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.
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.
Do you know the difference between myocardial infarction and myocardial ischemia? See answer here.
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.