Second Degree Atrioventricular Block-Mobitz Type I (Wenckebach)
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
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
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
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]])
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
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
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
Electrical Correlates of the Normal EKG
PR Interval
Depolarization of Atrium (P-Wave)
Conduction Through AV Node
Conduction Through His Bundle
Conduction Through Bundle Branches
Conduction Through Fascicles
Conduction Through Terminal Purkinje Fibers
QRS
Ventricular Depolarization
T-Wave
Ventricular Repolarization
Blood Supply to Cardiac Conduction System
Blood Supply to the Sinoatrial (SA) Node
Right Coronary Artery: 60% of patients
Left Circumflex Artery: 40% of patients
Blood Supply to the Atrioventricular (AV) Node
Right Coronary Artery: 90% of patients
Left Circumflex Artery: 10% of patients
Blood Supply to the His Bundle
Right Coronary Artery: main blood supply
Septal Perforators of the Left Anterior Descending Coronary Artery: minor contribution
Main/Proximal Left Bundle Branch
Left Anterior Descending Artery: main blood supply
Right Coronary Artery: collateral flow
Left Circumflex Artery: collateral flow
Left Anterior Fascicle
Septal Perforators of the Left Anterior Descending Coronary Artery: main blood supply
AV Nodal Artery: 50% of patients
Left Posterior Fascicle
Proximal Left Posterior Fascicle
AV Nodal Artery: main blood supply
Septal Perforators of the Left Anterior Descending Coronary Artery: in some cases
Distal Left Posterior Fascicle
Anterior and Posterior Septal Perforating Arteries (Dual Blood Supply)
Right Bundle Branch
Septal Perforators of the Left Anterior Descending Artery: main blood supply
Right Coronary Artery: some collateral flow (depending on dominance of the system)
Left Circumflex Artery: some collateral flow (depending on dominance of the system)
Diagnosis
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Clinical Differentiation of Second Degree Atrioventricular Block-Mobitz Type I (Wenckebach) from Second Degree Atrioventricular Block-Mobitz Type II
General Comments: differentiation between Weckebach and Mobitz type II cannot be made in the setting of a 2:1 block (since every other beat is non-conducted and one cannot observe for possible PR prolongation that is observed in Wenkebach)
Second Degree Atrioventricular Block-Mobitz Type I (Wenckebach) (see Second Degree Atrioventricular Block-Mobitz Type I, [[Second Degree Atrioventricular Block-Mobitz Type I]]): progressive PR prolongation precedes the non-conducted P-wave
Second Degree Atrioventricular Block-Mobitz Type II: PR remains unchanged prior to the non-conducted P-wave
