Mobitz type II second-degree AV block is a high-risk infranodal conduction disturbance characterized by intermittent failure of AV conduction without preceding PR prolongation, carrying significant risk of sudden progression to complete heart block and requiring permanent pacemaker implantation regardless of symptoms per ACC/AHA/HRS guidelines. [1]
The following figure illustrates the ECG patterns distinguishing Mobitz Type I (Wenckebach) from Mobitz Type II second-degree AV block:
1. History
- Syncope or presyncope — the hallmark symptom; Mobitz originally noted progression to Stokes-Adams attacks [3]
- Lightheadedness, dizziness, fatigue, exercise intolerance
- Dyspnea on exertion or at rest (if low cardiac output)
- Palpitations — awareness of "skipped beats" or pauses
- Timing: episodic vs. persistent; exertional worsening suggests infranodal disease [4]
- Ask about recent medication changes, tick exposure (Lyme-endemic areas), recent cardiac surgery or catheterization [1]
- Prior history of MI (especially anterior), structural heart disease, or known bundle branch block
2. Alarm Features
- Syncope or near-syncope — suggests hemodynamically significant pauses or impending complete heart block
- Wide QRS complex (>120 ms) — indicates diffuse His-Purkinje disease with high risk of sudden progression [4]
- Hemodynamic instability: hypotension, altered mental status, signs of cardiogenic shock
- New heart failure symptoms (pulmonary edema, peripheral edema)
- Chest pain — consider acute MI (especially anterior) as the etiology
- Progression to high-grade or complete AV block on telemetry [5]
3. Medications
Offending agents to review:
- Beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin — primarily slow AV nodal conduction, but when second-degree block occurs on these agents, it often reveals severe underlying infranodal disease rather than being truly drug-induced [6]
- Class Ic (flecainide, propafenone) and Class III antiarrhythmics (amiodarone, sotalol) — can worsen infranodal conduction [7-8]
- Fingolimod — sphingosine-1-phosphate receptor modulator causing bradycardia [7]
Key points:
- Therapeutic doses of AV-nodal blocking agents are rarely the sole cause of Mobitz II; ~50% of patients have recurrence after drug discontinuation [6-7]
- Per ACC/AHA/HRS guidelines, in patients on chronic stable doses of medically necessary beta-blockers or antiarrhythmics, it is reasonable to proceed to permanent pacing without waiting for drug washout (Class IIa) [1]
Contraindicated in Mobitz II (without a pacemaker):
- AV-nodal blocking agents (beta-blockers, non-DHP CCBs, digoxin)
- Atropine — generally ineffective and may paradoxically worsen infranodal block by increasing the sinus rate and overwhelming the diseased His-Purkinje system [1][8]
4. Diet
- No specific dietary triggers or recommendations unique to Mobitz II
- Correct electrolyte abnormalities: hyperkalemia and hypomagnesemia can exacerbate conduction disease
- Adequate hydration to maintain preload, especially in the setting of bradycardia-related low cardiac output
- Avoid excessive caffeine or alcohol if contributing to arrhythmia burden
5. Review of Systems
- Cardiovascular: chest pain, dyspnea, orthopnea, PND, lower extremity edema, palpitations
- Neurologic: syncope, presyncope, dizziness, confusion (low cardiac output)
- Constitutional: fatigue, exercise intolerance, malaise
- Musculoskeletal: muscle weakness (screen for neuromuscular diseases — myotonic dystrophy, Kearns-Sayre syndrome) [1]
- Dermatologic: erythema migrans rash (Lyme disease) [1]
- Respiratory: cough, granulomatous disease symptoms (sarcoidosis) [1]
6. Collateral History and Family History
- Family history of sudden cardiac death, pacemaker implantation, or conduction disease
- Hereditary neuromuscular diseases: myotonic dystrophy type 1, Emery-Dreifuss muscular dystrophy, limb-girdle muscular dystrophy, Kearns-Sayre syndrome — all associated with progressive conduction system disease [1]
- Lamin A/C gene mutations — associated with progressive AV block and risk of sudden death [1]
- Social history: outdoor activities in tick-endemic areas (Lyme), IV drug use (endocarditis)
7. Risk Factors
- Advanced age — degenerative fibrosis of the conduction system is the most common cause [1]
- Chronic hypertension and diabetes mellitus [1][9]
