First Degree Atrioventricular Block (First Degree Heart Block)

Definition of Atrioventricular Block

  • Atrioventricular Block: delayed/intermittent/completely absent transmission of the impulse from atria to the ventricles (either transiently or permanently) due to anatomical/functional impairment of the cardiac conduction system

Classification of Atrioventricular Blocks


Etiology of First Degree Atrioventricular Block

Normal Variant

  • First Degree Atrioventricular Block is Observed in a Small Percentage of Patients Without Apparent Heart Disease
    • Study of Healthy Aviators [MEDLINE]: PR intervals as long as 280 msec have been reported

Increased Vagal Tone

  • Athletic Training
  • Carotid Sinus Massage
  • Hypersensitive Carotid Sinus Syndrome
  • Pain
  • Sleep

Familial Atrioventricular Block

  • Cardiac Sodium Channnel (SCN5A) Gene Mutation: autosomal dominant
  • Cardiac Transcription Factor (CSX/NKX2-5) Gene Mutation

Hereditary Neuromuscular Degenerative Disease

  • Becker Muscular Dystrophy
  • Erb’s Dystrophy
  • Kearns-Sayre Syndrome
  • Myotonic Dystrophy (see Myotonic Dystrophy, [[Myotonic Dystrophy]])

Degenerative Conduction System Disease

  • Lenegre’s Disease
    • Epidemiology: occurs in younger patients (may be hereditary)
    • Physiology: progressive, fibrotic, sclerodegenerative disease of the conduction system
    • Clinical: frequently associated with slow progression to complete heart block
  • Lev’s Disease
    • Epidemiology: occurs in older patients
    • Physiology: fibrosis/calcification extending from any of the left-sided fibrous structures adjacent to the conduction system into the conduction system itself
      • Fibrosis of the Top of the Muscular Septum: commonly causes right bundle branch block with left anterior fascicular block
      • Calcification of the Mitral Valve Ring or the Central Fibrous Body: may be the most common cause of complete heart block with a narrow QRS complex
      • Aortic Valve Calcification: may invade the bundle of His, right bundle branch, left bundle branch, and/or left anterior fascicle -> QRS complex may be prolonged

Infiltrative Disease

Metabolic Abnormality

  • Hypercalcemia (see Hypercalcemia, [[Hypercalcemia]]): may occur with severe hypercalcemia
  • Hyperkalemia (see Hyperkalemia, [[Hyperkalemia]]): usually with plasma potassium >6.3 meq/L
  • Hypokalemia (see Hypokalemia, [[Hypokalemia]])
  • Hypoxia (see Hypoxemia, [[Hypoxemia]])

Ischemic Heart Disease

  • Acute Myocardial Ischemia/Infarction (see Coronary Artery Disease, [[Coronary Artery Disease]])
    • Epidemiology
      • Incidence of High-Degree Atrioventricular Block (data from 4 large randomized thrombolytic trials: GUSTO-I, GUSTO-III, ASSENT-II, and GUSTO-IIb) [MEDLINE]: 6.9% (inferior wall MI: 9.8%; anterior wall MI: 3.2%)
      • Incidence of Complete Heart Block (data from TRACE trial) [MEDLINE]: 5.1% (and 81% occurred within the first 2 days after acute MI)
      • Incidence of Bundle Branch Block (data from TRACE trial) [MEDLINE]: 8%
  • Chronic Ischemic Heart Disease

Myocarditis (see Myocarditis, [[Myocarditis]])

Rheumatologic Disease

Thyroid Disease

  • Hyperthyroidism (Severe)/Thyrotoxic Periodic Paralysis (see Hyperthyroidism, [[Hyperthyroidism]])
  • Hypothyroidism (Severe) (see Hypothyroidism, [[Hypothyroidism]])

