommended for the management of paroxysmal AV Block.

## Primary Prevention
There are no established measures for the primary prevention of paroxysmal AV Block.

## Secondary Prevention
There are no established measures for the secondary prevention of paroxysmal AV Block.