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Don't Be Fooled by U Waves

Pregerson, Brady MD

doi: 10.1097/01.EEM.0000731780.53166.c8
    U waves, abdominal pain, chest pain, hyperkalemia

    A woman in her 30s was brought in by EMS for 12 hours of constant generalized abdominal and chest pain with intermittent vomiting. The pain was nonpleuritic and nonexertional, and she had no syncope, palpitations, shortness of breath, fever, cough, diarrhea, or other complaints. She reported being an alcoholic, and initially demanded pain medication but fell asleep soon after.

    Her vital signs were normal except for a pulse in the high 40s to low 50s. The exam was normal except for mild epigastric tenderness. Her ECG is shown.

    What is the most likely cause of her ECG findings? Hyperkalemia? Hypokalemia, ACS, or normal variant? Gamma-hydroxybutyrate (GHB) toxicity?

    The answer: Normal variant.

    The patient's ECG showed junctional bradycardia with a rate in the mid- to high 40s with U waves best noted in V4-V6. The differential diagnosis of an upright U wave includes ischemia, hypokalemia, hypercalcemia, bradycardia, normal variant, and GHB toxicity. Of these, the clinical picture would best fit ischemia or hypokalemia, but the QU interval was not prolonged, so hypokalemia was not likely the cause. Given her age and 12 hours of constant chest pain, ischemia was also unlikely, leaving bradycardia or normal variant as the most likely explanation. The only differential diagnosis of junctional bradycardia that possibly fit was ischemia. Troponin and potassium should obviously be checked.


    The troponin I in this case was <0.01 (99% URL <0.030: troponin-i immunoassay, Abbott), potassium was 2.9 mmol/L, magnesium was 1.3 mEq/L, and PO4 was 2.5 mg/dL. U waves were suggestive but far from diagnostic of hypokalemia, and a potassium of 2.9 is not very low and would be expected to cause only subtle effects on the ECG, if any at all.

    Because the QU interval was normal, there was likely another cause of the U waves besides hypokalemia. A careful physician would only be convinced that these U waves were due to hypokalemia if the U waves resolved after the potassium was normalized and at the same heart rate. No definitive cause for the junctional bradycardia was found, but it resolved on its own and the heart rate normalized gradually over six hours, leading to suspicion that it was due to increased vagal tone. The patient was very sleepy for most of her ED stay. The possibility of GHB toxicity was considered a possible unifying diagnosis, but the patient continued to insist that she only used alcohol.

    Case Lessons

    • Hyperkalemia can cause almost any ECG finding and definitely junctional bradycardia but not U waves.
    • Hypokalemia can cause or contribute to U waves but not junctional bradycardia.
    • ACS can cause many ECG findings, including U waves and junctional bradycardia but not likely clinically.
    • Normal variant U waves can be normal and are more common if bradycardic. High vagal tone can rarely cause junctional bradycardia.
    • GHB toxicity can cause U waves and increased vagal tone resulting in junctional bradycardia but was not confirmed by the patient.

    An Aside

    If you haven't yet read the OMI manifesto, you should. It's long, but everyone should know at least the basics of why the current STEMI criteria miss about a third of occlusion MIs that would benefit from emergent reperfusion and how you can diagnose those patients and get them the care they need. Read it here:

    This post was peer-reviewed by Stephen W. Smith, MD, of Dr. Smith's ECG Blog (

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    Dr. Pregersonis an emergency physician at Tri-City Hospital and Scripps Coastal, both in Oceanside, CA. He is the author of the EMN blog Bradycardia at, and of Emergency Medicine 1-Minute Consult, Tarascon Emergency Department Quick Reference Guide, A to Z Emergency Pharmacopoeia & Antibiotic Guide, Don't Try This at Home, and Think Twice: More Lessons from the ER. Follow him on Twitter@EM1MinuteGuru, and visit his website

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