AV Blocks and Degrees of AV Block

• AV blocks involve issues with the transmission of impulses

Degrees of AV Block

• First-degree AV block is defined by a delay in the transmission of sinus impulses to the ventricle, resulting in a prolonged PR interval
• Second-degree AV block occurs when some, but not all, sinus impulses reach the ventricles, leading to dropped beats
• Third-degree AV block, none of the sinus impulses get through to the ventricles, leading to dropped beats and AV dissociation
• Atrial and ventricle activity dissociates from one another
• Permanent pacemakers may be required in third degree

AV Blocks

• Underlying rhythm is always sinus
• P waves are sinus P waves
• Issues involve complete or partial interruption in impulse transmission to the ventricles
• The block occurs either at the AV node or bundle branches

First Degree AV Block

• Rate can occur at any rate
• Regularity depends on the underlying rhythm
• P waves are upright, matching, and there is one per QRS
• PR interval is prolonged which is greater than 0.20 seconds
• QRS interval is less than 0.12 seconds
• Contributing factors can be AV node ischemia, digitalis toxicity, or medication side effects
• There are typically no adverse effects
• Treatment involves removing the cause

Type I Second Degree AV Block (Wenckebach/Mobitz 1)

• Atrial rate is 60-100, while the ventricular rate varies
• Irregular regularity with groups of beats, followed by a pause
• P waves are upright, matching with one blocked
• PR intervals gradually prolong until a QRS is dropped
• QRS remains less than 0.12 seconds
• Causes are related to MI, digitalis toxicity, or medications
• Typically no adverse effects, but monitor for worsening block
• Usually, no treatment is needed, but atropine may be given if the heart rate slows and symptoms occur

Type II Second Degree AV Block

• Atrial rate is 60-100, the ventricular rate varies
• Regularity can be regular, irregular, or RBI (regular but irregular)
• P waves are upright, matching with some blocked
• PR interval is constant on conducted beats
• QRS is less than 0.12 seconds if there is no bundle branch block (BBB); greater than or equal to 0.12 seconds if BBB
• Causes involve MI, conduction system lesion, hypoxia, or medications
• Adverse effects include decreased cardiac output, and potentially progressing to third-degree AV block
• Treatments involve atropine, epinephrine, pacemaker, oxygen, or dopamine

2:1 AV Block

• Atrial rate is 60-100, the ventricular rate is half the atrial rate
• Regular regularity
• P waves are upright, matching two per QRS
• PR interval is constant on conducted beats
• QRS is less than 0.12 seconds
• Causes include MI, medications, hypoxia, or conduction system lesion
• The adverse effect is decreased cardiac output
• Treatments include atropine, epinephrine, pacemaker, dopamine, and oxygen

Third-Degree AV Block

• Atrial rate is 60-100, the ventricular rate is 20-60 typically
• Regular regularity
• P waves are upright, matching, and dissociated from QRS complexes
• PR interval varies
• QRS is less than 0.12 seconds if the AV junction is the pacemaker, or greater than 0.12 seconds if the ventricle is the pacemaker
• Causes include MI, conduction system lesion, medications, or hypoxia
• An adverse effect is decreased cardiac output
• Treatments include pacemaker, atropine, epinephrine, dopamine, and oxygen

Differentiating AV Blocks

• First-degree AV block: there are no dropped QRS complexes, and the PR interval is greater than 0.20 seconds
• Wenckebach: PR interval gradually prolongs until a QRS is dropped
• Type II: PR interval is constant, but some QRS complexes are dropped
• 2:1 AVB: there are two P waves for every QRS complex
• Third-degree AV block: PR interval varies, R-R interval is constant, and some QRS complexes are dropped

Rhythm Summary of AV Blocks

• 1:1 P:QRS ratio, >0.20 second PR interval, 3rd degree AVB (AV dissociation)
• 1:1 until 1 QRS drops P:QRS ratio with increases in PR interval until QRS dropped leads to 2nd degree Mobitz I

• 1:1 P:QRS ratio, PR constant when conducted 2nd degree Mobitz II

• No relationship P:QRS and PR:QRS ratio usually mean AV dissociation