Infra-Hisian Block

# Overview
Infra-Hisian block is defined as an impaired conduction in the electrical system of the heart that occurs below the atrioventricular node.

# Historical Perspective
- In 1899, Dr. Wenckebach described the progressive delay between atrial and ventricular contraction and the eventual failure of a P wave to reach the ventricles.
- Dr. Mobitz then divided the second degree AV block into two subtypes.
- In 1905, Dr. John Hay discovered the second degree of AV block.
- While Dr. Hay was examining a patient who complained of a slow pulse and had dyspnea on exertion for more than 2 years, he noticed the heart rate dropping from 80 beats to 40 beats per minute.
- Dr. Hay noted the a waves and the arterial pulse to remain stable in the beginning. However, recording the pulsation several times resulted in "a" waves that were not followed by c wave. The a-c jugular wave interval was used as a measurement of AV conduction.
- Dr. Hay figured out that the pause following a wave was due to failure of ventricular muscles to respond to a stimulus.

# Classification
- Infra-hisian block describes the block of distal conduction system.
- Types of infra-hisian block are shown in the following table:
- Out of all these types of infra-hisian block, Mobitz II heart block is considered as the most important because of the possible progression to a complete heart block.

# Pathophysiology

## Normal Cardiac Conduction
- The normal cardiac conduction proceeds in a way so as to allow time for the atrium to relax during atrial diastole.
- The electrical impulse generated in the SA node travels through the internodal pathways towards the AV node.
- The conduction through the AV node is slowed down as it travels through it. This decrease in velocity of conduction allows time for the atrium to contract ahead of the ventricle so that the blood from the atria can fill up the ventricles through the atrioventricular valves.
- As the impulse flows through the compact AV node, it rapidly conducts through the ventricular myocardial cells. Once the depolarization is complete, the ventricle relaxes during diastole in preparation for the next impulse.

## Anatomy
- The conduction system of heart consists of specialized cells designed to conduct electrical impulse faster than the surrounding myocardial cells.
- Anatomically, the AV node is divided into three regions as follows:
Transitional cell zone: This is the region where the internodal atrial pathways merge with the compact AV node.
Compact AV node: This region is located at the apex of the triangle of Koch, which is formed by the ostium of coronary sinus, tricuspid annulus and the tendon of Todaro.
Penetrating portion of the AV bundle: This region enters the tendon of Todaro and runs within the fibrous body of the membranous interventricular septum and eventually divides at the crest of the muscular interventricular septum into right and left branches.
- Transitional cell zone: This is the region where the internodal atrial pathways merge with the compact AV node.
- Compact AV node: This region is located at the apex of the triangle of Koch, which is formed by the ostium of coronary sinus, tricuspid annulus and the tendon of Todaro.
- Penetrating portion of the AV bundle: This region enters the tendon of Todaro and runs within the fibrous body of the membranous interventricular septum and eventually divides at the crest of the muscular interventricular septum into right and left branches.
- The left bundle branch penetrates the membranous portion of the interventricular septum and divides into several smaller branches. Parts of the left bundle branch include a pre-divisional segment, anterior fascicle/hemibundle and posterior fascicle/hemibundle. Rarely a median fascicle is present in some hearts.
The anterior fascicle supplies the anterior papillary muscle and the Purkinje network of the antero-lateral surface of the left ventricle.
The posterior fascicle supplies the posterior papillary muscle and the Purkinje network of the postero-inferior surface of the left ventricle.
Left bundle branch receives its blood supply from left anterior descending artery.
- The anterior fascicle supplies the anterior papillary muscle and the Purkinje network of the antero-lateral surface of the left ventricle.
- The posterior fascicle supplies the posterior papillary muscle and the Purkinje network of the postero-inferior surface of the left ventricle.
- Left bundle branch receives its blood supply from left anterior descending artery.

