Heart block
A heart block can be a blockage at any level of the electrical conduction system of the heart
Electrical conduction system of the heart
The normal intrinsic electrical conduction of the heart allows electrical propagation to be transmitted from the Sinoatrial Node through both atria and forward to the Atrioventricular Node. Normal/baseline physiology allows further propagation from the AV node to the ventricle or Purkinje Fibers...

 (shown in the diagram on the right).
  • Blocks that occur within the sinoatrial node
    Sinoatrial node
    The sinoatrial node is the impulse-generating tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. It is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava...

     (SA node) are described as SA nodal blocks.
  • Blocks that occur within the atrioventricular node
    Atrioventricular node
    The atrioventricular node is a part of the electrical control system of the heart that coordinates heart rate. It electrically connects atrial and ventricular chambers...

     (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
    Bundle of His
    The bundle of His, known as the AV bundle or atrioventricular bundle, is a collection of heart muscle cells specialized for electrical conduction that transmits the electrical impulses from the AV node to the point of the apex of the fascicular branches...

  • 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 infra-Hisian 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
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

 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
Cardiac ectopy
Ectopic beat is a disturbance of the cardiac rhythm frequently related to the electrical conduction system of the heart, in which beats arise from fiber or group of fibers outside the region in the heart muscle ordinarily responsible for impulse formation, i.e., the Sinus node...

 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 three basic types of AV nodal block
Atrioventricular block
An atrioventricular block involves the impairment of the conduction between the atria and ventricles of the heart.The causes of pathological AV block are varied and include ischaemia, infarction, fibrosis or drugs. Certain AV blocks can also be found as normal variants, such as in athletes or...

  • First degree AV block
  • Second degree AV block
    • Type I second degree AV block (Mobitz I) (also known as Wenckebach block)
    • Type 2 second degree AV block (Mobitz II) - usually but not always below the AV node and therefore infraHisian - see below
  • Third degree AV block (Complete heart block)

Types of infra-Hisian 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 bundle branch block
    Left bundle branch block is a cardiac conduction abnormality seen on the electrocardiogram . In this condition, activation of the left ventricle is delayed, which results in the left ventricle contracting later than the right ventricle....

    • Left anterior fascicular block
    • Left posterior fascicular block
  • Right bundle branch block
    Right bundle branch block
    A right bundle branch block is a defect in the heart's electrical conduction system.During a right bundle branch block, the right ventricle is not directly activated by impulses travelling through the right bundle branch. The left ventricle however, is still normally activated by the left bundle...

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