Atrioventricular Block

Atrioventricular (AV) block is a bradyarrhythmia caused by delay, or interruption, in the electrical conduction between the atria and the ventricles. Atrioventricular block occurs due to either anatomic or functional impairment, and is classified into 3 types. The 1st-degree block is due to delayed conduction through the AV node. The 2nd-degree block is characterized by progressive conduction delay or intermittently blocked conduction. The 3rd-degree block involves total interruption in conduction between the atria and ventricles, causing complete AV dissociation. Patients may be asymptomatic or may present with syncope, chest pain, dyspnea, and bradycardia depending on the severity of the block. Electrocardiography (ECG) establishes the diagnosis, and treatment is based on the type of block and hemodynamic stability of the patient.

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Epidemiology and Etiology

Epidemiology

  • More common in the elderly
  • 3rd-degree block is the rarest.
  • 1st-degree and Mobitz type 1 2nd-degree atrioventricular (AV) block can be seen in healthy patients.

Etiology

  • Pathologic:
    • Idiopathic fibrosis and sclerosis of the conduction system (approximately 50% of cases)
    • Myocardial infarction (approximately 40% of cases)
    • Cardiomyopathy:
      • Hypertrophic obstructive cardiomyopathy 
      • Sarcoidosis 
      • Amyloidosis
    • Myocarditis:
      • Lyme disease
      • Systemic lupus erythematosus
      • Infective endocarditis
      • Other bacterial, viral, and parasitic causes
    • Congenital heart disease:
      • Atrial and AV septal defects
      • Neonatal lupus 
    • Familial disease
    • Hypothyroidism and hyperthyroidism
    • Hyperkalemia
    • Tumors and trauma
  • Physiologic:
    • Increased vagal tone (due to athletic training, sleep, pain, carotid sinus massage, carotid sinus hypersensitivity syndrome)
    • Heart tissue changes related to aging
  • Iatrogenic:
    • Cardiac surgery (aortic valve surgery)
    • Transcatheter cardiac procedures
    • Cardiac ablation therapy
    • Medications:
      • Beta blockers 
      • Non-dihydropyridine calcium channel blockers 
      • Digoxin 
      • Adenosine 
      • Antiarrhythmics (amiodarone, quinidine, procainamide, disopyramide)

Pathophysiology and Classification

Atrioventricular (AV) block is a delay, or interruption, in the electrical impulse as it passes from the atria to the ventricles through the AV node or the His-Purkinje system. Atrioventricular block is classified based on the severity of the disruption.

Schematic of the electrical system of the heart. Atrioventricular block may occur within the AV node, His bundle, or the bundle branches. Blocks at the level of the AV node or the His bundle will generally be narrow. Infrahisian (below the His bundle) blocks result in wide QRS complexes.

Image by Lecturio.

First-degree AV block

  • Delayed or prolonged conduction through the AV node; not considered a true block
  • Defined as a prolonged PR interval (> 0.2 sec)
  • May be non-pathologic

Second-degree AV block

Second-degree AV block is further divided into 2 subtypes:

  • Mobitz type 1 (also known as “Wenckebach”):
    • Progressive increase in the delay of conduction until a block forms at the AV node → conduction is not transmitted through the AV node → the QRS complex is “dropped”
    • Usually the result of excess vagal tone of the AV node in athletes
  • Mobitz type 2: 
    • Intermittent block of conduction through the AV node → unchanged PR intervals with intermittent QRS complexes “dropped”
    • Almost always results from conduction system disease below the level of the AV node
    • Can deteriorate into 3rd-degree AV block

Third-degree (complete) AV block

  • Complete failure of conduction between the atria and ventricles
  • Results in AV dissociation: 
    • Atrial activation and ventricular activation are independent of each other.
    • Occurs because no atrial impulses reach the ventricles
  • An escape pacemaker occurs distal to the block:
    • Junctional escape rhythm from above the His bundle
    • Ventricular escape rhythm from below the His bundle
  • Atrial and ventricular contractions become uncoordinated → poor movement of blood → poor cardiac output → heart failure, hemodynamic instability, or cardiovascular collapse

