What causes the U wave in an electrocardiogram (ECG)?

Causes of U Wave in ECG

The U wave in an electrocardiogram (ECG) is primarily a mechanoelectric phenomenon that occurs after repolarization is completed, appearing as a low-amplitude, low-frequency deflection after the T wave. 1

Characteristics of the U Wave

• The U wave is frequently absent in limb leads and most evident in leads V2 and V3 1
• Normal U wave amplitude is approximately 0.33 mV or 11% of the T wave 1
• U wave presence is heart-rate dependent - rarely present at rates >95 bpm and enhanced during bradycardia (present in 90% of cases at heart rates <65 bpm) 1
• The U wave typically appears following the T wave with the TU junction along the baseline or fused with it 2

Main Theories of U Wave Origin

Several theories exist regarding the genesis of the U wave:

1. Mechanoelectric Phenomenon:

   • The U wave represents an electric-mechanical phenomenon occurring after ventricular repolarization is completed 1
   • May be related to mechanical forces in the ventricular wall during ventricular filling 3, 4

2. Purkinje Fiber Repolarization:

   • Delayed repolarization of the intraventricular conducting system (Purkinje fibers) 2, 3
   • This is considered the most likely explanation for physiological U waves 3, 5

3. Papillary Muscle Repolarization:

   • Delayed repolarization of the papillary muscles 2

4. M-Cell Repolarization:

   • Prolonged repolarization in the cells of the mid-myocardium ("M-cells") 2, 6

Pathological U Wave Changes

Increased U Wave Amplitude

• Hypokalemia: Traditionally associated with increased U-wave amplitude, often with ST depression and decreased T-wave amplitude 1

  • In severe hypokalemia (K+ <2.7 mmol/L), U-wave amplitude may exceed T-wave amplitude 1
  • Recent evidence suggests this may be due to fusion of U wave with T wave rather than true U-wave amplitude increase 1

• Cardioactive Drugs: Various medications with quinidine-like effects can cause increased U-wave amplitude 1

• Increased Sympathetic Tone: Can cause fusion of U wave with T wave 1

Inverted U Wave

• Inverted U waves in leads V2 through V5 are abnormal 1
• May appear transiently during:
  • Acute myocardial ischemia 1
  • Hypertension 1
• Associated with higher blood pressure, history of hypertension, higher heart rate, cardiac disease, and left ventricular hypertrophy 6
• Negative U waves are highly specific for the presence of heart disease 6
• Associated with increased risk of all-cause mortality, cardiac death, and cardiac hospitalization in men 6

Clinical Significance and Recognition

• Abnormal U waves are often subtle and rarely an isolated ECG abnormality 1
• Often overlooked by ECG readers and automated systems 1
• Statements concerning the U wave should be included in ECG interpretation when 1:
  1. The U wave is inverted
  2. The U wave is merged with the T wave
  3. U-wave amplitude exceeds T-wave amplitude

Relationship to Long QT Syndrome

• Fusion of U wave with T wave occurs in markedly prolonged QT intervals, such as in congenital and acquired long-QT syndromes (LQTS) 1
• This fusion can complicate accurate measurement of the QT interval 1

Clinical Pitfalls

• Distinguishing between a true U wave and a T wave with two peaks (T1-T2) can be challenging 2
• The traditional view that the end of T wave represents the end of myocardial repolarization may need revision, as T and U together might be considered one repolarization complex 4
• In hyperkalaemia, U waves typically disappear, but discordant U waves may rarely coexist with hyperkalaemia in patients with chronic kidney disease 5