## Pathophysiology of Mobitz type II second degree AV block
- Mobitz type II second degree AV block is characterized by a PR interval that remains unchanged with occasional dropped beats prior to a P wave that fails to conduct to the ventricles as compared to the gradually prolonging PR interval in Mobitz type I.
- ECG findings include intermittently non-conducted P waves not preceded by PR prolongation and not followed by PR shortening.
- It almost always results from a disease of the conduction system below the level of AV node, occurring in the bundle of His in approximately 20% of the cases and in the bundle branches in the remainder.
- Depending upon the location of the block, patients having bundle branch involvement also have axis shifts and QRS widening.
- At least two-thirds of the patients with Mobitz type II second degree AV block have bifascicular or even trifascicular disease.
- In the presence of 2:1 AV block, Mobitz type I and Mobitz type II second degree AV block cannot be differentiated on the basis of electrocardiographic findings. In such cases, every other P wave is non-conducted without a chance to observe the constant PR interval that is characteristic of Mobitz type II second degree AV block.
- The conduction delay seen in Mobitz type II second degree block is almost always at the infra-nodal level involving the distal conduction system (His bundle (20%), bundle branches or/and fascicles).
- Although often both the terms, infranodal block or infrahisian block are applied to Mobitz type II second degree AV block, they are not synonymous with it.
- Infranodal block and infra-Hisian block are terms which refer to the anatomic location of the block, whereas
- Mobitz II refers to an electrocardiographic pattern associated with block at these levels.

## Pathophysiology of LBBB
- Unlike right bundle branch block (RBBB), left bundle branch block completely modifies the way of depolarization of the conduction system of the heart.
- In LBBB the activation of interventricular septum is from right to left due to uninterrupted conduction in the RBB.
- Then the electrical impulse propagates inferiorly to the left resulting in delayed depolarization and activation of the left ventricle especially the left lateral wall.
- In LBBB, the right to left activation of the septum causes a small negative deflection (Q wave) in lead V1 and a positive deflection (R wave) in lead V6.
- The right ventricle depolarizes earlier than the left ventricle giving an R wave in lead V1 and an S wave in lead V6.
- Subsequent delayed depolarization of the left ventricle results in an S wave in lead V1 and another R wave in lead V6.

## Pathophysiology of RBBB
- Right bundle branch block occurs when the electrical impulse is not conducted along the right bundle branch.
- As the conduction along the left bundle branch remains unaffected, the electrical impulse travels normally within the septum from left to right.
- However, the right ventricular contraction occurs comparatively slowly giving the characteristic 'M' pattern on the electrocardiogram.

### Genetics
- Familial cases of right bundle branch block have been observed in 4 Lebanese families and the abnormality was mapped to chromosome 19.
- There is a subset of patients with Brugada syndrome who have mutations in SCN5A, the gene encoding for the voltage-gated cardiac sodium channel.

### Associated Syndromes
- Duchenne muscular dystrophy
- Myotonic dystrophy: Other EKG findings include:
First-degree AV block
Left anterior fascicular block
Intraventricular conduction delay
Arrhythmias
- First-degree AV block
- Left anterior fascicular block
- Intraventricular conduction delay
- Arrhythmias
- Stokes-Adams attacks
- Kearns-Sayre Syndrome
- Brugada syndrome

### Pseudo Right Bundle Branch Block
Brugada syndrome:
- Brugada syndrome is due to a channelopathy mediated by the SCN5A gene.
- The RBBB pattern seen in patients of Brugada syndrome is not actually RBBB but instead it is due to a repolarization abnormality. Therefore, the RBBB like pattern seen in Brugada syndrome is referred to as a 'pseudo right bundle branch block'.
- EKG findings include ST-segment elevation in leads V1-V3.
- Cocaine consumption and/or the use of the antiarrhythmic propafenone may unmask the EKG findings seen in Brugada syndrome.

