Blood Tests for Bradycardia (Heart Rate 40 bpm)
Order laboratory tests based on clinical suspicion for underlying reversible causes, specifically: thyroid function tests, serum potassium, arterial blood gas (pH), and Lyme titer if epidemiologically relevant. 1
Core Laboratory Panel
The 2018 ACC/AHA/HRS guidelines provide a Class IIa recommendation (reasonable to perform) for targeted laboratory testing in bradycardia patients, emphasizing that testing should be guided by clinical suspicion rather than routine comprehensive screening 1:
Thyroid function tests (TSH, free T4) - Hypothyroidism is a common reversible cause of sinus node dysfunction and should be assessed at least once in all patients with suspected bradycardia 1
Serum potassium - Both hyperkalemia and hypokalemia can cause bradyarrhythmias and conduction disturbances 1
Arterial blood gas or venous pH - Acidosis, hypoxemia, and hypercarbia are potentially reversible metabolic causes of bradycardia 1
Lyme disease serology - Indicated when there is clinical suspicion based on geographic exposure history, as Lyme carditis can cause high-grade AV block 1, 2, 3
Additional Context-Specific Testing
Beyond the core panel, consider these tests based on specific clinical scenarios 1:
Complete blood count - To assess for anemia or infection that may contribute to symptoms 1
Serum creatinine with eGFR - Renal dysfunction can affect drug metabolism and electrolyte balance 1
Troponin (high-sensitivity) - If acute myocardial ischemia or infarction is suspected as the cause of new-onset AV block 1
Drug levels - For digoxin, lithium, or antiarrhythmic medications if the patient is taking these agents 1
Common Reversible Causes to Screen For
The guidelines emphasize identifying treatable etiologies before considering permanent pacing 1:
Medications: Beta-blockers, calcium channel blockers (non-dihydropyridine), digoxin, antiarrhythmics, lithium 1
Electrolyte abnormalities: Hyperkalemia, hypokalemia, hypoglycemia 1
Metabolic conditions: Hypothyroidism, hypothermia 1
Infections: Lyme disease, Legionella, viral myocarditis 1, 3
Ischemia: Acute MI or ongoing myocardial ischemia 1
Hypervagotonia: Sleep apnea, increased vagal tone 1
Clinical Pitfalls to Avoid
Do not order comprehensive laboratory panels routinely - The evidence does not support routine extensive testing; target your workup based on history, physical exam findings, and ECG characteristics 1
Distinguish physiologic from pathologic bradycardia - Athletic training and sleep can cause benign bradycardia that requires no intervention 1, 4, 5
Correlate laboratory findings with symptoms - Asymptomatic bradycardia with normal labs may not require aggressive intervention 4, 5, 6
Consider medication review before extensive testing - Drug-induced bradycardia is common and reversible; a careful medication history may obviate the need for extensive laboratory workup 1, 5
Integration with Other Diagnostic Modalities
Laboratory testing should complement, not replace, other essential diagnostic steps 1, 2:
12-lead ECG is mandatory to characterize the type of bradycardia (sinus bradycardia vs. AV block) 1, 2
Transthoracic echocardiography is recommended (Class I) for patients with high-grade AV blocks to assess for structural heart disease 1, 2
Ambulatory monitoring may be needed if symptoms are intermittent and correlation with bradycardia episodes is uncertain 1