Iatrogenic

  • Alcohol (Ethanol) Septal Ablation for Hypertrophic Cardiomyopathy (see Hypertrophic Cardiomyopathy, [[Hypertrophic Cardiomyopathy]])
    • Epidemiology: complete heart block occurs in 14-22% of cases
    • Physiology: ethanol infusion into the first septal perforating branch of the left anterior descending (LAD) coronary artery -> infarction/thinning of the proximal interventricular septum
  • Cardiac Surgery
    • Epidemiology: complete heart block occurs in 1-5.7% of cases
    • Risk Factors for Post-Cardiac Surgery Complete Heart Block
      • Aortic Valve Annular Calcification
      • Aortic Valve Surgery
      • Bicuspid Aortic Valve
      • Female Gender
      • Pre-Existing Conduction System Disease (RBBB or LBBB)
    • Procedures
      • Aortic Valve Replacement of a Calcified Aortic Valve
      • Closure of Ventricular Septal Defect (see Ventricular Septal Defect, [[Ventricular Septal Defect]])
      • Mitral Valve Replacement of a Calcified Mitral Valve
  • Catheter Ablation for Arrhythymia: following ablation for reentrant arrhythmias when the reentrant pathway lies within or near the AV node
  • Left Anterior Descending (LAD) Coronary Artery Stenting
    • Epidemiology: rare
    • Physiology: due to stent-related occlusion of septal perforator artery -> septal infarction
  • Swan-Ganz Catheter Interference with Right Bundle Branch Conduction in Setting of Pre-Existing Left Bundle Branch Block (LBBB) (see Swan-Ganz Catheter, [[Swan-Ganz Catheter]])
  • Trans-Catheter Aortic Valve Replacement (TAVR) (see Aortic Stenosis, [[Aortic Stenosis]])
    • Epidemiology: approximately 33% of patients require a permanent pacemaker within 30 days of TAVR
      • There may be a higher rate of atrioventricular block with self-expanding implanted aortic valves, as compared to balloon expandable versions
    • Predictors of Post-TAVR Atrioventricular Block
      • Pre-existing cardiac conduction disturbance
      • Narrow left ventricular outflow tract
      • Increased severity of mitral annular calcification appear to be predictors of this complication
  • Trans-Catheter Closure of Ventricular Septal Defect (VSD) (see Ventricular Septal Defect, [[Ventricular Septal Defect]])
    • Amplatzer Ventricular Septal Defect Occluder: likely due to the right ventricular retention disk overlapping the ventricular conduction system as it passes above or anterosuperiorly to the ventricular septal defect

Drugs/Toxins

  • Adenosine (Adenocard) (see Adenosine, [[Adenosine]])
  • Amiodarone (Cordarone) (see Amiodarone, [[Amiodarone]])
  • Beta Blockers (se eβ-Adrenergic Receptor Antagonists, [[β-Adrenergic Receptor Antagonists]])
    • Agents
      • Labetalol (Normodyne, Trandate) (see Labetalol, [[Labetalol]])
      • Metoprolol (Lopressor, Toprol) (see Metoprolol, [[Metoprolol]])
      • Propanolol (see Propanolol, [[Propanolol]])
    • Physiology: antagonism of catecholamine-induced increase in heart rate
  • Calcium Channel Blockers (see Calcium Channel Blockers, [[Calcium Channel Blockers]])
    • Agents
      • Diltiazem (Cardizem, Tiazac, Dilt-CD) (see Diltiazem, [[Diltiazem]])
      • Verapamil (Isoptin, Verelan, Verelan PM, Calan, Bosoptin, Covera-HS) (see Verapamil, [[Verapamil]]): probably the most common calcium channel blocker associated with atrioventricular blocks
    • Physiology: atrioventricular nodal blockade, since calcium channels are especially concentrated in the sinoatrial and atrioventricular nodes within the heart
  • Digitalis Intoxication (see Digitalis, [[Digitalis]])
    • Physiology: digitalis is a cardiac glycoside -> inhibits myocardial Na+/K+ ATPase and increases vagal activity
  • Digoxin (Lanoxin) (see Digoxin, [[Digoxin]])
    • Physiology: digoxin is a cardiac glycoside -> inhibits myocardial Na+/K+ ATPase and increases vagal activity
  • Disopyramide (Norpace) (see Disopyramide, [[Disopyramide]])
    • Physiology: modulates the sodium channel
    • Clinical: may produce block in the more distal His-Purkinje system
  • Mad Honey Intoxication (see Mad Honey, [[Mad Honey]])
    • Physiology: grayanotoxin-contaminated honey made from Rhododendron Ponticum and other plant species from the Ericaceae and Sapindaceae families -> increased cardiac sodium channel permeability
  • Nerium Oleander Intoxication (see Nerium Oleander, [[Nerium Oleander]])
    • Physiology: contains oleandrin and other less well-studied cardiac glycosides
  • Procainamide (Pronestyl) (see Procainamide, [[Procainamide]])
    • Physiology: modulates the sodium channel
    • Clinical: may produce block in the more distal His-Purkinje system
  • Quinidine (Quinaglute, Quinidex) (see Quinidine, [[Quinidine]])
    • Physiology: modulates the sodium channel
    • Clinical: may produce block in the more distal His-Purkinje system

