The heart electrical signals are initiated in its natural pacemaker - the sinoatrial
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.