What is the workup for a patient with Mobitz Type 2 (second-degree atrioventricular) AV (atrioventricular) block?

Workup for Mobitz Type II AV Block

All patients with Mobitz Type II second-degree AV block require immediate cardiac monitoring, urgent cardiology consultation, and preparation for permanent pacemaker implantation, as this is a Class I indication regardless of symptoms due to unpredictable progression to complete heart block. 1

Immediate Actions

• Place transcutaneous pacing pads immediately upon recognition, as Mobitz Type II carries high risk of sudden progression to complete heart block 1
• Initiate continuous cardiac monitoring until permanent pacemaker is placed 1
• Assess hemodynamic stability including blood pressure, signs of low cardiac output, and symptoms of syncope or presyncope 1

Diagnostic Workup

Essential Studies

• 12-lead ECG to confirm the diagnosis—look for constant PR intervals before and after blocked P waves (distinguishing it from Mobitz Type I which shows progressive PR prolongation) and evaluate QRS width, as wide QRS suggests more extensive His-Purkinje disease 1, 2
• Transthoracic echocardiography (Class I recommendation) to assess for underlying structural heart disease, cardiomyopathy, or valvular abnormalities 1
• Exercise stress testing to evaluate for exercise-induced worsening of AV block and assess chronotropic response 3, 1
• Ambulatory ECG monitoring (Holter or event monitor) to document progression to higher-grade block and assess burden of conduction abnormalities 3

Laboratory Evaluation

• Electrolyte panel including potassium, calcium, and magnesium to rule out reversible metabolic causes 1
• Consider thyroid function tests as both hypothyroidism and hyperthyroidism can cause Mobitz Type II 4
• Cardiac biomarkers (troponin) if acute myocardial infarction is suspected, as ischemic heart disease is a primary cause 4

Advanced Imaging (Based on Initial Results)

• Cardiac MRI may be considered if echocardiogram and laboratory results suggest infiltrative cardiomyopathy (sarcoidosis, amyloidosis), myocarditis, or to assess for myocardial scar 3, 4

Critical Diagnostic Pitfalls to Avoid

• Do not confuse with Mobitz Type I (Wenckebach): Mobitz Type II has constant PR intervals, occurs in the His-Purkinje system with unreliable escape rhythms, and requires pacemaker; Mobitz Type I shows progressive PR prolongation, occurs at the AV node, and is generally benign 1, 5
• Beware of pseudo-AV block: Concealed His bundle or junctional extrasystoles can mimic Mobitz Type II—look for premature QRS complexes that may be hidden 2, 6
• 2:1 AV block cannot be classified as Mobitz I or II on surface ECG alone; electrophysiologic study or stress testing may be needed to determine the level of block 2
• Vagal surge mimicry: Simultaneous sinus slowing with AV block may resemble Mobitz Type II but is typically benign vagal phenomenon—true Mobitz Type II requires stable sinus rate 2, 7

Acute Management While Awaiting Pacemaker

For Hemodynamically Unstable Patients

• Atropine 0.5 mg IV every 3-5 minutes up to maximum 3 mg total dose may be attempted, though often ineffective since block occurs below the AV node in the His-Purkinje system 1, 8
• Urgent transvenous temporary pacing for patients not responding to atropine or with hemodynamic compromise, with transcutaneous pacing as bridge 1
• Access via femoral, internal jugular, or subclavian veins to right ventricular apex 1

Atropine Caveats

• Doses <0.5 mg may paradoxically worsen AV block via central vagal stimulation 9, 8
• Atropine is unreliable for Mobitz Type II as the block is infranodal 1, 8

Definitive Management

• Permanent pacemaker implantation is mandatory for all patients with Mobitz Type II, even if asymptomatic, due to unpredictable progression to complete heart block with risk of Adams-Stokes attacks and sudden death (Class I indication) 1, 10
• Dual-chamber pacemakers should be programmed to maintain native AV conduction when possible to prevent pacing-induced ventricular dysfunction 1

Special Clinical Scenarios

• Post-cardiac surgery or TAVR: If Mobitz Type II persists beyond 7-10 days postoperatively, permanent pacemaker is indicated; most postoperative AV block recovers within this timeframe 1, 4
• Neuromuscular diseases (myotonic dystrophy, Kearns-Sayre syndrome): Consider pacemaker with defibrillator capability if meaningful survival >1 year expected 1, 4
• Infiltrative cardiomyopathies (cardiac sarcoidosis, amyloidosis): Consider pacemaker with defibrillator capability 1, 4
• Acute MI setting: Represents Class Ia indication for temporary transvenous pacing, followed by permanent pacemaker if block persists 1

Referral

• Immediate electrophysiologist referral is essential for all confirmed cases of Mobitz Type II 3