Other

  • Cardiac Cysts
  • Cardiac Trauma
  • Cardiac Tumors
  • Cardiomyopathy (see Congestive Heart Failure, [[Congestive Heart Failure]])
  • Congenital Heart Disease (see Congenital Heart Disease, [[Congenital Heart Disease]])
  • Endocarditis with Valve Ring Abscess (see Endocarditis, [[Endocarditis]])
  • Mitochondrial Myopathy
  • Myocardial Bridging
  • Nail-Patella Syndrome
  • Neonatal Lupus Syndrome
    • Physiology: trans-placental passage of anti-Ro/SSA or anti-La/SSB antibodies from mother
  • Phase IV Block (Bradycardia-Related Block)

Physiology

First Degree Atrioventricular Block with Normal QRS Duration

  • Conduction Delay in Atrium, AV Node, Bundle of His, or Infra-Hisian Specialized Conduction System: conduction delay in more than one of these sites has been reported to occur in 20-80% of cases
    • Atrial Site of Conduction Delay: site of conduction delay in 3% of first degree block cases
      • Ebstein Anomaly (see Ebstein Anomaly, [[Ebstein Anomaly]])
      • Endocardial Cushion Defects
    • AV Nodal Site of Conduction Delay: most common site of conduction delay in first degree block
      • Etiologies of First Degree Block with Increased Atrial-His Time (Determined by His Bundle Electrocardiography)
    • Bundle of His Site of Conduction Delay: slowed conduction in this site can result in split His potentials, but the PR interval is rarely prolonged
      • Disopyramide (Norpace) (see Disopyramide, [[Disopyramide]])
      • Procainamide (Pronestyl) (see Procainamide, [[Procainamide]])
      • Quinidine (Quinaglute, Quinidex) (see Quinidine, [[Quinidine]])
    • Infra-Hisian Specialized Conduction System Site of Conduction Delay (Bundle Branches, Fascicles, Terminal Purkinje Fibers): conduction is equally slowed in both the right and left conducting systems in these cases (as the PR interval is normal if one bundle is intact)
      • Disopyramide (Norpace) (see Disopyramide, [[Disopyramide]])
      • Procainamide (Pronestyl) (see Procainamide, [[Procainamide]])
      • Quinidine (Quinaglute, Quinidex) (see Quinidine, [[Quinidine]])

First Degree Atrioventricular Block with Wide QRS Duration

  • Conduction Delay in AV Node, Bundle of His, or Infra-Hisian Specialized Conduction System: 66% of cases have conduction delay in at least 2 sites (Example: as RBBB usually has a normal PR interval, when the PR interval is prolonged, there must also be a second site of delay in the AV node, His bundle, or left bundle branch)
    • AV Nodal Site of Conduction Delay: occasional site of conduction delay in these cases
    • Bundle of His Site of Conduction Delay: occasional site of conduction delay in these cases
    • Infra-Hisian Specialized Conduction System Site of Conduction Delay (Bundle Branches, Fascicles, Terminal Purkinje Fibers): most common site of conduction delay in these cases

Diagnosis

Electrocardiogram (EKG) (see Electrocardiogram, [[Electrocardiogram]])

