Electrocardiogram (ECG, EKG) library
I think you will find this site extremely helpful. Remember in 3rd degree heart block the "P" waves and the "QRS" and no correlation at all........is is a slow AV dissociation. The R to R intervals may be regular, the PR interval will be wildly inconsistent. But it is important to realize that not all patients with AV dissociation have complete heart block. For example, patients with ventricular tachycardia have AV dissociation, but not complete heart block; in this example, AV dissociation is due to the ventricular rate being faster than the intrinsic sinus rate. On electrocardiography (ECG), complete heart block is represented by QRS complexes being conducted at their own rate and totally independent of the P waves
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). ECG Video: AV Block - Complete (Third Degree) - YouTube 3rd Degree Atrioventricular Block - YouTube Differentiating AV Blocks - YouTube
- 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 organic nature of block(eg, 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.
Medscape: Medscape Access
(requires registration but it is free and a great resource!)
I hope this helps.