What does the presence of a U wave on an electrocardiogram (ECG) indicate?

U Waves on Electrocardiogram: Clinical Significance

The presence of a prominent U wave on an electrocardiogram (ECG) most commonly indicates hypokalemia, but can also be associated with bradycardia, certain medications, ischemic heart disease, and left ventricular hypertrophy. 1

Normal U Wave Characteristics

• A normal U wave is a small, rounded deflection following the T wave
• Typically measures approximately 0.33 mV or about 11% of the T-wave amplitude
• Most evident in precordial leads V2 and V3
• Usually in the same direction as the T wave

Pathological U Wave Associations

Electrolyte Abnormalities

• Hypokalemia: Most common pathological cause of prominent U waves
  • At potassium levels between 3.0-3.5 mEq/L: U waves become more prominent
  • At potassium levels below 2.5 mEq/L: Sagging ST segments, low T waves, and prominent U waves 2
  • At very low potassium levels: T wave may become merely a notch on the upstroke of a giant U wave 1, 2
  • This configuration can be mistakenly interpreted as ST depression and QT prolongation 2

Heart Rate Effects

• Bradycardia enhances U-wave amplitude
  • Present in 90% of cases with heart rates <65 bpm
  • Rarely present at heart rates >95 bpm 1

Inverted U Waves

• Abnormal finding when present in leads V2 through V5
• Clinical associations include:
  • Acute myocardial ischemia (transient appearance)
  • Arterial hypertension, particularly with left ventricular hypertrophy 1, 3
  • Left ventricular hypertrophy with or without heart failure 3
  • Chronic cor pulmonale 3

Other Associations

• QT interval prolongation (congenital and acquired long-QT syndromes) 1
• Medication effects (certain antiarrhythmics, digoxin)
• Beat-to-beat U wave polarity variability has been observed in some cases of heart failure with electrolyte disturbances 4

Clinical Pearls and Pitfalls

Important Distinctions

• Differentiate from hyperkalemia: While prominent U waves suggest hypokalemia, hyperkalemia typically presents with peaked T waves, QRS widening, and PR prolongation 5
• T wave with two peaks (T1-T2) vs. true U wave: These can be confused and require careful interpretation 6

Clinical Approach

1. When prominent U waves are identified, check serum potassium levels immediately
2. Look for associated ECG changes that may indicate severity of hypokalemia
3. Examine for other causes if electrolytes are normal (bradycardia, medications, structural heart disease)
4. Pay special attention to inverted U waves, which are always abnormal and may indicate ischemia or hypertensive heart disease

Monitoring Considerations

• U waves may normalize after correction of the underlying cause
• In hypokalemia, ECG typically returns to normal after potassium repletion 2
• Serial ECGs can help monitor effectiveness of treatment

The origin of the U wave remains somewhat controversial, with several proposed mechanisms including repolarization of the Purkinje fiber system, delayed repolarization of papillary muscles, afterpotentials from mechanical feedback, and prolonged repolarization in mid-myocardial cells 6.