Atrioventricular Nodal Blocks
An atrioventricular nodal block is an impairment of the conductive system between the heart’s atria and ventricles. Under normal conditions, an electrical impulse generated by the sinoatrial node first makes the atria contract and then travels further into atrioventricular node from which it is sent into the ventricles making them contract. Atrioventricular block is a condition during which the transfer of this impulse through the atrioventricular node is impaired which desynchronizes the contraction of atria and ventricles.
There are three types of atrioventricular block:
- First degree AV block – the conduction of the impulse through the atrioventricular node is simply slowed down.
- Second degree AV block – during this type of AV block some of the impulses generated by sinoatrial node are completely blocked. There are two types of this block:
o Mobitz 1 – the time it takes for the impulse to travel through the atrioventricular node gradually lengthens, until an impulse is completely blocked.
o Mobitz 2 – the time it takes for the impulse to travel through the atrioventricular node remains unchanged, while some of the impulses generated by SA node are blocked. The amount of blocked impulses can vary. Sometimes the ratio of blocked to unblocked impulses is 1 to 5, however, as the condition of the patient worsens, it can change to 1 to 4, 1 to 3, etc. The transition between Mobitz 2 and third degree AV block is the most dangerous for the person, since this causes a large portion of the impulses to be blocked by the atrioventricular node, leading to prolonged asystole.
- Third degree AV block – no impulses generated by SA node can reach the ventricles. This causes the atrioventricular node to take the place of sinoatrial node and start generating impulses. This leads to complete desynchronization of the atria and ventricles as the atria contract with one rhythm and ventricles contract with another (AV node cannot generate impulses faster than 60 times per minute).