Bloodwork for First-Degree Heart Block
In patients with first-degree heart block, order laboratory tests based on clinical suspicion for underlying reversible causes, including thyroid function tests, electrolytes (particularly potassium), Lyme titer if epidemiologically relevant, and pH if metabolic disturbance is suspected. 1
Recommended Laboratory Testing
The 2018 ACC/AHA/HRS bradycardia guidelines provide a Class IIa recommendation (reasonable to perform) for targeted laboratory testing in patients with bradycardia or conduction disorders based on clinical suspicion for potential underlying causes. 1
Core Laboratory Panel
Order the following tests based on clinical context:
Thyroid function tests (TSH): Hypothyroidism can cause or exacerbate conduction abnormalities and is a readily reversible cause of first-degree AV block. 1, 2
Serum electrolytes (particularly potassium and magnesium): Electrolyte disturbances, especially hyperkalemia, can prolong AV conduction and are immediately correctable. 1, 2
Lyme titer: In endemic areas or with appropriate exposure history, Lyme carditis is an important reversible cause of AV block that requires specific antimicrobial therapy. 1
pH/arterial blood gas: If metabolic acidosis is suspected clinically, as acid-base disturbances can affect cardiac conduction. 1
Additional Testing in Specific Clinical Scenarios
If structural heart disease or heart failure is suspected, expand the workup to include:
- Complete blood count (to identify anemia) 1, 3
- Serum creatinine and blood urea nitrogen (to assess renal function) 1, 3
- Liver function tests 1, 3
- Fasting blood glucose or hemoglobin A1c 1, 3
- Lipid profile 1, 3
This expanded panel is recommended by ACC/AHA heart failure guidelines when there is concern for underlying cardiomyopathy or heart failure, as first-degree AV block can be associated with worse outcomes in patients with structural heart disease. 4
Important Clinical Context
First-degree AV block in asymptomatic patients without structural heart disease does not routinely require extensive laboratory evaluation. 1 However, the clinical significance depends heavily on the underlying cardiac substrate—first-degree AV block is associated with increased heart failure hospitalization and mortality in patients with established coronary artery disease. 4
Key Pitfalls to Avoid
Don't order routine cardiac imaging or extensive laboratory panels in asymptomatic patients with isolated first-degree AV block and no clinical evidence of structural heart disease. This represents a Class III (no benefit) recommendation. 1
Consider medication review: Many drugs can prolong the PR interval (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics). Drug toxicity or interactions may be the culprit rather than intrinsic conduction disease. 2
In younger patients with first-degree AV block, consider acute rheumatic fever if there are other suggestive clinical features, as conduction abnormalities occur in a subset of these patients. 5
Recognize that marked first-degree AV block (PR ≥300 ms) may cause symptoms similar to pacemaker syndrome and warrants more thorough evaluation even in the absence of higher-grade block. 6