Cardiology

Background

Atrioventricular (AV) block refers to impaired conduction of electrical impulses from the atria to the ventricles through the AV node and His-Purkinje system. The severity of the block can range from slowed conduction (first-degree) to intermittent failure (second-degree) or complete dissociation between atrial and ventricular activity (third-degree or complete heart block). Clinical consequences vary from asymptomatic bradycardia to syncope and sudden cardiac arrest. 

II) Classification/Types

By Degree of Block: 

• First-degree AV block: Prolonged PR interval (>200 ms) without dropped beats 

• Second-degree AV block: 

• Mobitz Type I (Wenckebach): Progressive PR prolongation with eventual dropped QRS 

• Mobitz Type II: Fixed PR interval with intermittent non-conducted P waves 

• Third-degree (complete) AV block: No atrioventricular conduction; atria and ventricles beat independently 

By Location: 

• Supra-Hisian (nodal): Typically benign (e.g., Mobitz I) 

• Infra-Hisian (infranodal): Typically pathological (e.g., Mobitz II, complete block) 

Pathophysiology 

AV block results from disruption in impulse conduction due to abnormalities at the AV node or distal conduction system. In first-degree and Mobitz I blocks, conduction delay is usually at the AV node. Mobitz II and complete blocks often reflect disease in the His-Purkinje system and carry a higher risk for progression and adverse events. Ischemia, fibrosis, medications, or infiltrative processes may impair conduction. 

Epidemiology 

• Common in older adults due to age-related fibrosis 

• First-degree AV block is frequently seen in healthy individuals 

• Mobitz I often seen in athletes and during sleep 

• Mobitz II and third-degree block are less common but more clinically significant 

• One of the leading indications for permanent pacemaker implantation 

Etiology 

I) Causes

• Degenerative conduction disease (Lev’s or Lenègre’s disease) 

• Myocardial infarction, especially inferior or anterior 

• Medications: beta-blockers, digoxin, calcium channel blockers, amiodarone 

• Electrolyte abnormalities: hyperkalemia 

• Infectious/inflammatory: Lyme disease, endocarditis, myocarditis 

• Infiltrative diseases: sarcoidosis, amyloidosis, hemochromatosis 

• Congenital AV block (maternal lupus antibodies) 

• Post-cardiac surgery or catheter ablation 

II) Risk Factors

• Advanced age 

• Coronary artery disease 

• Structural heart disease 

• History of cardiac surgery or catheter ablation 

• Chronic use of AV nodal blockers 

• Autoimmune or infiltrative disease history 

Clinical Presentation 

I) History (Symptoms)

• Often asymptomatic in first-degree or Mobitz I 

• Fatigue, lightheadedness 

• Syncope or near-syncope (especially in Mobitz II or complete block) 

• Dyspnea or worsening heart failure 

• Palpitations (irregular or slow heart rate) 

II) Physical Exam (Signs)

• Bradycardia 

• Irregular or regular rhythm depending on block type 

• Variable intensity of S1 heart sound 

• Cannon A waves in jugular venous pulse (in complete heart block) 

• Signs of hypoperfusion (cool extremities, hypotension) in advanced blocks 

Differential Diagnosis (DDx) 

• Sinus bradycardia 

• Atrial fibrillation with slow ventricular response 

• Sick sinus syndrome 

• Carotid sinus hypersensitivity 

• Vasovagal syncope 

• Electrolyte disorders (e.g., hyperkalemia) 

• Medication-induced bradyarrhythmia 

Diagnostic Tests 

Initial Work-Up 

• 12-lead ECG: Diagnostic tool to identify type of AV block 

• Holter monitor or event recorder: For intermittent blocks or syncope 

• Electrolytes, TSH, cardiac biomarkers: Rule out reversible causes 

• Drug review: Evaluate for AV nodal blocking agents 

• Echocardiography: Assess structural heart disease 

Advanced Testing 

• Electrophysiology study (EPS): For unclear cases or to localize site of block 

• Cardiac MRI or PET scan: If infiltrative or inflammatory disease suspected 

Treatment 

I) Acute Management

• Asymptomatic or stable first-degree/Mobitz I: Observation 

• Symptomatic or unstable patients: 

• Atropine 0.5 mg IV every 3–5 min (max 3 mg) 

• Dopamine or epinephrine infusion if unresponsive 

• Temporary pacing (transcutaneous or transvenous) for Mobitz II or complete block 

II) Chronic Management

• Remove or adjust AV nodal blockers 

• Treat reversible causes (e.g., Lyme disease, hypothyroidism) 

• Permanent pacemaker for: 

• Symptomatic Mobitz I or II 

• Asymptomatic Mobitz II or complete block 

• Block following anterior MI or with wide QRS 

Medications 

Device Therapy 

• Permanent Pacemaker (PPM): Mainstay for Mobitz II and complete AV block 

• Dual-chamber pacing: Preferred to maintain AV synchrony 

• Temporary pacing: In acute unstable high-grade blocks 

• ICD: Only if concurrent ventricular arrhythmia or reduced EF (e.g., post-MI with EF <35%) 

Patient Education, Screening, Vaccines 

Education 

• Recognize symptoms of bradycardia (fatigue, syncope) 

• Understand the function and follow-up for pacemaker 

• Avoid drugs that impair AV conduction 

• Maintain regular follow-up 

Screening/Prevention 

• Periodic ECG in elderly or those on AV nodal blockers 

• Medication reconciliation to avoid bradyarrhythmics 

• Monitor electrolytes and thyroid function in susceptible patients 

Vaccinations 

• No AV block-specific vaccines 

• Routine immunizations as per age and comorbidities 

Consults/Referrals 

• Cardiology: For pacemaker evaluation 

• Electrophysiology: For diagnostic EPS or complex pacing needs 

• Infectious disease: If Lyme or myocarditis suspected 

• Endocrinology: For metabolic or hormonal causes 

Follow-Up 

Short-Term 

• Monitor symptom resolution 

• Reassess ECG and labs post-intervention 

• Temporary pacemaker removal after stabilization 

Long-Term 

• Regular device checks every 3–6 months 

• Monitor for lead/device complications 

• Reassess structural disease or comorbid progression 

Prognosis 

• First-degree and Mobitz I: Generally benign 

• Mobitz II and complete block: High risk of progression and sudden death if untreated 

• Excellent with pacemaker placement for symptomatic or high-grade AV blocks 

• Prognosis influenced by underlying etiology (e.g., MI, infiltrative disease)