# Causes

## Mobitz type II second degree AV block causes
- Mobitz type II second degree AV block is rarely seen in the patients without any underlying heart disease.
- The most common causes of Mobitz type II second degree AV block include:
Reversible causes (both pathologic and iatrogenic)
Idiopathic causes similar to other degrees of AV block such as idiopathic progressive cardiac conduction disease with myocardial fibrosis and/or sclerosis affecting the conduction system.
- Reversible causes (both pathologic and iatrogenic)
- Idiopathic causes similar to other degrees of AV block such as idiopathic progressive cardiac conduction disease with myocardial fibrosis and/or sclerosis affecting the conduction system.
- Details of all the possible etiologies are given in the table below:

## Life Threatening Causes
Life-threatening conditions can result in death or permanent disability within 24 hours if left untreated.
- Acute myocardial infarction
- Acute rheumatic fever
- Bacterial endocarditis
- Myocarditis
- Severe hypothermia

## Common Causes
- Acute rheumatic fever
- Bacterial endocarditis
- Calcific aortic stenosis
- Digoxin
- Dilated cardiomyopathy
- Diltiazem
- Enhanced vagal tone
- HCM
- Hypertension
- Iatrogenic after surgical correction of VSD, tetralogy of Fallot, and endocardial cushion defect
- Inferior ST elevation MI
- Massive calcification of the mitral annulus
- Myocarditis
- Normal variants
- Penetrating and non-penetrating trauma of the chest
- Sclerodegenerative disease of the electrical conduction system
- Verapamil
-  blockers

## Causes by Organ System

## Causes in Alphabetical Order
- For causes of Left bundle branch block, click here.
- For causes of Right bundle branch block, click here.

# Epidemiology and Demographics

## Prevalence
- In United States, the prevalence of second-degree AV block is believed to be 3 in 100,000 individuals.
- Nearly 3% of the patients with underlying structural heart disease develop some form of second-degree AV block.
- The male-to-female ratio of second-degree AV block is 1:1.

## Gender
- Men and women are affected equally by second degree AV block.

# Risk Factors
- Common risk factors associated with second degree AV block include the following:
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
- 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

# Natural History, Complications and Prognosis

## Natural History
- Mobitz II second degree AV block is due to the block inferior to the AV node (infra-Hisian structures) and it rapidly progresses to a complete heart block in which no escape rhythm may emerge.

## Complications
- Complete heart block
- Stokes-Adams syndrome
- Syncope
- Dizziness
- Chest pain
- Death

## Prognosis
- Mobitz II, as it involves the infra-nodal structures, carries the risk of progression to complete heart block and carries an unfavorable prognosis.

# Diagnosis

## Diagnostic Study of Choice
- Electrocardiography (ECG) is employed to determine the type of second-degree atrioventricular (AV) block present.
- Follow-up ECGs and cardiac monitoring are appropriate.
- Routine imaging studies are not required. However, if myocarditis is a concern, echocardiography may be indicated.
- If myocardial ischemia is a concern, a chest radiograph may be indicated.

## History and Symptoms
- History from patients with second degree AV block should involve asking about the following:
Congenital heart disease
Current heart condition
Recent or previous cardiac procedures
History of medications
- Congenital heart disease
- Current heart condition
- Recent or previous cardiac procedures
- History of medications
- Most people with Wenckebach (Type I Mobitz) do not show symptoms.
- If the sinus rate is slow and only few beats are conducted (higher grade blocks) there may be a significantly reduced cardiac output.
- Usual symptoms in such patients include:
Light-headedness
Dizziness
Fainting
Fatigue
Heart failure symptoms
Pre-syncope
Syncope
- Light-headedness
- Dizziness
- Fainting
- Fatigue
- Heart failure symptoms
- Pre-syncope
- Syncope

## Physical Examination
- Patients with Mobitz II can appear asymptomatic as well. However, in more cases they may be in distress or progress to the more severe third degree AV block.
- Patients may appear pale in cases of bradycardia with decreased cardiac output.
- Bradycardia with an irregular pulse
- Lightheadedness
- Hypotension
- Syncope or presyncope
- Jugular venous distension
- Bibasilar crackles in patients with exacerbated heart failure
- Peripheral edema

