Third-Degree AV Block
Complete Heart Block
Third-degree AV block or complete heart block is the most clinically significant AV block associated with ACLS. Complete heart block occurs when the electrical impulse generated in the SA node in the atrium is not conducted to the ventricles.
When the atrial impulse is blocked, an accessory pacemaker in the ventricles will typically activate a ventricular contraction. This accessory pacemaker impulse is called an escape rhythm.
Because two independent electrical impulses occur (SA node impulse & accessory pacemaker impulse), there is no apparent relationship between the P waves and QRS complexes on an ECG.
Characteristics that can be seen on an ECG include:
- P waves with a regular P to P interval
- QRS complexes with a regular R to R interval
- The PR interval will appear variable because there is no relationship between the P waves and the QRS Complexes
In the image above note that the p-waves are independent of the QRS complexes. Also note the 4th QRS complex (impulse) looks different from the others. This is because it is from a different accessory pacemaker in the ventricle than the other QRS complexes.
Common Causes
The most common cause of complete block is coronary ischemia and myocardial infarction. Reduced blood flow or complete loss of blood flow to the myocardium damages the conduction system of the heart, and this results in an inability to conduct impulses from the atrium to the ventricles.
Those with third-degree AV block typically experience bradycardia, hypotension, and in some cases hemodynamic instability.
The treatment for unstable third-degree AV block in ACLS is transcutaneous pacing.
Below is a short video which will help you quickly identify third-degree AV block on a monitor.
Please allow several seconds for the video to load.
This is a fantastic site. Easy to understand and straight to the point.
Thank you for these videos on the blocks it makes it so much easier to analyze what you are seeing.
Is this Diane from Winthrop?
I am fairly new to evaluating arrythmias. I am having trouble in these rythms distinguishing between T waves and P waves to know when I am missing a QRS. Any hints for me to “get this?”
If the rhythm is regular, you can use calipers or some sort of measuring device. Look at the best of what you think is a p-wave. Then measure the distance to what you think is the next p-wave. Since the rhythm is regular, the p-waves should be at a regular interval.
In the case of third degree heart block, there is a disassociation between the atrium and the ventricles so it may be very difficult to tell the difference between a p-wave and a t-wave. However, they will still be regular. Therefore, whichever wave is at the same rate as the QRS will be your t-wave and the other will be your p-wave.
Taking time to look at the complete block really helps. This is exactly what I’ve been looking for. You definitely don’t learn them all overnite! Thanks again.
This is a very easy site to learn from and money well spent!