Management of Type 2 Second-Degree AV Block (Mobitz Type II)
The next step is to order an echocardiogram (option A), as this is essential initial evaluation for Mobitz Type II second-degree AV block, which represents infranodal conduction disease requiring comprehensive cardiac assessment before definitive pacemaker therapy. 1
Immediate Diagnostic Workup
Mobitz Type II second-degree AV block is an abnormal finding that mandates further evaluation, as it represents infranodal disease (within or below the His-Purkinje system) in the vast majority of cases. 1
The recommended initial evaluation includes: 1
- Echocardiogram - to assess for structural heart disease, ventricular function, and underlying cardiac pathology
- Ambulatory ECG monitor (Holter) - to assess burden of block and detect progression
- Exercise ECG test - to evaluate response to increased heart rate demands
Why Echocardiogram First
The echocardiogram serves multiple critical purposes in this clinical scenario: 1
- Identifies structural heart disease that may be causing the conduction abnormality (cardiomyopathy, infiltrative disease, congenital abnormalities)
- Assesses ventricular function which impacts prognosis and urgency of intervention
- Guides pacing decisions by identifying patients with impaired cardiac function who may be at higher risk
Based on echocardiographic findings, additional testing such as laboratory work and cardiac MRI may be considered. 1
Why Not the Other Options
Serum electrolytes (option B): While electrolyte abnormalities can cause first-degree AV block or Mobitz Type I (Wenckebach) block at the AV node level, Mobitz Type II block is infranodal in 90% of cases and represents organic, degenerative conduction disease rather than a reversible metabolic derangement. 2, 3 Electrolytes may be checked as part of the workup but are not the priority first step.
Lyme serology (option C): Although Lyme disease can affect the cardiac conduction system, it typically causes AV nodal block (first-degree or Mobitz Type I) rather than infranodal Mobitz Type II block. 4 Lyme serology would be considered if there is appropriate epidemiologic exposure and clinical context, but is not the routine first step.
Nuclear stress test (option D): While exercise testing is recommended, a standard exercise ECG test is preferred over nuclear imaging to assess the chronotropic response and AV conduction behavior with exertion. 1 Exercise-induced progression of AV block (not due to ischemia) indicates His-Purkinje disease with poor prognosis. 4
Critical Management Points
Referral to electrophysiologist is essential once initial workup is complete, as Mobitz Type II block requires permanent pacemaker implantation regardless of symptoms. 1
Infranodal blocks require pacing regardless of form or symptoms because they represent disease in the His-Purkinje system with risk of progression to complete heart block. 2
Common pitfall: Confusing Mobitz Type II with vagally-mediated AV block - true Type II block should not have sinus slowing before the blocked beat, as simultaneous sinus slowing and AV nodal block suggests a benign vagal surge rather than infranodal disease. 2
Permanent pacing indications: Persistent second-degree AV block in the His-Purkinje system with bilateral bundle branch block, or transient advanced second-degree infranodal AV block with associated bundle branch block. 1