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Bradycardia: a common cause of U waves : Emergency Medicine News


cardiology, ECG, U waves


A woman in her 40s with a history of hypertension came to the emergency room with vomiting, right flank pain and shortness of breath. She said all of her symptoms, even the SOB, were similar to a previous episode of pyelonephritis.

She had no chest pain, syncope, palpitations, fever, cough, dysuria, or other symptoms. His vital signs were normal except for elevated blood pressure of 187/100 mm Hg, and an ECG was performed, which is shown.

Troponin I was less than 0.03 ng/mL (99% URL

The computer read it as sinus bradycardia at 53 bpm, possible left ventricular hypertrophy (LVH), and a previous, probably recent, myocardial infarction.

ECG results

ECG showed less than 1 mm ST segment elevation in V2-V3 associated with mild T inversion/biphasic T waves. T wave inversion was also seen in lead III, where it is often normal when the QRS is mostly negative, but here the QRS was mostly positive, making it more likely abnormal. U waves were also observed in leads V2-V5.

A small U wave may be a normal variant, but is considered definitely abnormal if it is greater than 1.5 mm or greater than 25% of the height of the previous T wave in either lead, which appeared to be the case here, unless in Lead V3. Causes of abnormal U waves include ischemia, low potassium, magnesium, and calcium, central nervous system disasters, LVH, mitral valve prolapse, and certain medications (i.e., say digoxin, amiodarone, quinidine and neuroleptics).

What was the most likely cause of the ECG findings in this patient: acute coronary syndrome, drug toxicity, hypokalemia, or hyperventilation? Acute coronary syndrome was unlikely based on overall clinical presentation and unimpressive ST changes on ECG. Drug toxicity was also unlikely unless the patient was taking digoxin, amiodarone, quinidine, or a neuroleptic.

Hyperventilation is not a known cause of U waves, although it can alter and contribute to hypokalemia. However, hypokalemia isolated by cell displacement is rarely severe enough to cause ECG changes. Hypokalemia was the correct diagnosis. This is one of the most common causes of U waves, and vomiting can certainly cause hypokalemia via contraction alkalosis.

Case lessons

Common causes of true hypokalemia are diuretics, diarrhea, and drinking (the three Ds). Excessive vomiting can cause contraction alkalosis, which can also cause potassium to drop. In fact, one of the most common laboratory abnormalities in gastroenteritis is hypokalemia.

Adrenaline release is a common cause of mild hypokalemia, often accompanied by mild leukocytosis, hyperglycemia, and compensatory metabolic acidosis. (I call this adrenaline tetrad.) If potassium is particularly low, check the magnesium level as you will usually need to replenish both to fix either one.

This article has been 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 located in Oceanside, California. He is the author of 1 Minute Emergency Medicine Consultation, Tarascon Emergency Department Quick Reference Guide, A to Z Emergency Pharmacopoeia and Antibiotics Guide, Don’t Try It at Home, and Think twice: no more lessons from urgency. Follow him on Twitter@EM1MinuteGuru, and visit his website at Read his past columns on