REBEL ECG of the Week #4

62 year old male with chief complaint of weakness.  Patient had a complicated hospital course including necrotizing fasciitis which required surgical debridement and IV antibiotics.  Patient was discharged home with oral antibiotics and returned to the ED with a chief complaint of weakness, abdominal pain, and 3 weeks of loose bowel movements.

BP: 100/51     HR 93     RR 16     Temp 97.2     O2 Sat 98% on RA

ECG from triage is shown…

Before reading on, try to come up with your own interpretation of this ECG before moving on to the final impression

Hypokalemia U Wave

  • Rate: Ventricular rate 81
  • Rhythm: Normal Sinus Rhythm
  • Axis: Normal Axis
  • Final ECG Interpretation: U waves consistent with hypokalemia

Hypokalemia is the most common electrolyte abnormality encountered in clinical practice (i.e. K+ <3.6 mmol/L seen in over 20% of hospitalized patients) [1]. As serum potassium levels decline, the transmembrane potassium gradient is decreased causing an elevation in the resting membrane potential and a prolongation of the action potential (Phase 3 repolarization).  Therefore, the earliest ECG changes associated with hypokalemia is decreased T wave amplitude [2].  At extreme, low levels of potassium a positive deflection after the T wave (U wave) can be seen.  The U wave is best seen in the mid-precordial leads (i.e. V2 and V3) [2].

Action Potential

What are some electrocardiographic findings associated with hypokalemia? [2]

  • Flattening of T wave
  • T wave inversion
  • ST segment depression
  • Prominent U waves
  • Prolongation of QT (U) Interval
  • Ventricular tachycardia
  • Torsades de pointes

Case Conclusion:  Patient was diagnosed with C. Diff Colitis.  Initial K+ was 1.3 shown on the ECG above.  Potassium was aggressively replaced and ECG changes in lead V2 are shown as the potassium started to correct below.

Hypokalemia U Wave


  1. El-Sherif N et al. Electrolyte Disorders and Arrhythmogenesis. Cardiol J 2011. PMID: 21660912
  2. Diercks DB et al. Electrocardiographic Manifestations: Electrolyte Abnormalities. JEM 2004. PMID: 15261358
Cite this article as: Salim Rezaie, "REBEL ECG of the Week #4", REBEL EM blog, November 29, 2013. Available at:

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