Third-degree AV block or complete heart block is the most clinically significant AV block associated with PALS. 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 the impulse is generated from a different accessory pacemaker in the ventricle than the other QRS complexes.
Complete heart block is caused by extensive injury or disease to the conduction system of the heart.
The most common cause of complete block in adults 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.
- Cardiac surgery
- Hypoxia/acidosis (more common in infants and children)
- Congenital complete heart block (more common in infants and children)
- Medications (beta-blockers, calcium-channel blockers, digoxin)
Complete heart block is basically the end point of both types of second-degree block. Either the cells of the conduction system progressively fatigue until complete block occurs, or there is a sudden conduction system failure related to myocardial infarction. All causes of both types of the second-degree block can ultimately lead to complete heart block. Below are some of the more common causes.
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 is transcutaneous pacing.