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What happen when T vwave repeat?

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When a T Wave Appears to "Repeat" - The U Wave

When a second small positive deflection appears after the T wave on ECG, it is called the U wave. This is the "repeat" or extra bump you see following the T wave.

What is the U Wave?

The U wave is a small, rounded deflection (≤1 mm) that follows the T wave and normally has the same polarity as the T wave (both upright in most leads). It is best seen in leads V2-V3.
Origin (still debated):
  • Late repolarization of the Purkinje system
  • Delayed repolarization of mid-myocardial M cells
(Guyton and Hall Textbook of Medical Physiology)

ECG showing a prominent U wave in hypokalemia (lead V3):

Prominent U wave in hypokalemia - lead V3
(Harrison's Principles of Internal Medicine, 22E)

Normal vs. Abnormal U Wave

FeatureNormal U waveAbnormal (Prominent) U wave
Amplitude≤1 mm>1 mm (larger than T wave)
PolaritySame as T waveCan be inverted
AppearanceBarely visibleClearly visible, sometimes merges with T

Causes of a Prominent (Large) U Wave

  1. Hypokalemia - most common cause
    • As K+ drops, the U wave becomes progressively larger
    • At K+ ~3.5 mEq/L: prominent U wave present, ST depression
    • At K+ ~2.5 mEq/L: T wave inverts, U wave very prominent, PR interval lengthens
    • Important trap: The U wave can be mistaken as part of the T wave, causing a falsely prolonged QT interval to be measured (e.g., true QT = 0.4s but appears as 0.6s)
  2. Drugs - dofetilide, amiodarone, sotalol, quinidine
  3. Bradycardia
  4. Hypercalcemia, hypomagnesemia
(Ganong's Review of Medical Physiology; Harrison's 22E)

Why Prominent U Waves Are Clinically Important

Very prominent U waves indicate prolonged ventricular repolarization. This is a marker of increased susceptibility to Torsades de Pointes (TdP) - a dangerous polymorphic ventricular tachycardia that can degenerate into ventricular fibrillation and sudden cardiac death.
(Harrison's Principles of Internal Medicine, 22E - T Wave and U Wave section)

ECG diagram showing U wave following T wave:

U wave following T wave on ECG
(Ganong's Review of Medical Physiology - Hypokalemia ECG changes)

Key Clinical Takeaway

If you see what looks like a "repeated T wave" on an ECG:
  1. Check the patient's serum potassium (hypokalemia is #1 cause)
  2. Review medications (especially antiarrhythmics)
  3. Measure the true QT interval carefully - do not include the U wave in your QT measurement
  4. Be alert for risk of Torsades de Pointes
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