AV (Nodal) Blocks

First Degree AV Block

First degree AV block - a PR interval greater than 0.2s - is a result of a slowdown in the electrical impulse, usually at the AV node. It can be found in individuals with normal hearts, though can be found in conjunction with other cardiac issues (such as the other AV blocks). First Degree AV block alone rarely results in a significant clinical event, and almost never treated on its own.

Second Degree AV Block, Type 1

Second Degree, Type 1 (Wenckeback, Mobitz I) AV block is a result of prolongation of the refractory period in the AV node, resulting in the characteristic PR lengthening followed by a dropped QRS. The pattern of P waves to QRS complexes (e.g. 4:3 - or 4 P waves before 1 dropped QRS complex) can either be consistent or variable.

Like first degree AV block, most patients are asymptomatic and uncommonly will progress to signs of shock. Unstable Second Degree, Type 1 AV block is usually a result of an MI or electrolyte imbalance.

Second Degree AV Block, Type 2

Second Degree, Type 2 (Hay, Mobitz II) AV block is a result of abnormalities at or below the AV Node/Bundle of His. Unlike Type 1, there's no PR prolongation and is almost always considered abnormal. Often, it can be attributed to an MI or to cardiac-targeting medications. At the very least, a cardiology consult is warranted if the AV block is new. Wider QRS are not uncommon.

Advanced/High-Grade AV Block

A High Grade or Advanced AV Block is simply a Second Degree, Type II AV Block with more than 1 dropped QRS (i.e. more than 1 interruption in electrical impulse), resulting in prolonged episodes with no ventricular impulses. It's particularly precarious as it could progress to a Third Degree AV Block imminently so close monitoring is required. Wider QRS are not uncommon.

2:1 AV Block

A 2:1 AV Block is a Second Degree AV Block, but at this ratio it is not possible to quickly tell if it's a Type 1 or Type 2 block. However, in the emergency department it's better to assume that it's the more "dangerous" Second Degree, Type 2 AV block, and requires close and careful monitoring.

Third Degree (Complete) AV Block

Third Degree/Complete AV Block is complete dissociation of the P wave to the QRS complex. This usually manifests as a regular P wave line (at the usual P wave generation rate of 60-100) and a regular QRS complex line caused by the AV Node or Ventricles generating a pulse. If the AV Node generates an impulse, you'll see narrow junctional QRS complexes at a rate of 40-60; if the ventricles generate an impulse, you'll see much slower, wide ventricular QRS complexes.

Complete AV Block is often a result of structural changes to the heart or an MI. If a Complete AV Block has narrow junctional QRS complexes, there may still be enough cardiac output for patient's to not experience any major symptoms. However, Complete AV Block with wide ventricular QRS complexes often results in inadequate cardiac output and these patient's are usually unstable. In both of these cases, if there's no clear reversible cause of the heart block, these patients will eventually need a pacemaker - either within ED if they are unstable, or later in the cardiology lab if they are stable.

Managing Bradycardia from AV Blocks

Most AV blocks (with the exception of 1st degree AV blocks) have the potential to cause severe bradycardia. In the emergency setting, these are managed as per ACLS guidelines.