A heart block is a disease of the electrical conduction system of the heart, during which the impulses that control the beating of the heart are slowed down or completely blocked.
A heart block can affect the conduction system on any level:
- Blocks that affect the sinoatrial (SA) node are called SA nodal blocks.
- Blocks that affect the atrioventricular (AV) node are called AV nodal blocks.
- When blocks affect the conduct system below AV node they are called infra-Hisian blocks.
- Blocks that affect ether the left or the right bundle branches are called bundle branch blocks.
Sinoatrial Nodal Blocks
Sinoatrial node is a small mass of specialized cardiac muscle fibers responsible for the initial impulse that is carried through the atria, internodal atrial pathways, and finally into the atrioventricular node. The impulse then travels down the bundle of His through the bundle branches, reaching the Purkinye fibers, which depolarize the ventricles causing them to contract.
SA blocks cause the electrical impulses to be delayed or blocked before they reach the atria and the rest of the conduction system, delaying atrial contraction and consequently slowing down the beating of the whole heart (sinus bradycardia). In contrast, AV blocks affect only the AV node delaying the ventricular contraction, while the contraction of atria remains normal.
There are three forms of sinoatrial block
- First-degree SA block – is characterized by a delay between the moment that the impulse was generated by sinoatrial node and the time it reaches the atria. This type of block cannot be seen on an electrocardiogram (ECG).
- Second-degree SA block – is registered on ECG as occasional absence of the whole PQRST complex, which can be felt by the patient as if their heart “skipped a beat”. Second-degree SA block has two types:
o Type I –the P-QRS complexes cluster more and more until the whole P-QRS complex is dropped.
o Type II - a whole P-QRS complex is occasionally dropped without gradual clustering.
- Third-degree SA block – is characterized by complete inability for sinoatrial node to excite the heart, and this causes the atrioventricular node to take on the role of sinoatrial node by generating impulses that make the heart contract (no faster than 60 times per minute).