What is the role of lab testing in the diagnosis of third-degree atrioventricular (AV) block?

Updated: Jul 05, 2018
  • Author: Akanksha Agrawal, MBBS; Chief Editor: Jeffrey N Rottman, MD  more...
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Answer

For most patients with illness serious enough to cause third-degree atrioventricular (AV) block (complete heart block), a complete blood cell (CBC) count is indicated to screen for coincident problems (eg, anemia, infection) that may require emergency intervention. The presence of fever or an elevated white blood cell (WBC) count should be evaluated with blood cultures because endocarditis can be complicated by heart block.

Serum concentrations of electrolytes, including potassium and magnesium, should be measured to look for metabolic imbalance, indications of renal insufficiency or failure, and particularly for severe hyperkalemia. The prothrombin time and activated partial thromboplastin time should also be routinely obtained.

A digoxin level should be obtained for patients on digoxin or in whom ingestion of digoxinlike compounds (eg, lily of the valley, oleander, foxglove, Bufonidae toads) is suspected. The same should be done for any other drugs the patient is taking that are capable of causing AV block. Note that the presence of a detectable digoxin level following a nondigoxin cardiac glycoside ingestion can only confirm the presence of such a toxin. The digoxin level does not correlate to the degree of cardiac glycoside toxicity following nondigoxin-induced cardiac glycoside ingestions.

Myocarditis-related laboratory studies should be performed in patients suspected of having myocarditis. Such studies include Lyme titers, human immunodeficiency virus (HIV) serologies, enterovirus polymerase chain reaction (PCR), adenovirus PCR, and Chagas titers, as clinically appropriate.

Lyme titers should be obtained from all patients who may have been exposed to Lyme disease. Because cardiac manifestations of Lyme disease are delayed, Lyme-induced heart block can occur during any season. The decision to perform serologic testing for Lyme disease or any of the collagen vascular diseases depends on other associated history and findings.


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