The three types of heart block are defined as follows: First degree ... s electrical system. Because third-degree heart block can be intermittent, an ECG may not diagnose it.

Third-degree atrioventricular nodal block, also known as third-degree heart block or complete heart block, occurs when no action potentials conduct through the AV node. This results in the P waves ...

This ECG has ... beats (second degree type I AV block), however at times P waves are randomly not conducted (2:1 AV block). Note the irregular R-R intervals making 3rd degree heart block unlikely.