Heart block (ATRIOVENTRICULAR (AV) BLOCK)

In this condition conduction between the atria and ventricle is impaired.

There are three forms of AV heart block

1 . First-degree heart block

2, Second degree (partial) heart block

3- Third-degree (complete) heart block

FIRST-DEGREE HEART BLOCK

It is a simple prolongation of the PR interval to more than 0.22 seconds.

Every impulse from the atria is conducted to the ventricle but with delay.

First-degree heart block implies prolonged conduction without block, each atrial impulse is conducted to the ventricles but there is a delay in AV conduction leading to prolonged PR interval on ECG.

Site:– The site of slowed conduction may be within the atrium, AV node, bundle of His, or bundle branches.

Causes

Enhanced vagal stimulation such as in inferior wall MI

Drugs e.g, digoxin, beta-blockers, verapamil, diltiazem and amiodarone.

Myocarditis, Addison disease

Congenital heart diseases such as ASD

Rheumatic fever

It can also occur in normal individuals such as children and athletes.

Clinical features

It cannot be diagnosed clinically and its recognition depends on observing a PR interval of >0.20 s in ECG.

Management

It is a benign condition and requires no treatment

Its only importance is as an index of drug toxicity and as a precursor of the more advanced degrees of heart block.

Second-degree heart block

Second-degree heart block implies intermittent conduction; some impulses from the atria are conducted to ventricles whereas others are not.

There are 2 main types of second-degree heart block

Mobitz type 1

In this condition, there is a progressive prolongation of successive PR intervals followed by a dropped beat (non conducted P wave). This is also known as Wenckebach’s phenomenon. In this AV block, there is a conduction defect in the AV node and AV conduction time (PR interval) progressively lengthens before the blocked beat. Pulse is irregular clinically.

Prognosis is good in first-degree and in Mobitz type 1.

The site of the block is mainly an AV node. QR$ complex is normal in morphology (not wide) because there is no delay in intraventricular depolarization.

Causes

Inferior wall MI

Acute rheumatic fever

Myocarditis

Degenerative conductive system disease.

Drugs such as digoxin, beta-blockers,s and calcium channel blockers

Hyperkalemia

Mobitz type 2

In this condition, the PR interval of the conduct impulses remains constant but some P waves are not conducted which is more P waves than QRS complex.

The risk of complete heart block in this type is greater than Mobitz type 1. The site of the block is intranodal in location and the QRS complexes are wide

Mobitz type II AV block is abrupt and is not preceded by lengthening of AV conduction time. It is usually due to blocking within the bundle of His.

Can lead to complete heart block

Management

Treat the cause

Atropine if needed

Pacing may be required

COMPLETE HEART BLOCK

A complete heart block is an advanced form of block. No impulse from the atria reaches the ventricles. Cardiac action is maintained by an escape rhythm.

Escape rhythm arising in the bundle of His produces narrow QRS complexes at the rate of 50-60 beats per minute. Escape rhythm arising below the His bundle produces broad complexes and at the rate of 15-40 beats/min.

Exercise does not increase the heart rate

ETIOLOGY OF COMPLETE HEART BLOCK

Congenital

Acquired

Idiopathic fibrosis

Myocardial infarction/ischemia

Infections, aortic root abscess in infective endocarditis

Chaga’s disease

Lyme disease

Sarcoidosis

Amyloidosis,

Neoplasia

Trauma e.g. cardiac surgery

Drugs e.g. digoxin, beta-blockers, amiodarone

Connective tissue disease: SLE, Rheumatoid arthritis

SYMPTOMS OF HEART BLOCK

Symptoms of heart block develop due to bradycardia

Exercise intolerance

Easy fatigability

Dyspnea on exertion

Syncope

Dizziness

MANAGEMENT

Acute inferior wall myocardial infarction is often complicated by transient AV block which may respond to atropine. if it fails temporary pacemaker should be inserted. In many cases, the AV block will resolve within 7-10 days. The patient may develop heart failure or hypotension due to a slow heart rate in an already damaged heart requiring a pacemaker.

Mobilz II or complete heart block as a result of anterior wall infarction is usually a sign of extensive myocardial damage and carries a poor prognosis.

Asystole often occurs and a temporary pacemaker should be inserted prophylactically as soon as possible.

2nd-degree heart block- Mobitx type-1

2nd-degree heart block- Mobitx type-2