What is the first test to order in a patient with Mobitz type II second-degree atrioventricular block and acute kidney injury due to dehydration?

First Test to Order: Serum Electrolytes

In a patient with Mobitz type II second-degree AV block and acute kidney injury due to dehydration, you should order serum electrolytes first (Option B). This is the most appropriate initial test because electrolyte abnormalities—particularly hyperkalemia, hypokalemia, and hypomagnesemia—are reversible causes of AV block that must be identified and corrected immediately, especially in the setting of acute kidney injury where electrolyte disturbances are common 1, 2, 3.

Clinical Reasoning

Why Electrolytes Come First

• Acute kidney injury creates high risk for life-threatening electrolyte abnormalities that can worsen or cause AV block, particularly hyperkalemia which directly affects cardiac conduction 1, 3
• Correction of electrolyte imbalances is recommended as a Class I indication in patients with arrhythmias, and identifying reversible causes takes priority before proceeding to permanent interventions 1, 2
• The patient received aggressive IV fluid resuscitation (1-liter bolus plus 100 mL/hr maintenance), which can further shift electrolytes and must be monitored 3
• His medications include lisinopril-hydrochlorothiazide and glipizide, both of which can cause electrolyte disturbances (hyperkalemia from ACE inhibitor, hypokalemia from thiazide, hypoglycemia affecting potassium) 2, 3

The Mobitz Type II Context

While Mobitz type II is a serious arrhythmia that typically requires permanent pacemaker placement regardless of symptoms 1, 3, 4, you must first rule out reversible causes:

• Mobitz type II occurs in the His-Purkinje system and has high risk of progression to complete heart block, making it a Class I indication for permanent pacing 1, 3, 4
• However, electrolyte abnormalities must be excluded first because they represent potentially reversible causes that could be contributing to or causing the conduction abnormality 2, 3
• The patient is currently stable (no chest pain, improved symptoms, normal vital signs, oxygen saturation 95%), allowing time for this rapid initial assessment 3
• You have already placed transcutaneous pacing pads, which is the appropriate immediate safety measure 3

Why Not the Other Options?

Echocardiography (Option A)

• Echocardiography is indeed recommended as part of the evaluation for Mobitz type II to assess for structural heart disease 3, 4
• However, it should come after electrolytes because structural findings won't change the immediate management, whereas electrolyte abnormalities require urgent correction 3, 4
• The echocardiogram helps with risk stratification and pacemaker planning but doesn't address reversible causes 4

Lyme Serologies (Option C)

• Lyme disease can cause AV block, but this is not the first-line test in acute management 2
• Lyme-related cardiac involvement typically presents with fluctuating degrees of AV block and would be considered after excluding more common and immediately life-threatening causes 2
• This patient has multiple risk factors for electrolyte disturbances making that a much higher priority 3

Nuclear Stress Test (Option D)

• Completely inappropriate in this acute setting with confirmed Mobitz type II block 3
• Stress testing could be dangerous as exercise can worsen AV block in His-Purkinje disease 1, 2
• The patient needs pacemaker evaluation, not ischemia assessment, as the primary concern 3, 4

Critical Management Algorithm

1. Order serum electrolytes immediately (potassium, magnesium, calcium, sodium) along with repeat creatinine to assess kidney function 1, 2, 3

2. Correct any identified electrolyte abnormalities urgently while maintaining continuous cardiac monitoring 1, 3

3. Proceed with echocardiogram to assess for structural heart disease and ventricular function 3, 4

4. Consult cardiology/electrophysiology (already done per scenario) for permanent pacemaker planning, as Mobitz type II is a Class I indication for pacing even if electrolytes are corrected 1, 3, 4

5. Consider ambulatory ECG monitoring to assess burden of block if pacemaker placement is delayed 3, 4

Important Caveats

• Do not delay pacemaker consultation while waiting for test results—the cardiology consult should proceed in parallel 3, 4
• Atropine is often ineffective for Mobitz type II because the block occurs below the AV node in the His-Purkinje system, but may be attempted if the patient becomes symptomatic 3, 4
• Even if electrolytes are normal, this patient still needs a permanent pacemaker because Mobitz type II has unpredictable progression to complete heart block 1, 3, 4
• Maintain transcutaneous pacing pads in place until permanent pacing is established 3