First-Degree Heart Block - Delay in the conduction of an atrial impulse to the ventricle through the AV-node characterized by a prolonged P-R interval on an ECG.

Second-Degree Block - Intermittent blockage of atrial impulses at the A-V node. Conduction ratios are represented by the number of P-waves versus the number of ventricular responses in an ECG. Mobitz I is a type of second-degree heart block, also known as the Wenckebach phenomenon, in which the time between the atrial and ventricular contractions (P-R interval) becomes progressively longer until a P-wave is not conducted through the A-V node. Mobitz II is a type of second-degree heart block in which the time between the atrial and ventricular contractions is a consistent P-R interval, but an occasional P-wave is not conducted through the A-V node.

Complete Heart Block - A pathologic condition in which intrinsic conduction is blocked at any level in the A-V junction. Also referred to as third-degree heart block.