From the Guidelines
The U wave on an ECG signifies repolarization of the Purkinje fibers or the mid-myocardial M cells in the ventricles, and its presence and morphology can indicate various clinical conditions, including hypokalemia, hypomagnesemia, and myocardial ischemia. The U wave is normally upright, best seen in leads V2-V4, and has the same direction as the preceding T wave 1. Its amplitude is approximately 0.33 mV or 11% of the T wave, and its presence is heart-rate dependent, rarely present at rates greater than 95 bpm 1.
Key Points to Consider
- The U wave is enhanced by bradycardia and is present in 90% of cases at heart rates less than 65 complexes per minute 1.
- An increase in U-wave amplitude may be caused by cardioactive drugs with quinidine-like effects and by hypokalemia, and with advanced hypokalemia, the U-wave amplitude may exceed the T-wave amplitude in the same lead 1.
- Fusion of the U wave with the T wave can occur in association with an increase in sympathetic tone and in the presence of a markedly prolonged QT interval, such as in congenital and acquired long-QT syndromes (LQTS) 1.
- An inverted U wave in leads V2 through V5 is abnormal and may appear transiently during acute ischemia or in the presence of hypertension 1.
Clinical Implications
- Abnormal U waves can indicate important clinical conditions, such as myocardial ischemia, left ventricular hypertrophy, or cardiomyopathy 1.
- When evaluating U waves, it's essential to consider them in context with the patient's clinical presentation and other ECG findings 1.
- If abnormal U waves are detected, electrolyte levels should be checked, and cardiac evaluation may be warranted, especially if there are symptoms suggesting cardiac disease 1.
From the Research
U Wave Significance
The U wave on an electrocardiogram (ECG) is a small, rounded, and upward deflection that appears after the T wave 2. Its origin is still a topic of debate, with several theories attempting to explain its presence, including:
- Repolarization of the intraventricular conducting system or Purkinje fibers system
- Delayed repolarization of the papillary muscles
- Afterpotentials caused by mechanoelectrical hypothesis or mechanoelectrical feedback
- Prolonged repolarization in the cells of the mid-myocardium ("M-cells") 2, 3
Clinical Significance
The U wave can be an indicator of various cardiac conditions, such as:
- Coronary artery disease with ongoing myocardial ischemia or infarction
- Ventricular hypertrophy
- Congenital heart disease
- Primary cardiomyopathy
- Valvular defects 3 Abnormal U-waves, particularly negative U-waves, are highly specific for the presence of heart diseases and can be associated with increased risk of all-cause mortality, cardiac death, and cardiac hospitalization in men 3.
Electrolyte Disorders
Electrolyte disorders, such as hypokalemia, can alter cardiac ionic currents kinetics and promote proarrhythmic or antiarrhythmic effects 4. Hypokalemia is often associated with a prominent U wave, and its ECG manifestations can include:
- Dynamic changes in T-wave morphology
- ST-segment depression
- U waves, often best seen in the mid-precordial leads (V2–V4) 4, 5 Hyperkalemia, on the other hand, can affect the ECG in different ways, depending on the serum potassium level 4.
U Wave Characteristics
The morphology of the T-U wave is dependent on the geometrical position of the recording point with respect to the myocardium, and T and U form a continuum, being the resultant of one and the same process of repolarization of the ventricular myocardium 6. The U wave's characteristics, including its amplitude and polarity, can provide valuable information for diagnosis and prognosis in clinical practice 3.