First Test to Order: Serum Electrolytes
Order serum electrolytes immediately as the first test in this patient with Mobitz type II second-degree AV block and acute kidney injury from dehydration. 1
Clinical Reasoning
Why Electrolytes Take Priority
Acute kidney injury creates a high risk for life-threatening electrolyte abnormalities that can worsen or cause AV block, particularly hyperkalemia, which directly affects cardiac conduction. 1
Correction of electrolyte imbalances is a Class I indication in patients with arrhythmias, and identifying reversible causes takes priority before proceeding to permanent interventions. 1
The electrolyte panel should include potassium, magnesium, calcium, and sodium, along with repeat creatinine to assess kidney function. 1
The Context of Mobitz Type II
Mobitz type II occurs in the His-Purkinje system and has a high risk of progression to complete heart block, making it a Class I indication for permanent pacing. 1, 2, 3
Even if electrolytes are normal, this patient will still need a permanent pacemaker because Mobitz type II has unpredictable progression to complete heart block. 1
The block is almost always infranodal (below the AV node) and carries significant risk of Stokes-Adams arrest. 2, 4
Why Other Tests Are Not First Priority
Echocardiography
Echocardiography is recommended as part of the evaluation for Mobitz type II to assess for structural heart disease, but it should come after electrolytes. 1, 5
While echo is important for the complete workup, it will not change immediate management in this hemodynamically stable patient with a reversible metabolic derangement. 1, 5
Nuclear Stress Test
Stress testing could be dangerous as exercise can worsen AV block in His-Purkinje disease. 1
Exercise testing is diagnostic if Mobitz II second degree or third degree AV block develops during exercise even without syncope, but this is not appropriate as an initial test in a patient with already-documented Mobitz II. 6
Lyme Serologies
While Lyme disease can affect the cardiac conduction system, there is no clinical context suggesting this diagnosis in a patient with hypertension, diabetes, and acute dehydration. 7
This would be a consideration in endemic areas with appropriate clinical presentation, but not as the first test in this scenario.
Critical Management Points
You have already appropriately placed transcutaneous pacing pads and consulted cardiology. 1
Continuous cardiac monitoring is essential until permanent pacemaker placement. 1
The patient will require permanent pacemaker implantation regardless of the electrolyte results, as Mobitz type II is a Class I indication for pacing even in asymptomatic patients. 1, 5, 4
Common Pitfall to Avoid
- Do not delay checking electrolytes to obtain an echocardiogram first. In the setting of acute kidney injury and dehydration, life-threatening hyperkalemia could be present and immediately correctable, whereas the echo findings will not change the acute management but will guide long-term pacemaker programming. 1