In order to understand electrical abnormalities in the heart, a nurse must first understand how a healthy heart is supposed to function. Watch the video below for a very short overview of the normal electrical pathway in the heart.
In a patient who is experiencing some form of heart block, there is a portion of the electrical pathway that is not working correctly.
First Degree Heart Block
The electrical impulse flowing through the heart is delayed when moving from the atria to the ventricles in a first degree heart block. The PR interval is greater than 0.2 seconds long. The atria contract and then there is an abnormal delay before the ventricles contract. This is commonly seen in athletes. Medications can also cause patients to have a first degree heart block. Patients are usually asymptomatic.
Second Degree Heart Block
Mobitz Type I (AKA Wenckebach)
This type of second degree heart block does not happen as commonly. I think I've only seen it a couple of times in the six years I've been a nurse. With this type of heart block, the electrical signals get more and more delayed with each beat until one electrical signal does not reach the ventricles at all. You see the PR interval increasing with each beat and then one P wave alone. After the dropped beat, the electrical impulses start up again with P wave followed directly by a QRS complex. Patients can experience dizziness related to this type of heart block.
Mobitz Type II
With Mobitz Type II, it is similar to Type I in that the electrical impulses are not being transmitted to the ventricles efficiently. The main difference is that Type II is not as systematic as Type I; sometimes the electrical signals will continue to the ventricles and sometimes they won't. In the strip below, you can see there are two separate P waves that have no QRS complexes directly following them. Patients need pacemakers placed to treat this type of heart block.
Third Degree Heart Block (complete Heart Block)
Third degree heart block is also called complete heart block or complete AV block. This is because none of the electrical signals from the AV node reach the ventricles. When the ventricles do not receive any electrical impulses from the atria, they naturally beat at a much slower pace. This is not aligned with the beats of the atria. The atria continue to beat at the same pace and the ventricles continue to beat at the same pace, but they are completely unaware of each other. Each part of the heart has an intrinsic electrical pulsing system built into it in case other parts of the heart are not working.
- The intrinsic rate of the SA node is 60-100 beats per minute (so you should see about that many P waves per minute).
- The intrinsic rate of the AV node is 40-60 beats per minute, but this portion of the heart is where the electrical blockage occurs.
- The intrinsic rate of the Purkinje fibers is 20-40 beats per minute which is what is causing the QRS complex to form. You would expect to see a ventricular rate of 20-40 beats per minute because that is how frequently the Purkinje fibers are going to fire.
This type of heart block is a medical emergency and can result in cardiac arrest and death. A patient needs to be temporarily externally paced and then have a pacemaker inserted to treat this type of heart block.