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
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)
Trans-Catheter Aortic Valve Replacement (TAVR) (see 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
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
Diltiazem (Cardizem, Tiazac, Dilt-CD) (see Diltiazem)
Verapamil (Isoptin, Verelan, Verelan PM, Calan, Bosoptin, Covera-HS) (see 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
Physiology: grayanotoxin-contaminated honey made from Rhododendron Ponticum and other plant species from the Ericaceae and Sapindaceae families -> increased cardiac sodium channel permeability
Endocarditis with Valve Ring Abscess (see 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
Complete Failure of Transmission of Sino-Atrial (SA) Node Firing to the Ventricle: absence of conduction, resulting in complete dissociation of atrial and ventricular electrical activity
Site of Block: 61% of cases have block below the His bundle
AV Node (20% of cases)
Bundle of His (<20% of cases)
Bundle Branch Purkinje System
Origin of Escape Rhythm: the ventricular escape rhythm can originate anywhere from the AV node to the bundle branch Purkinje system
AV Dissociation: while all cases of complete heart block have AV dissociation, not all cases of AV dissociation are due to complete heart block
Example: AV dissociation can occur in ventricular tachycardia (VT), where ventricular rate is faster than the sinus rate
Example: AV dissociation can occur in accelerated junctional tachycardia, where ventricular rate is faster than the sinus rate
Blood Supply to the 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)
Complete AV Dissociation: no relationship between P waves and QRS complexes
Isorhythmic AV Dissociation: atrial and ventricular rates are so close to each other that the P waves appear to be normally conducting
Diagnosis of this requires close inspection of a long rhythm strip (to detect P-R interval variation) or pharmacologic acceleration of the atrial/sinus rate
Absence of Fusion Complexes
Variable QRS Duration: depends on the site of the block and the site of the escape rhythm pacemaker (pacemaker above His bundle produces a narrow-complex escape rhythm, while pacemaker at or below His bundle produces a wide-complex escape rhythm)
Block at Level of AV Node: escape is typically junctional at around 45-60 beats per min
Patient is usually hemodynamically stable
Heart rate increases in response to exercise and atropine
Block Below AV Node: escape arises from His bundle or bundle branch Purkinje system and is usually <45 beats per min
Patient is usually hemodynamically unstable
Heart rate does not increase in response to exercise and atropine
Variable, depending on site of block and escape rhythm
Treatment
General Management
External/Transvenous Pacemaker: for emergent/short-term treatment
Permanent Pacemaker: for long-term treatment
Treatment of Complete Heart Block Associated with Swan-Ganz Catheter Interference with Right Bundle Branch Conduction in Setting of Pre-Existing Left Bundle Branch Block
Usually resolves with removal of catheter
Treatment of Complete Heart Block After Cardiac Surgery
Time course for recovery is variable
Many patients recover within 48 hrs of surgery
If no recovery occurs by post-op day 4-5, a permanent pacemaker should be implanted
Treatment of Complete Heart Block Associated with Ischemia/Infarction
Anterior Wall Myocardial Infarction with His-Purkinje (infranodal) Block: occlusion of the left anterior descending coronary artery (particularly proximal to the first septal perforator) usually requires permanent pacemaker implantation
Inferior Wall Myocardial Infarction with AV Nodal Block: often resolves within hrs-days (particularly with early coronary revascularization)
