What is the best way to differentiate between a second degree AV block type II and a 3rd degree block?

Question

Analyzing AV blocks is a common pitfall for students when first learning ECG interpretation, specifically differentiating between second degree type II and 3rd degree AV blocks.

Answer 1

In Mobitz type 1 second-degree AV block, the PR interval progessively lengthens with each beat and the QRS complex is dropped, which is also called Wenckeback. Mobitz type 2 second-degree AV block, the PR interval remains constant. Beats are intermittently nonconducted and QRS complexes dropped, usually in a repeating cycle of every 3rd or 4th P wave. In third-degree AV block, there is no relationship between P waves and QRS complexes. The P wave rate is greater than the QRS rate. Most patients require a pacemaker.

Answer 2

Second Degree AV Block Type II: non-conducted P waves with a fixed PR interval in conducted beats.

Third Degree AV Block: complete disassociation between P waves and QRS complexes with independent atrial and ventricular rhythms.

Answer 3

The easiest way to identify a second degree type II AV block is a consistent PR interval. If the PR interval is consistent then it can’t be a 3rd degree block (or a second degree type I for that matter). If the PR interval is all over the place (not in a pattern of getting successively longer then have a dropped QRS like in a second degree type I) and there are many more p waves than QRS complexes then it’s a 3rd degree block (complete heart block).