What is Heart Block ?

 HEART BLOCK

Heart block is the blockage of impulses generated by SA node in the conductive system. Because of the blockage, the impulses cannot reach the cardiac musculature, resulting in ectopic arrhythmia. Based on the area affected, the heart block is classified into two types:

What is Heart Block ?


1. Sinoatrial block

2. Atrioventricular block.

Sinoatrial Block – AV Nodal Rhythm

Sinoatrial block is the failure of impulse transmission from SA node to AV node. It is also called sinus block. During sinoatrial block, heart stops beating. Immediately, AV node takes over the pacemaker function and produces the impulses. This leads to AV nodal (atrioventricular) rhythm.

Sinoatrial block is due to the defect in intermodal fibers and it occurs suddenly. Initially, the heart stops for a while. Then after few seconds, the AV node becomes the pacemaker and the heart starts beating decreased rate of 40 to 60/minute. Impulses may be discharged from any part of AV node, viz.

1. In upper nodal rhythm, the impulses are discharged from the upper part of AV node. In this rhythm, the P wave of ECG is inverted. QRS complex and T wave are normal

2. In middle nodal rhythm, the impulses are by the middle part of AV node. Here, all the chambers of

the heart contract simultaneously. P wave of ECG is absent as it merges with QRS complex

3. In lower nodal rhythm, the impulses are produced by the lower part of AV node. In this condition, ventricular contraction occurs prior to atrial contraction as the impulses reach the ventricles prior to the atria. In ECG, QRS complex appears prior to P wave and R-P interval is obtained instead of P-R interval. It is called reversed heart block.

Atrioventricular Block

Atrioventricular block is the heart block in which the impulses are not transmitted from atria (from AV node) to ventricles because of defective conductive system.

Atrioventricular block is of two categories:

1. Incomplete heart block

2. Complete heart block.

1. Incomplete Heart Block

Incomplete heart block is the condition in which the transmission of impulses from atria to ventricles is

slowed down and not blocked completely. Impulses reach ventricles late.

Incomplete heart block is of four types:

i. First degree heart block

ii. Second degree heart block

iii. Wenckebach phenomenon

iv. Bundle branch block.

i. First degree heart block

First degree heart block is the heart block in which the conduction of impulses through AV node is very

slow, i.e. the AV nodal delay is longer. It is also called delayed conduction. In ECG, the P-R interval is very much prolonged and is more than 0.2 second. First degree heart block is common in young adults

and trained athletes. It is also caused by rheumatic fever and some drugs. It does not produce any symptom.

ii. Second degree heart block

Second degree heart block is the type of heart block in which some of the impulses produced by SA node fail to reach the ventricles. It is also called the partial heart block. When some of the impulses from SA node fail to reach the ventricles, one ventricular contraction occurs for every 2, 3 or 4 atrial contractions, i.e. 2 : 1, 3 : 1 or 4 : 1. In ECG, the ventricular complex (QRST) is missing

accordingly. During frequent development of second degree heart block, bradycardia occurs.

iii. Wenckebach phenomenon or syndrome

Wenckebach phenomenon is a type of heart block characterized by progressive increase in AV nodal

delay, resulting in missing of one beat. Afterwards, the conduction of impulse is normal or slightly delayed. In ECG, the progressive lengthening of P-R interval is noticed till QRST complex disappears.

iv. Bundle branch block

Bundle branch block (BBB) is the heart block that occurs during dysfunction of right or left branch of

bundle of His. During this type of block, the impulse from atria reaches unaffected ventricle first. Then, from here, the impulse travels to the affected side. So, ECG shows normal ventricular rate, but the QRS complex is prolonged or deformed.

2. Complete Heart Block

(Third degree heart block)

Complete heart block is the condition in which the impulses produced by SA node cannot reach the

ventricles. It is also called complete atrioventricular block or third degree heart block. Because of this, the ventricles beat in their own rhythm, independent of atrial beat. It is called idioventricular rhythm.

Complete heart block occurs due to any one of the following causes:

i. Disease of AV node, which leads to AV nodal block

ii. Defective conductive system below the level of AV node, causing infranodal block.

i. AV nodal block

In this type of block, a part of AV node is defective and the unaffected part becomes the pacemaker.

Rhythmicity of AV node is about 45 to 60/minute.

ii. Infranodal block

Infranodal block is the heart block in which the impulses from SA node are blocked in the branches of bundle of His (below the level of AV node). In this condition  distal part of the conductive system (i.e. the Purkinje fibers) becomes the pacemaker. The rhythmicity of Purkinje fibers is about 35/minute. Sometimes, a part of ventricular musculature becomes the pacemaker and the ventricular rate in such conditions is about 20/ minute.

Third degree heart block is the serious one since it decreases the pumping action of the heart. Very often,

it results in Stokes-Adams syndrome. It may also cause heart failure.

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