Clinical Presentation

  • 1st-degree and Mobitz type 1 2nd-degree AV block are usually asymptomatic. 
  • Mobitz type 2 2nd-degree AV block and 3rd-degree AV block may present with:
    • Chest pain
    • Bradycardia
    • Nausea 
    • Presyncope or syncope (Stokes-Adams syndrome)
    • Hypotension 
    • Pallor
    • Blurry vision
    • Confusion
    • Fatigue
    • Dyspnea
    • Dizziness
    • Sudden cardiac arrest
    • Patients with 3rd-degree block may have cannon A-waves (atrial-wave): 
      • Waves seen occasionally in the jugular veins
      • Due to the right atrium contracting against a closed tricuspid valve

Diagnosis

The diagnosis is made by electrocardiography (ECG), and the findings depend on the type of AV block.

First-degree AV block

  • All P waves are followed by a QRS complex.
  • PR interval > 0.2 sec

Twelve-lead ECG showing 1st-degree AV block: Notice the uniformly prolonged PR intervals. There are no “dropped” QRS complexes.

Image: “12-lead ECG ” by the Department of Cardiac Surgery, Tor Vergata University of Rome, Italy. License: CC BY 2.0.

Second-degree AV block, Mobitz type 1 (Wenckebach)

  • Irregular rhythm
  • PR interval progressively lengthens.
  • Failure of the AV node to conduct the atrial impulse results in a missing QRS complex.
  • AV node conduction resumes with the next beat, and repeats the cycle.
  • Mnemonic: “Longer, longer, longer, drop! This must be a Wenckebach!”
  • 2:1 AV block is difficult to differentiate from Mobitz type 2, but the following may be clues:
    • A PR interval > 0.3 sec
    • Enhanced AV nodal conduction with atropine
    • Worsened with carotid massage by slowing AV nodal conduction

Second-degree AV block, Mobitz 1: The PR interval progressively lengthens in an irregular rhythm until a QRS complex is “dropped.” Arrows: P waves; red lines: progressively prolonging PR interval.

Image by Lecturio.

Second-degree AV block, Mobitz type 2

  • Consistent, unchanging PR intervals
  • An abrupt failure of P wave conduction results in a missing QRS complex.
  • The rhythm on either side of the drop is normal.
  • Can form a pattern (e.g., a P:QRS conduction ratio of 3:1)
  • May progress to high-grade 2nd-degree AV block, where multiple P waves in a row are blocked

Second-degree AV block, Mobitz 2: ECG shows that the impulse from the SA node is periodically “dropped,” resulting in a normal P wave followed by a drop of the QRS complex and T wave. The red arrows indicate P waves.

Image by Lecturio.

Third-degree AV block

  • No relationship between the P waves and QRS complexes
  • Variable PR interval
  • QRS, P-P, and R-R intervals are constant.
  • QRS complexes are due to junctional or ventricular escape rhythms:
    • Junctional rhythms will have a rate of 4060 beats/min.
    • Ventricular rhythms will have a rate of 2040 beats/min.
    • QRS complexes may be narrow or wide, depending on the site of occurrence.

Third-degree AV block: The atria and ventricles are out of sync and follow their own pacemakers. In this ECG, there is complete asynchronicity between P waves and QRS complexes.

Image by Lecturio.

Management

First-degree AV block

  • No treatment needed in asymptomatic patients
  • Evaluate for underlying conditions and reversible causes. 
  • Regular follow-up with ECG to monitor for progression
  • Pacemaker placement is usually not indicated; exceptions include:
    • Patients with wide QRS complexes due to a conduction delay below the AV node, which may progress to 2nd- and 3rd-degree heart block
    • Symptomatic patients with “pseudo-pacemaker syndrome” due to the loss of AV synchrony: 
      • Due to atrial contraction against a closed mitral valve or when the atrial contraction occurs right after ventricular systole
      • Leads to hemodynamic changes (systemic hypotension, elevated pulmonary artery pressure, cannon A-waves)

Second-degree AV block, Mobitz type 1 (Wenckebach)

  • Asymptomatic (most patients): same as 1st-degree AV block treatment 
  • Symptomatic:
    • Stable: 
      • Continuous monitoring with transcutaneous pacing pads in place in case of deterioration 
      • Identify and treat reversible causes.
      • If persistent and there are no reversible causes, then permanent pacemaker placement
    • Unstable: 
      • Atropine 
      • If no response, then temporary transcutaneous pacing
      • Dopamine infusion is an alternative.
      • As above, pacemaker placement is pursued if there is no reversible etiology.

