node, or SA node, and travel through the atria to reach the atrioventricular node, or AV

node. The AV node is the gateway to the ventricles. The AV node passes the signals onto the bundle

of His. This bundle is then divided into left and right bundle branches which conduct the

impulses toward the apex of the heart. The signals are then passed onto fascicular branches,

and spread through millions of Purkinje fibres over the ventricular myocardium.

Heart block is a group of diseases characterized by presence of an obstruction, or a “BLOCK”

in the heart electrical pathway. A block may slow down the conduction of electrical impulses,

OR, in more severe cases, completely stop them. Heart blocks are classified by location

where the blockage occurs. Accordingly, there are: SA nodal blocks, AV nodal blocks, intra-Hisian

blocks, bundle branch blocks and fascicular blocks.

Of these, AV nodal blocks, or AV blocks, are most clinically significant. In fact, very

commonly, the term “heart block “, if not specified otherwise, is used to describe

AV blocks. In AV blocks, the electrical signals are slow to reach the ventricles, or completely

interrupted before reaching the ventricles. There are three degrees of AV block:

First-degree AV block: the electrical signals are SLOWED as they pass from the SA node to

the AV node, but all of them eventually reach the ventricle. On an ECG, this is characterized

by a longer PR interval of more than 5 small squares. First-degree AV blocks rarely cause

symptoms or problems and generally do NOT require treatment.

Second-degree AV blocks are divided further into type I and type II:

- In type I, the electrical signals are delayed further and further with each heartbeat until

a beat is missing completely. On an ECG, this is seen as PROGRESSIVE prolongation of PR

interval followed by a P wave WITHOUT a QRS complex. This is known as a “blocked”

P wave or a “dropped” QRS complex. The cycle then re-starts over. As this usually

repeats in regular cycles, there is a fixed ratio between the number of P waves and the

number of QRS complexes per cycle. The number of QRS complexes always equals the number

of P waves MINUS one. In this example, there are four P waves for every three QRS complexes.

This is a “4 to 3” heart block. Second-degree type I blocks are usually mild and no specific

treatment is indicated. - In type II second degree blocks, some of

the electrical signals do NOT reach the ventricles. On an ECG, this is seen as intermittent non-conducted

P-waves. The PR interval, however, remains CONSTANT in conducted beats. In majority of

cases, the successfully conducted QRS complexes may appear broader than usual. In some type

II blocks, there is a fixed number of P waves per QRS complex. In this example, there are

three P waves for every QRS complex and the condition is described as “3 to 1” heart

block. However, as the nature of type II block is unstable, this ratio is likely to change

over time. Second- degree type II is less common than second-degree type I but is much

more dangerous as it frequently progresses to complete heart block or cardiac arrest.

Implantation of an artificial pacemaker is recommended for treatment of this type of

AV blocks. Third-degree AV blocks are also referred to

as complete heart blocks. In this condition, NONE of the electrical signals from the atria

reach the ventricles. With NO input coming from the atria, the ventricles usually try

to generate some impulses on their own. This is known as an “ESCAPE rhythm”. On an

ECG, two independent rhythms can be seen: a regular P wave pattern represents atrial

rhythm; and a regular, but UNUSUALLY slow QRS pattern represents the escape rhythm.

The PR interval is variable as there is NO relationship between the 2 rhythms. Patients

with third-degree heart blocks are at high risk of cardiac arrest. They require immediate

treatment, cardiac monitoring and pacemaker implantation.