Sometimes there are disruptions the depolarization conduction system. These disruptions are called heart blocks.
If you remember, in a "normal healthy" heart the pacer impulse originates in the SA Node and is conducted through the atria until it hits the AV Node. From the AV Node it is conducted through the bundle of his, leading through the right and left bundle branches, terminating at the the perjikie fibers.
A heart block can occur at any of these points.
SA Node:
Sinus Block occurs when one of the following takes place:
The SA node does not generate and impulse for a PQRST cycle(resulting in a 'skipped beat' on the EKG).
Or
The SA Node generates a depolarization impulse, however it is blocked from leqving the SA Nose, so surrounding tissues are not stimulated to depolarize.
In an EKG, this will appear as a flat baseline segment (an extended "pause" between PQRST cycles).
Sick Sinus Syndrome
The SA Node is under distress and paces at a very slowly and no automaticity foci in the atria, AV Junction, or ventricles are attempting to pace the atria because they are also in distress.
AV Block
This is when the depolarization impulse from the SA node is parially or completely blocked at the AV node, bundle of HIS, or the bundle branches.
First Degree AV Block-
This is not a true heart block, it is merely a slowing of the impulse conduction through the atria. This results in a delay between atrial depolarization and ventricular depolarization. On the EKG reading this is manifested by a PR Interval lasting at least 0.2 seconds.
Second Degree AV Block-
There are two sub-types of heart block in this catergory:
Type I (AKA Wenckebach)
This type occurs when when conduction is slowed at the AV Node in progressive degrees until the a 'P' wave is entirely blocked from producing a QRS response in the ventricles. This is manifested on the EKG strip as a progressively lengthening PR interval until there is a 'P' wave without a QRS complex following it (there is just s baseline segment, followed by another 'P' wave belonging to the next PQRST complex, and the cycle of increasing PR interval length starts again).
Type II (AKA Mobitz)
In this type of heart block, the disruption of the depolarization impulse occurs in the Bundle of His or the Bundle Branches. 'P' waves from the SA Node are blocked at a consistent ratio (such as 4 'P' waves to 1 QRS complex, or 3 'P' waves to 1 QRS complex). On an EKG strip, the PR interval is consistent in the PQRST complex; however, there may be several 'P' waves that do not initiate ventricular depolarization.
These two subtypes of heart block are distinct from each other. However, in some instances they can be confused with each other:
Imagine you pick up an EKG strip and you see the following pattern repeated: two P waves followed by a QRS complex. You don't notice a variable PR interval but, because of the ratio of two atrial depolarizations to one ventricular depolarization, it is difficult to tell.
This rhythm is an 2:1 AV block. There is a method to determine wheither the block is in the AV node or the bundle brances using the vagal manuver to increase parasympathetic stimulation (slowing) of the AV node so that (if it is in the AV node) the ratio of P waves to QRS complexes changes or (if it is in the branches) it may eliminate the block.
This procedure can be helpful, however it can also make the rhythm worse. In a hospital setting is recommended to get a test called an EP to help diagnose this type of block.
Third Degree AV Block-
In this type of block, the atria and ventricles are completely isolated from each other electrically. Both the atria and the ventricles have their own, independent pacers that 'beat' at an inherent rate (the ventricles usually much slower) and there is NO association between the P wave and the QRS complex (AV dissociation). A patient with this type of heart block can deteriorate very quickly, monitor closely and keep in mind that the patient will most likely require an artificial pacemaker to maintain cardiac output.
Ventricular Block
Bundle Branch Block-
When either of the bundle branches (right or left) is damaged, conduction to the ventricle it serves is delayed. On an EKG strip you may see this as a 'Split' or 'Joined' QRS complex. It is wide (often greater then 3 mm) and may have two peaks (one signifying the depolarization of each ventricle).
This will appear most clearly in the chest leads on the far right or left of the heart (V1, V2, V5, V6).
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