  • Definition of First Degree Atrioventricular Block: PR interval prolonged >0.2 sec (200 msec)
    • At Slow Heart Rates: PR interval prolonged >0.21 sec (210 msec)
    • PR Interval >300 msec (Especially with Normal QRS Duration): suggests an AV nodal site of the conduction delay

PR Interval

  • Normal PR Interval (P-Wave + PR Segment): 0.12-0.20 sec = 120-200 msec = 3-5 small boxes
    • PR Interval Shortens with Increased Heart Rate: due (in part) to rate-related shortening of the cardiac action potential

Atropine (see Atropine, [[Atropine]])

  • Dose: 1 mg IV
  • Variable Effect in Patient with Atrioventricular Block: since vagal tone slows AV nodal conduction but has little effect on the infranodal conduction system
    • AV Nodal Site of Conduction Delay: atropine enhances AV nodal conduction and shorten the PR interval
    • Infranodal Site of Conduction Delay: atropine will accelerate the SA node firing (and heart rate), encroaching on the refractory period and exacerbating the conduction delay

Exercise

  • Increased Adrenergic Tone During Exercise Increases the Sinus Rate and Accelerates AV Nodal Conduction: therefore, results in similar effects to atropine testing

Vagal Maneuvers

  • Vagal Maneuvers Generally Act to Slow AV Nodal Conduction: however, this effect may be obscured since the sinus rate is also slowed (which allows more time for the AV node and the infranodal conduction system to recover from the refractory period and to conduct more normally)

Electrophysiologic (EP) Study

  • His Bundle Electrocardiography: most accurate means of determining the site of conduction delay

Clinical Manifestations

Cardiovascular Manifestations

  • Isolated First Degree Atrioventricular Block with No Apparent Heart Disease
    • Clinical: asymptomatic
  • First Degree Atrioventricular Block with “Pacemaker Syndrome”
    • Physiology: first degree atrioventricular block associated with loss of atrioventricular synchrony
      • Contraction Against a Closed Mitral Valve
      • Atrial Contraction Occurring Shortly after Ventricular Systole with Associated Incomplete Atrial Filling
    • Clinical Features

Treatment

Evaluation of New Onset First Degree Atrioventricular Block

  • Evaluate for Other Associated Cardiac Disease
    • Muscular Dystrophy-Related Cardiomyopathy
    • Infiltrative Cardiomyopathy
    • Other Dilated Cardiomyopathies
  • Eliminate Other Etiologies of AV Nodal Block
    • AV Nodal Blocking Medications
    • Myocardial Ischemia

Determination of Site of Conduction Disturbance

  • First Degree Atrioventricular Block with PR Interval >300 msec (Especially with Normal QRS Duration)
    • Suggests an AV Nodal Site of Conduction Delay: no further evaluation is required
  • First Degree Atrioventricular Block with Wide QRS
    • Suggests Significant Possibility That the Site of Conduction Delay is Below the AV Node: due to the unpredictable progression to second/third degree atrioventricular block, His bundle electrocardiography should be considered
      • HV Interval Prolongation >100 msec: permanent pacemaker placement is indicated

Indications for Permanent Pacemaker in First Degree Atrioventricular Block

  • First Degree Atrioventricular Block with Wide QRS and His Bundle Electrocardiography Demonstrating HV Interval Prolongation >100 msec: probable indication for permanent pacemaker placement (although this is controversial)
  • First Degree Atrioventricular Block with “Pacemaker Syndrome”
  • First Degree Atrioventricular Block with Concurrent Neuromuscular Disease: due to the unpredictable progression of atrioventricular conduction disturbances
    • Erb’s (Limb-Girdle) Dystrophy
    • Kearns-Sayre Syndrome
    • Myotonic Dystrophy (see Myotonic Dystrophy, [[Myotonic Dystrophy]])
    • Peroneal Muscular Atrophy

References

  • Analysis of the electrocardiograms obtained from 1000 young healthy aviators; ten year follow-up. Circulation. 1954 Sep;10(3):384-400 [MEDLINE]