Second-degree AV block, Mobitz type 2

  • Same treatment as with symptomatic Mobitz type 1 patients
  • Requires careful monitoring because 2nd-degree AV block, Mobitz type 2 can progress to 3rd-degree AV block 
  • Will often lead to pacemaker placement unless there is a reversible cause

Third-degree AV block

  • Treatment of stable and unstable patients is the same as Mobitz type 2 2nd-degree AV block.
  • Dobutamine may also be used in unstable patients presenting with heart failure.
  • Will require pacemaker placement unless there is a reversible cause

Differential Diagnosis

  • Junctional rhythm: a rhythm originating in the AV junction due to slowing of the sinoatrial (SA) node or failure of electrical impulses to reach the ventricles. Electrocardiography findings include narrow QRS complexes, bradycardia, retrograde (or no) P waves, or AV dissociation. The P wave morphology, rate, and relationship to the QRS complex will help determine if the junctional rhythm is related to the AV block. Treatment is based on the cause and the stability of the patient. 
  • Sinus node dysfunction: a spectrum of SA node abnormalities causing alterations in electrical impulse formation and conduction, resulting in abnormal atrial rates. Symptoms include light-headedness, syncope, and alternating periods of bradycardia and tachycardia. Electrocardiography findings of severe bradycardia, sinus pauses or arrests, junctional escape rhythm, or alternating bradycardia and tachycardia will differentiate sinus node dysfunction from AV block. A pacemaker is usually needed.
  • Sinus bradycardia: a sinus rhythm with a heart rate < 60 beats/min, which may result from vagal tone, medications, or underlying conditions. Symptoms can include dizziness, chest pain, and dyspnea. Electrocardiography shows a slow, regular rhythm; narrow QRS complexes; and normal P wave morphology. Normal PR intervals and a lack of “dropped” beats differentiates sinus bradycardia from AV block. Treatment depends on the patient’s stability and the underlying cause.

References

  1. Sandesara, C.M., & Olshansky, B. (2017). Atrioventricular block. In Dizon, J.M. (Ed.), Medscape. Retrieved November 20, 2020, from https://emedicine.medscape.com/article/151597-overview
  2. Sauer, W.H. (2020). Etiology of atrioventricular block. In Saperia, G.M. (Ed.), Uptodate. Retrieved November 20, 2020, from https://www.uptodate.com/contents/etiology-of-atrioventricular-block
  3. Sauer, W.H. (2020). First degree atrioventricular block. In Saperia, G.M. (Ed.), Uptodate. Retrieved November 20, 2020, from https://www.uptodate.com/contents/first-degree-atrioventricular-block
  4. Sauer, W.H. (2019). Second degree atrioventricular block: Mobitz type II. In Saperia, G.M. (Ed.), Uptodate. Retrieved November 20, 2020, from https://www.uptodate.com/contents/second-degree-atrioventricular-block-mobitz-type-ii
  5. Sauer, W.H. (2020). Second degree atrioventricular block: Mobitz type I (Wenckebach block). In Saperia, G.M. (Ed.), Uptodate. Retrieved November 20, 2020, from https://www.uptodate.com/contents/second-degree-atrioventricular-block-mobitz-type-i-wenckebach-block
  6. Sauer, W.H. (2020). Third degree (complete) atrioventricular block. In Saperia, G.M. (Ed.), Uptodate. Retrieved November 20, 2020, from https://www.uptodate.com/contents/third-degree-complete-atrioventricular-block
  7. Mitchell, L.B. (2019). Atrioventricular block. [online] MSD Manual Professional Version. https://www.msdmanuals.com/professional/cardiovascular-disorders/arrhythmias-and-conduction-disorders/atrioventricular-block

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