Third-degree Atrioventricular Block

Third-degree Atrioventricular Block

Third-degree AV block, also known as complete heart block, is a medical condition in which the impulse generated in the SA node in the atrium does not propagate to the ventricles.

Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram (ECG).

• The P waves with a regular P to P interval represents the first rhythm.
• The QRS complexes with a regular R to R interval represent the second rhythm. The PR interval will be variable, as the hallmark of complete heart block is no apparent relationship between P waves and QRS complexes.

One of the pathognomonic characteristic of this block is the absolute absence of the opportunity for atrial impulses to enter and capture the ventricles (unlike AV dissociation with functional block) due to the organic nature of block (e.g., fibrosis, calcification, or infiltration of the node). It means that, in the presence of complete heart block, fusion or capture beats will never be seen. Patients with third-degree AV block typically experience bradycardia (an abnormally low measured heart rate), hypotension, and at times, hemodynamic instability. In some cases, exercising may be difficult, as the heart cannot react quickly enough to sudden changes in demand or sustain the higher heart rates required for prolonged activity.