The AV node is a group of specialized conducting cells found in the middle of the heart. The AV node is responsible for transmitting electrical pulses from the top part of the heart to the bottom part of the heart. Second degree AV block refers to a condition where the transmission of impulses through the AV node is slower than expected with occasional times where impulses are not transmitted at all.
Second degree AV block is often diagnosed incidentally on an electrocardiogram or some other heart rhythm recording. There are 2 different forms of second degree AV block. The first form, type I or Wenkebach, refers to a situation where impulse transmission to the AV node progressively slows with each successive beat until a point where a single impulse is no longer transmitted. Type I AV block in many instances can be a normal finding caused by enhanced vagus nerve activity. The vagus nerve is a large nerve that travels from the brain throughout the body. It has branches to many portions of the heart, including the AV node. When the vagus nerve is activated, it can slow transmission through the AV node, resulting in either first-degree AV block or type 1 second degree AV block. This can often be seen during sleep in completely normal, healthy individuals. Less commonly type I AV block may be a manifestation of more diffuse electrical conduction system disease.
Type 2 AV block refers to a situation where impulse transmission through the AV node occasionally is blocked without any warning. Type 2 AV block is more concerning and can be a sign of impending complete heart block. More frequent monitoring and/or consideration of a pacemaker may be warranted in patients with type 2 AV block, especially in those with advanced forms.