- Coronary artery disease — especially anterior MI (LAD territory supplies the His bundle and bundle branches) [1][10]
- Pre-existing bundle branch block (RBBB or LBBB) — strongest ECG predictor of progression [9]
- Prior cardiac surgery or transcatheter aortic valve replacement (TAVR) [8]
- Infiltrative diseases: cardiac sarcoidosis, amyloidosis [1]
- Autoimmune/inflammatory: SLE, rheumatoid arthritis [8]
- Neuromuscular diseases [1]
8. Differential Diagnosis
- Mobitz Type I (Wenckebach) — progressive PR prolongation before dropped beat; generally benign and AV nodal in origin [1][5]
- 2:1 AV block — cannot be classified as Mobitz I or II from a single strip; requires additional maneuvers or longer monitoring to determine level of block [3][8]
- High-grade (advanced) AV block — ≥2 consecutive non-conducted P waves; treated similarly to Mobitz II [8]
- Complete (third-degree) heart block — complete AV dissociation with escape rhythm
- Pseudo-Mobitz II — vagally mediated block with sinus slowing before the dropped beat (sinus rate is NOT constant, distinguishing it from true Mobitz II) [3]
- Blocked premature atrial complexes (PACs) — P waves hidden in T waves mimicking dropped beats
9. Past Medical History
- Prior MI (especially anterior wall)
- Known structural heart disease, cardiomyopathy, or valvular disease
- Prior cardiac surgery or catheter ablation
- Known bundle branch block or bifascicular block
- Neuromuscular disease
- Sarcoidosis, amyloidosis, or other infiltrative disease
- History of Lyme disease
- Prior episodes of syncope or documented bradycardia
10. Physical Exam
Vital signs:
- Bradycardia — often with regular rhythm interrupted by pauses
- Hypotension if hemodynamically significant
Focused exam:
- Irregular pulse with intermittent pauses (dropped beats)
- Cannon A waves in JVP — when atria contract against a closed tricuspid valve during a blocked beat
- Variable S1 intensity (though less variable than in complete heart block since conducted beats have fixed PR)
- Signs of heart failure: elevated JVP, pulmonary crackles, peripheral edema
- Neurologic exam: assess for neuromuscular disease (myotonia, ptosis, muscle wasting)
- Skin: erythema migrans (Lyme), lupus rash
11. Lab Studies
- BMP/CMP — potassium (hyperkalemia), calcium, magnesium, renal function
- Troponin — rule out acute MI as the cause
- TSH — hypothyroidism can contribute to conduction disease [1]
- Digoxin level — if on digoxin [1][8]
- Lyme serologies — in endemic areas or with suggestive history [1]
- ACE level, inflammatory markers — if sarcoidosis suspected
- CK, aldolase — if neuromuscular disease suspected
12. Imaging
- Echocardiogram — assess LV function, structural heart disease, valvular disease, infiltrative cardiomyopathy; essential before pacemaker implantation to determine if CRT-D vs. standard pacemaker is needed [1]
- Chest X-ray — cardiomegaly, pulmonary edema, hilar lymphadenopathy (sarcoidosis)
- Cardiac MRI — if infiltrative cardiomyopathy (sarcoidosis, amyloidosis) is suspected; late gadolinium enhancement can identify myocardial fibrosis
- Coronary angiography or CT angiography — if ischemic etiology suspected
13. Special Tests
- Electrophysiology study (EPS) — may be considered to determine the level of block (intra- vs. infra-Hisian) when the ECG is ambiguous; HV interval ≥70 ms indicates significant infranodal disease [1][8]
- Exercise stress testing — infranodal block worsens with exercise (increased sinus rate overwhelms diseased His-Purkinje system), while AV nodal block improves [1]
- Ambulatory ECG monitoring (Holter, event monitor) — to capture intermittent block and correlate with symptoms [1]
- Carotid sinus massage / pharmacologic challenge (atropine, isoproterenol) — may help localize the level of block [1]
14. ECG
Diagnostic criteria for Mobitz Type II: [3-4]
- Fixed, constant PR intervals in conducted beats — both before and after the dropped beat
- Intermittent non-conducted P waves (dropped QRS) without preceding PR prolongation
- Constant PP intervals (stable sinus rate) — sinus slowing before the block suggests vagal mechanism, NOT true Mobitz II [3]
- Pause duration = exactly 2 PP intervals
- Usually associated with a wide QRS (bundle branch block pattern), indicating infranodal disease [4]
