• AV block is a block (delay or disruption) anywhere along the conduction from the atria to the ventricles such as the AV node, His-Purkinje sytem, or atrial myocytes.
• AV block can be caused by congenital or acquired conditions.
• Acquired causes of AV block include:
Enhanced vagal tone, due to emotional stress, pain, micturition, or in carotid hypersensitivity
Degenerative diseases: Lev's in elderly and Lenegre's in younger patients
Drugs: digoxin, beta blockers, calcium channel blockers, amiodarone
Acute ischemia, often inferior MI
Extention of aortic calcification in severe aortic stenosis
Infiltrative diseases: amyloidosis, sarcoidosis, hemochromatosis, malignancy
Infection: endocarditis, myocarditis (Lyme, rheumatic fever, Chagas, TB)
Rheumatic diseases: scleroderma, SLE, RA
Muscular dystrophies
Iatrogenic: TAVR, LV cath, AV nodal ablation, aortic valve replacement
Long QT syndrome (functional AV block due to prolonged ventricular refractoriness)
• AV block can progress from first (delayed conduction) to second (intermittently failed conduction) to third (persistently failed conduction) degree.
• Generally, site of AV block can be predicted based on certain ECG features and responsiveness to sympathetic (exercise) and vagal (carotid massage) stimuli:
AV node: more prolonged PR interval, narrow QRS, responsive to vagal and sympathetic stimuli and atropine
His-Purkinje: widened QRS, presence of bundle branch block
• First-degree AV block occurs when there is a delay in conduction anywhere from the atria to the ventricles such as the AV node, His-Purkinje sytem, or atrial myocytes.
• On ECG, PR intervals are prolonged (≥ 200 ms) at constant length and all P waves are conducted (no dropped beat).
• Site of first-degree AV block is suggested by certain ECG features:
AV node: PR ≥ 300 ms, variable PR intervals, narrow QRS
His-Purkinje: presence of bundle branch block
Atria: prolonged P wave duration with left or right atrial enlargement pattern
• Second-degree AV block occurs when there is intermittent failure in conduction through the AV node or His-Purkinje system.
• On ECG, there are intermittent dropped QRS's following P's.
• Second-degree AV block is classified into different types based on ECG patterns:
Mobitz type I: progressively prolonged PR intervals until non-conducted P with dropped QRS
Mobitz type II: constant (normal or prolonged) PR intervals with intermittent non-conducted P's
Fixed 2:1 AV block: alternating conducted and non-conducted P's
High grade AV block: 3:1 AV block or higher
• Vagal maneuver only worsens type I (associated with AV nodal block), but not type II (associated with His-Purkinje block) by activating parasympathetic stimulation of the AV node.
• In typical Mobitz type I second-degree (or Wenckebach) AV block:
PR intervals are progressively prolonged until P fails to conduct resulting in dropped QRS
PR interval immediately following the dropped QRS is shorter than the PR interval preceding the dropped QRS, it resets to the baseline PR interval and restarts the cycle (Wenkebach periodicity or group beating)
Prolongation of the PR interval occurs at progressively decreasing increments
• In atypical Wenckebach:
PR interval of the last conducted P has an increased (or greatest) increment
PR intervals have very little or no incremental change in duration
PR interval shortens then lengthens in the middle of the Wenkebach sequence
A junctional escape beat follows non-conducted P wave resulting in apparently shortened PR
• Site of type I second-degree AV block is suggested by certain ECG features:
AV node: narrow QRS
His-Purkinje: presence of bundle branch block, short baseline PR interval, small PR interval increment
• In Mobitz type II second-degree AV block, PR intervals of conducted P waves are either normal or prolonged but constant, followed by non-conducted P waves and dropped QRS's.
• PR interval following the dropped QRS is identical to PR interval preceding the dropped QRS. This is different from Mobitz type I, where PR interval immediately following the dropped QRS is shorter than the PR interval preceding the dropped QRS.
• Site of type II second-degree AV block is often the His-Purkinje including the His bundle and bundle branches.
• In fixed 2:1 AV block, there is alternating conducted and non-conducted P waves (a 2:1 P:QRS ratio).
• Because there is no prolongation of PR interval, it's diffcult to determine whether 2:1 AV block is a type 1 or type 2 second degree AV block therefore 2:1 AV block forms its own category. And either type 1 or type 2 second degree AV block can progress to fixed 2:1 AV block.
• Site of fixed 2:1 AV block is suggested by certain ECG features:
AV node: constant PR, PR ≥ 300 ms, variable PR intervals, narrow QRS, before/after Wenkebach
His-Purkinje: PR < 160 ms, widened QRS
• In high grade (or advanced second-degree) AV block), there is at least 2 non-conducted P waves for every conducted P wave (a 3:1 P:QRS ratio).
• High grade AV block has the highest risk of progressing to complete AV block.
• Third-degree AV block (also referred to as complete heart block or CHB) occurs when all conduction between the atria to the ventricles fail.
• Because there is no impulse conducted from above, either a junctional or ventricular automaticity focus will take over pacemaking activity and send out impulses thus giving rise to junctional or ventricular escape rhythm, respectively.
• Meanwhile, SA node still sends impulses to the atria causing atrial depolarization and sinus P waves that pace independently from QRS’s (AV dissociation).
• Site of third-degree AV block is suggested by certain ECG features:
AV node: junctional escape rhythm with narrow QRS
His-Purkinje: ventricular escape rhythm with widened QRS
• If the site of complete block is within the AV node, a junctional automaticity focus takes over pacemaking activity thus giving rise to junctional escape rhythm.
• Because impulses from the junctional focus travel down the His-Purkinje pathway, the resulting QRS complexes are normal and narrow.
• Meanwhile, SA node still sends impulses to the atria causing atrial depolarization and sinus P waves that pace independently from QRS’s (AV dissociation).
• If the site of complete block is within the His-Purkinje system, a ventricular automaticity focus takes over pacemaking activity thus giving rise to ventricular escape rhythm.
• Because impulses from the ventricular focus don't travel down the His-Purkinje pathway, but rather spread through adjacent ventricular myocytes, the resulting QRS complexes are widened.
• Meanwhile, SA node still sends impulses to the atria causing atrial depolarization and sinus P waves that pace independently from QRS’s (AV dissociation).
