What is the most appropriate test to order for a 55-year-old man with hypertension, diabetes, and hyperlipidemia, presenting with lightheadedness, Mobitz Type II second-degree AV block, and dehydration?

Diagnostic Approach for Mobitz Type II Second-Degree AV Block

In a 55-year-old man with hypertension, diabetes, and hyperlipidemia presenting with Mobitz Type II second-degree AV block, transthoracic echocardiography is the most appropriate initial test to order.

Initial Evaluation and Management

• Transthoracic echocardiography is strongly recommended (Class I recommendation) for patients with newly identified Mobitz type II atrioventricular block to assess for underlying structural heart disease 1
• The patient's presentation with lightheadedness and confirmed Mobitz Type II second-degree AV block on ECG warrants immediate evaluation for underlying cardiac pathology 1
• Transcutaneous pacing pads were appropriately placed as Mobitz Type II AV block has higher risk of progression to complete heart block and hemodynamic compromise 1, 2

Rationale for Echocardiography

• Mobitz Type II second-degree AV block is almost always located below the AV node in the His-Purkinje system, making it more likely to progress to complete heart block 2, 3
• Echocardiography can identify underlying structural heart disease that may be contributing to the conduction disorder, including:
  • Ischemic heart disease
  • Cardiomyopathy
  • Valvular disease
  • Left ventricular dysfunction 1
• This patient has multiple cardiovascular risk factors (hypertension, diabetes, hyperlipidemia) that increase the likelihood of underlying structural heart disease 1

Laboratory Testing Considerations

• Laboratory tests based on clinical suspicion for potential underlying causes are reasonable (Class IIa recommendation) 1
• Electrolyte panel is appropriate to rule out electrolyte abnormalities that could contribute to conduction disorders, particularly given the patient's history of kidney injury and dehydration 1
• Thyroid function tests may be considered if there is clinical suspicion for thyroid dysfunction 1
• Lyme disease serology would only be indicated if there is clinical suspicion or relevant exposure history 1

Additional Testing Considerations

• Nuclear stress testing would not be the first-line test in this scenario, though it may be considered later if ischemic heart disease is suspected after initial evaluation 1
• Disease-specific advanced imaging (CT, MRI, nuclear imaging) is reasonable only if structural heart disease is suspected but not confirmed by echocardiography 1

Clinical Pearls and Pitfalls

• Mobitz Type II AV block is frequently overdiagnosed; careful ECG interpretation is essential to distinguish it from atypical Wenckebach (Mobitz Type I) or pseudo-AV block from concealed extrasystoles 3, 4
• The presence of Mobitz Type II AV block in a patient with cardiovascular risk factors strongly suggests underlying structural heart disease and warrants thorough cardiac evaluation 1
• Mobitz Type II AV block is generally considered an indication for permanent pacemaker placement, particularly when symptomatic 1
• In rare cases, tachycardia-dependent Mobitz Type II AV block may occur and respond to rate control measures, but this should only be considered after structural disease is excluded 5

Conclusion

Transthoracic echocardiography is the most appropriate initial test for this patient with Mobitz Type II second-degree AV block, as it will help identify underlying structural heart disease that may be contributing to the conduction disorder and guide further management decisions.