- Narrow QRS Mobitz II is uncommon and may represent intra-Hisian block [8]
Key ECG pitfalls:
- 2:1 AV block cannot be classified as Mobitz I or II — look for longer rhythm strips showing variable conduction ratios [3][8]
- Pseudo-Mobitz II: look for sinus slowing before the dropped beat (vagal mechanism) [3]
The following figure illustrates the electrophysiologic distinction between infra-Hisian (Mobitz II) and AV nodal block:
15. Assessment
Mobitz type II is a high-risk conduction abnormality reflecting structural disease of the His-Purkinje system. [4-5] Key assessment points:
- Prognosis without treatment is poor — progression to complete heart block is common and sudden, associated with syncope, heart failure, and sudden death [1][4]
- A wide QRS indicates more diffuse conduction system disease and higher risk [4]
- Unlike Mobitz I (Wenckebach), which is generally benign and AV nodal, Mobitz II is almost always infranodal and warrants pacing regardless of symptoms [1]
- Atypical presentations: narrow QRS Mobitz II (rare, intra-Hisian), exercise-induced AV block [4]
- Always rule out reversible causes (acute MI, Lyme carditis, drug toxicity) before committing to permanent pacing, though most patients ultimately require a pacemaker [1]
16. Treatment Plan
Acute stabilization (ED):
- Continuous telemetry monitoring — mandatory until pacemaker implantation [5]
- Transcutaneous pacing pads applied immediately; activate if hemodynamically unstable
- Temporary transvenous pacing if hemodynamic compromise, symptomatic bradycardia, or prolonged pauses [1][12]
- Atropine is generally NOT effective for infranodal block and may worsen it; consider isoproterenol, dopamine, or epinephrine if temporizing is needed and ischemia risk is low (Class IIb) [8][12]
- IV aminophylline may be considered in the setting of acute inferior MI [12]
- Treat reversible causes: antibiotics for Lyme carditis, digoxin Fab for toxicity, correct hyperkalemia [1]
Definitive treatment:
- Permanent pacemaker implantation is a Class I recommendation for acquired Mobitz type II AV block not attributable to reversible or physiologic causes, regardless of symptoms [1]
- Asymptomatic Mobitz II with a wide QRS → Class I for pacing [4]
- Asymptomatic Mobitz II with a narrow QRS → Class IIa for pacing [4]
- If LVEF ≤35% and anticipated RV pacing >40%, consider CRT (cardiac resynchronization therapy) rather than standard RV pacing [1]
- In infiltrative cardiomyopathy (sarcoidosis, amyloidosis), consider pacemaker with defibrillator capability [1]
The 2018 ACC/AHA/HRS guideline algorithm for management of chronic AV block is shown below:
17. Disposition
Admission criteria:
- All patients with new Mobitz type II should be admitted with continuous telemetry monitoring until pacemaker implantation [5]
- Hemodynamic instability → ICU/CCU with temporary pacing capability
- Symptomatic patients (syncope, presyncope, heart failure) → monitored bed with cardiology consultation
Discharge criteria:
- Only after permanent pacemaker implantation with confirmed appropriate device function
- Reversible cause fully treated and AV block resolved (e.g., Lyme carditis with documented return to normal conduction) [1]
Specialist consultation:
- Cardiology/Electrophysiology — consult early for all Mobitz II patients for pacemaker planning
- Consider EP study if the level of block is uncertain (e.g., 2:1 block, narrow QRS) [1][8]
18. Follow Up / Return Precautions
Post-pacemaker follow-up:
- Device check within 2–12 weeks of implantation, then every 6–12 months
- Annual echocardiography if RV pacing burden is high (monitor for pacing-induced cardiomyopathy)
- Wound check at 1–2 weeks post-implantation
Return precautions (pre-pacemaker or if discharged with reversible cause):
- Return immediately for syncope, presyncope, chest pain, worsening dyspnea, or prolonged dizziness
- Avoid driving until pacemaker is implanted and functioning (per local regulations)
- Avoid strenuous activity
Patient counseling:
- Mobitz II carries a significant risk of sudden complete heart block — pacemaker implantation is life-saving and not optional in most cases [1]
- Expected recovery after pacemaker: most patients return to full activity within 4–6 weeks with arm movement restrictions in the first 2–4 weeks
References
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