What is the wave called just after the T (tachycardia) wave on an electrocardiogram (ECG)?

The U Wave on Electrocardiogram

The wave that appears just after the T wave on an electrocardiogram (ECG) is called the U wave, which is a mechanoelectric phenomenon resulting in a low-amplitude, low-frequency deflection. 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 about 11% of the T wave amplitude 1, 2
• U wave presence is heart-rate dependent:
  • Rarely present at rates greater than 95 bpm 1
  • Enhanced during bradycardia - present in 90% of cases at heart rates less than 65 complexes per minute 1, 2

Clinical Significance of the U Wave

• An inverted U wave in leads V2 through V5 is considered abnormal 1, 2
• Abnormal U waves may appear transiently during:
  • Acute myocardial ischemia 1
  • Hypertension 1, 3
• Increased U-wave amplitude can be associated with:
  • Hypokalemia (with severe hypokalemia, U-wave amplitude may exceed T-wave amplitude) 1, 2
  • Cardioactive drugs with quinidine-like effects 1
• Fusion of the U wave with the T wave can occur with:
  • Increased sympathetic tone 1
  • Markedly prolonged QT interval as in congenital and acquired long-QT syndromes 1, 2

Recognition in Clinical Practice

• Abnormal U waves are often subtle and rarely an isolated ECG abnormality 1
• U waves are frequently overlooked by ECG readers and automated systems 1
• According to guidelines, statements concerning the U wave should be included in ECG interpretation when 1:
  • The U wave is inverted
  • The U wave is merged with the T wave
  • U-wave amplitude exceeds T-wave amplitude

Theories on U Wave Origin

Several theories exist regarding the origin of the U wave, though none has gained universal acceptance 4, 3:

• Repolarization of the intraventricular conducting system (Purkinje fibers) 4, 5
• Delayed repolarization of the papillary muscles 4
• Afterpotentials caused by mechanoelectrical feedback 4, 5
• Prolonged repolarization in the cells of the mid-myocardium ("M-cells") 4, 3

Prognostic Implications

• Negative U waves have been associated with increased risk of all-cause mortality, cardiac death, and cardiac hospitalization, particularly in men 3
• Patients with negative U waves tend to have higher blood pressure, history of hypertension, higher heart rate, cardiac disease, and left ventricular hypertrophy compared to subjects with normal U waves 3

Common Pitfalls in U Wave Identification

• Confusion between a T wave with two peaks (T1-T2) and a true U wave 4
• Difficulty distinguishing the end of the T wave from the beginning of the U wave, especially with QT prolongation 5
• Some researchers suggest that T and U waves together should be considered as one repolarization complex rather than separate entities 6