Heart block

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Overview
A heart block is a disease in the electrical system of the heart. Heart block can cause lightheadedness, syncope (fainting), and palpitations. It can be either congenital or acquired. Acquired heart block is more common than congenital heart block. Damage to the heart muscle and its electrical system by diseases, surgery, or medicines can cause acquired heart block.

Types of heart block
A heart block can be a blockage at any level of the electrical conduction system of the heart.


 * Blocks that occur within the sinoatrial node (SA node) are described as SA nodal blocks.
 * Blocks that occur within the atrioventricular node (AV node) are described as AV nodal blocks.
 * Blocks that occur below the AV node are known as infra-Hisian blocks (named after the bundle of His).
 * Blocks that occur within the left or right bundle branches are known as bundle branch blocks.
 * Blocks that occur within the fascicles of the left bundle branch are known as hemiblocks.

Clinically speaking, most of the important heart blocks are AV nodal blocks and infrahisian blocks.

Types of SA nodal blocks
The SA nodal blocks rarely give symptoms. This is because if an individual had complete block at this level of the conduction system (which is uncommon), the secondary pacemaker of the heart would be at the AV node, which would fire at 40 to 60 beats a minute, which is enough to retain consciousness in the resting state.

Types of SA nodal blocks include:
 * SA node Wenckebach (Mobitz I)
 * SA node Mobitz II
 * SA node exit block

In addition to the above blocks, the SA node can be suppressed by any other arrhythmia that reaches it. This includes retrograde conduction from the ventricles, ectopic atrial beats, atrial fibrillation, and atrial flutter.

The difference between SA node block and SA node suppression is that in SA node block an electrical impulse is generated by the SA node that doesn't make the atria contract. In SA node suppression, on the other hand, the SA node doesn't generate an electrical impulse because it is reset by the electrical impulse that enters the SA node.

Types of AV nodal blocks
There are four basic types of AV nodal block:
 * First degree AV block
 * Second degree AV block
 * Type 1 second degree AV block (Mobitz I) (also known as Wenckebach phenomenon)
 * Third degree AV block (Complete heart block)

Types of infrahisian block
Infrahisian block describes block of the distal conduction system. Types of infrahisian block include:
 * Type 2 second degree heart block (Mobitz II)
 * Left bundle branch block
 * Left anterior fascicular block
 * Left posterior fascicular block
 * Right bundle branch block

Of these types of infrahisian block, Mobitz II heart block is considered most important because of the possible progression to complete heart block.