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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
Key points
- First-degree AV block: prolonged PR >200 ms, all P waves conducted — benign, no treatment
- Second-degree Mobitz I (Wenckebach): progressive PR prolongation then dropped QRS — usually benign, block at AV node level
- Second-degree Mobitz II: constant PR with sudden dropped QRS — dangerous, block below AV node (His-Purkinje), high risk of progression to complete heart block. Pacemaker indicated
- Third-degree (complete): no relationship between P waves and QRS, AV dissociation — pacemaker required
- Symptomatic bradycardia: atropine 0.5-1 mg IV first-line. If refractory: transcutaneous pacing, dopamine, or epinephrine infusion. Permanent pacemaker for irreversible causes
Overview
Bradycardia (HR <60 bpm) can be physiologic (trained athletes, sleep) or pathologic. AV conduction disorders are classified by the degree of block. First-degree is a conduction delay (PR >200 ms). Second-degree involves intermittent failure of conduction: Mobitz I (Wenckebach) with progressive PR prolongation is usually at the AV node level and generally benign; Mobitz II with constant PR and sudden dropped beats is at the infranodal level and carries high risk of progression to complete block. Third-degree (complete) heart block has complete AV dissociation. Causes include age-related fibrosis, ischemia (especially inferior MI for nodal block, anterior MI for infranodal), medications (beta-blockers, CCBs, digoxin, amiodarone), hyperkalemia, hypothyroidism, and infiltrative disease (sarcoid, amyloid, Lyme).
Clinical Features
Symptoms
Asymptomatic in many cases (first-degree, Mobitz I in athletes)
Lightheadedness, presyncope, or syncope (Stokes-Adams attacks)
Fatigue, exercise intolerance, dyspnea
Sudden cardiac death (complete heart block with no escape rhythm)
Signs
Bradycardia (regular or irregular depending on type)
Cannon A waves in JVP (complete heart block — atria contract against closed TV)
Variable S1 intensity (complete heart block — changing PR interval)
Hypotension if hemodynamically significant
Investigations
First-line
12-lead ECGFirst-degree: PR >200 ms, all P conducted. Mobitz I: progressive PR prolongation, grouped beating, dropped QRS. Mobitz II: constant PR, sudden dropped QRS, often with wide QRS escape (infranodal). Third-degree: P waves and QRS independent, regular P-P and R-R but no relationship. Narrow escape = junctional (40-60 bpm). Wide escape = ventricular (20-40 bpm, unreliable)
LabsBMP (K+, Mg2+, Ca2+), TSH, digoxin level if applicable, Lyme serologies if endemic area
Second-line
Continuous telemetryMonitor for progression (Mobitz II to complete block)
Ambulatory monitoringHolter or event monitor if intermittent symptoms
EchocardiogramAssess structural heart disease, infiltrative disease
Specialist
EP studyRarely needed — may assess infranodal conduction (HV interval) if diagnosis uncertain
1
Acute symptomatic bradycardia
- Atropine 0.5-1 mg IV, may repeat q3-5 min (max 3 mg). Effective for sinus bradycardia and AV nodal block (Mobitz I). Often INEFFECTIVE for Mobitz II and complete heart block (infranodal block)
- If atropine fails: transcutaneous pacing (bridge to transvenous), dopamine 5-20 mcg/kg/min IV, or epinephrine 2-10 mcg/min IV
- Transvenous temporary pacing for hemodynamically significant Mobitz II or complete heart block
- Discontinue offending medications (beta-blockers, CCBs, digoxin, amiodarone)
- Correct reversible causes: hyperkalemia (calcium, insulin/glucose, kayexalate), hypothyroidism (levothyroxine), Lyme (antibiotics)
2
Permanent pacemaker indications
- Third-degree heart block (symptomatic or asymptomatic with wide escape or ventricular pauses >3 sec)
- Mobitz II second-degree AV block (high progression risk)
- Symptomatic sinus node dysfunction (sick sinus syndrome) with documented bradycardia-symptom correlation
- Alternating bundle branch block (LBBB alternating with RBBB)
- Post-cardiac surgery or post-TAVR complete heart block that does not resolve
- Sinus node dysfunction: AAI or DDD pacemaker. AV block: DDD pacemaker. Chronic AF + slow rate: VVI pacemaker
3
Special situations
- Inferior MI: AV block is usually at nodal level (Mobitz I or transient complete block), often resolves spontaneously in 5-7 days. Atropine effective. Temporary pacing if hemodynamically significant
- Anterior MI: AV block is infranodal (Mobitz II or complete with wide escape) — implies massive septal necrosis, high mortality, temporary pacing urgently needed, often requires permanent pacemaker
USMLE Step 2 CK Exam Tips
- 1Mobitz I (Wenckebach) = progressive PR prolongation then drop = benign, no pacemaker needed unless symptomatic
- 2Mobitz II = constant PR then sudden drop = dangerous, needs pacemaker. Often has wide QRS (infranodal)
- 3Third-degree = complete AV dissociation, regular P-P and R-R but independent. Always needs pacemaker
- 4Atropine works for AV nodal block (sinus brady, Mobitz I) but NOT for infranodal block (Mobitz II, complete with wide escape)
- 5Inferior MI → AV nodal block (benign, transient). Anterior MI → infranodal block (serious, high mortality)
- 6Lyme disease is a classic reversible cause of heart block — serologies in any young patient with unexplained AV block in an endemic area
- 7Digoxin toxicity can cause almost any bradyarrhythmia + hyperkalemia. Treatment: digoxin-specific Fab antibody fragments (Digibind)
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