Heart Block

 In heart block the atrioventricular node is affected

 There are three types of heart block

  Fainting is the main symptom of heart block

A cardiac pacemaker is usually implanted

What is heart block?

To understand heart block you need to understand the mechanism of how the heart beats. There is a specialised system of electrical fibres in the heart with two main nodes or hubs from which electrical impulses spread along these fibres – the atrioventricular node (AV node) and the sino-atrial node.

In heart block it is the AV node which is affected. This node ensures that the heart beats in a synchronous way from the atria at the top to the ventricles at the bottom. This is called the sinus impulse.

Any abnormality in the conduction of this sinus impulse to the ventricles may signal the development of heart block. This can eventually lead to fainting attacks (syncope) or the heart stopping beating altogether (cardiac arrest).

The different types of heart block

There are three different types of heart block:

  First-degree AV block, more correctly called prolonged AV conduction

  Second-degree heart block

Third-degree heart block

Each of these types of heart block is characterised by the different ways in which the conducting system is disturbed.

Another type of heart block is called AV dissociation.

What causes heart block?

The AV node is supplied by two different parts of the nervous system – the parasympathetic and sympathetic. These are both parts of the autonomic nervous system – which controls involuntary actions in the body.

The sympathetic nervous system has nerves distributed to the heart, blood vessels, lungs, intestines and other organs in the abdomen. It governs functions in these areas by reflex action in balance with the parasympathetic nervous system, which supplies the same areas, but often provides the opposite function to the sympathetic nervous system.

The AV node is sensitive to what is called autonomic tone – the action of the two branches of the autonomic nervous system together.

Chronic slowing of AV nodal conduction can occur in highly trained athletes who have a very slow heart beat at rest.

A variety of diseases can also influence conduction by the AV node:

       Heart attack (myocardial infarction)

       Coronary spasm – which is constriction of the arteries supplying the heart

       Excess of the drug digoxin

       Excess of beta-blockers or calcium channel blockers – two drugs commonly used in people with heart disease

       Infections such as viral myocarditis – inflammation of the heart muscle caused by a virus

       Miscellaneous disorders such as Lyme disease, mononucleosis (glandular fever) and cancers, particularly something called mesothelioma of the heart

AV node block can also be congenital – meaning that you are born with it.

There are two uncommon muscular degenerative diseases which are responsible for isolated heart block in adults – Lev’s disease and Lenegre’s disease.

Hypertension (high blood pressure) and stenosis (tightening) of the aortic and mitral valves of the heart are specific disorders which will accelerate degeneration of the conducting system or cause the deposition of calcium or fibrosis of the system.

What are the symptoms of heart block?

The main symptom associated with heart block is fainting (syncope). The extent to which this occurs depends on the type and extent of the conducting disturbance.

How is heart block treated?

The most important aspect of managing someone who has heart block is whether or not the person needs a permanent cardiac pacemaker implanted.

Anyone who has symptoms associated with second or third-degree heart block should have a permanent pacemaker implanted.

There are other specialised circumstances in which a pacemaker may be required, but full investigation is needed by a specialist in this field before any decision can be made.