There are two types of second-degree AV blocks. The problem can be within the AV node or below the AV node. When it is below the AV node, the Bundle of His or bundle branch system is damaged and the rhythm is more concerning and can progress to third-degree AV block or ventricular standstill.

A defect in the AV node causes sinus rhythm with second-degree AV block, type I. Transmission takes longer with each successive impulse until one fails to be conducted to the ventricles. The PR interval gets progressively longer until some sinus impulses are blocked by the AV node. The rhythm usually occurs in a cyclic pattern such as 3:2, 5:4, etc. It is usually a temporary disorder and does not necessitate a pacemaker because it is not progressive.

A defect below the AV node in the Bundle of His or bundle branch system occurs due to ischemia or injury and causes second-degree AV block, type II. The sinus impulses are usually blocked at regular intervals, allowing only every second, third, or fourth impulse to be conducted to the ventricles. The PR intervals which are present will be constant. 2:1 conduction is common in second-degree, type II, resulting in a slow heart rate and symptoms. It is a progressive rhythm and pacemaker therapy should be implemented.​