## Laboratory Findings
Patients with second degree AV block should be checked for the following laboratory tests:
- Serum electrolytes
- Calcium
- Magnesium
- Myocardial enzymes in patients with myocardial infarction
- Myocarditis related laboratory tests as the following:
Lyme titres
HIV tests
PCR for enteroviruses
Chagas titres
- Lyme titres
- HIV tests
- PCR for enteroviruses
- Chagas titres

## Electrocardiogram
- There are intermittent blocked P waves.
- In the conducted beats, the PR intervals remain constant.
- The PR is fairly constant except that slight shortening may occur in the first beat after the blocked cycle. This is the result of improved conduction following the block.
- Most patients with type II second-degree AV block have associated bundle branch block.
- In these instances, the block is usually located distal to the His bundle. However, in approximately 27% to 35% of the patients, the lesion is located in the His bundle itself, and a narrow complex may be inscribed.
- 2:1 AV Block:
- Impossible to determine whether the second-degree AV block is type I or type II.
- A long rhythm strip is helpful to document any change in the behavior of the conduction ratio.
- When the atrial rate is increased by exercise or by atropine, the AV block in type I tends to decrease and that in type II tends to increase.
Shown below is an electrocardiogram of a 12 lead EKG with a 2:1 AV block.
Copyleft image obtained, courtesy of ECGpedia, 
Shown below is an electrocardiogram of a type II second degree AV block (Mobitz type II).
Copyleft image obtained, courtesy of ECGpedia, 

# Treatment

## Medical therapy for Mobitz II
- Correction of reversible causes of the block such as ischemia, medications, and vagotonic conditions should be considered.
- Treatment may also include medicines to control blood pressure and atrial fibrillation, as well as lifestyle and dietary changes to reduce the risk factors associated with heart attack and stroke.
- Treatment in emergency situations are atropine and an external pacer.

### Contraindicated medications
Second degree AV block(except in patients with a functioning artificial pacemaker) is considered an absolute contraindication to the use of the following medications:
- Adenosine
- Atenolol
- Betaxolol
- Bisoprolol
- Brimonidine tartrate and Timolol maleate
- Carteolol
- Diltiazem
- Disopyramide
- Dronedarone
- Flecainide
- Metoprolol
- Mexiletine
- Nadolol
- Nebivolol
- Penbutolol
- Pindolol
- Propranolol
- Sotalol
- Timolol
- Labetalol

## Surgery for Mobitz II

### Definitive treatment-Pacemaker insertion
- Type II Mobitz (symptomatic or asymptomatic) is by itself an indication for insertion of a pacemaker (definitive treatment). Other indications include::
Myotonic dystrophy
Kearns-Sayre syndrome
Erb's dystrophy
Peroneal muscular atrophy. These neuromuscular disorders have a high potential for unpredictable rapid progression to complete heart block.
- Myotonic dystrophy
- Kearns-Sayre syndrome
- Erb's dystrophy
- Peroneal muscular atrophy. These neuromuscular disorders have a high potential for unpredictable rapid progression to complete heart block.
- Implantation of permanent pacemakers in both asymptomatic and symptomatic patients is usually done. Asymptomatic Mobitz II are prone to be converted to symptomatic or third degree heart block. Thus, they should be considered for a pacemaker even if asymptomatic.
- A dual chamber DDD pacemaker is preferred over a single chambered VVI pacemakers as it maintains physiologic AV synchrony.
- A dual-chamber artificial pacemaker is a type of device that 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 and atrial fibrillation.

# Prevention

## Primary Prevention
- Effective treatment of hypertension and maintenance of normal blood glucose levels may be useful strategies in preventing the AV block.

# Differentiating Infra-Hisian Block from other Diseases