U-Wave on Electrocardiogram: Association with Hypokalemia
The appearance of a prominent U-wave on electrocardiogram is most commonly associated with hypokalemia (option b). 1, 2
Characteristics of the U-Wave
The U-wave is a mechanoelectric phenomenon that appears as a low-amplitude, low-frequency deflection occurring after the T wave. It has the following characteristics:
- Typically measures approximately 0.33 mV or 11% of the T-wave amplitude
- Most evident in precordial leads V2 and V3
- Often absent in limb leads
- Heart rate dependent - rarely present at rates >95 bpm
- Enhanced by bradycardia - present in 90% of cases with heart rates <65 bpm 1
U-Wave Changes in Electrolyte Disorders
Hypokalemia
- Causes increased U-wave amplitude, often with associated ST-segment depression and decreased T-wave amplitude
- In advanced hypokalemia (K+ <2.7 mmol/L), U-wave amplitude may exceed T-wave amplitude in the same lead
- At very low potassium levels (<2.5 mEq/L), the T-wave may become merely a notch on the upstroke of a giant U-wave 1, 2, 3
Hyperkalemia
- Typically presents with tall, peaked, narrow-based T waves
- Does not characteristically affect U-waves
- At levels >10.0 mmol/L, can cause sinus arrest, marked intraventricular conduction delay, ventricular tachycardia, and ventricular fibrillation 2
Other Electrolyte Abnormalities
- Hypomagnesemia may contribute to prominent U-waves, though this is often seen concurrently with hypokalemia 4
- Hypocalcemia and hypercalcemia primarily affect the ST segment and QT interval rather than the U-wave 2
Clinical Significance of U-Waves
Normal U-Waves
- Physiological U-waves are thought to be due to delayed repolarization of the Purkinje system 5
- Bradycardia enhances U-wave amplitude even in normal individuals 1
Abnormal U-Waves
- Inverted U-waves in leads V2 through V5 are abnormal 1
- May appear transiently during acute ischemia or in hypertension
- Often subtle and rarely an isolated ECG abnormality
- Negative U-waves are highly specific for heart disease and associated with increased risk of all-cause mortality, cardiac death, and cardiac hospitalization in men 6
Other Causes of Prominent U-Waves
- Cardioactive drugs with quinidine-like effects
- Increased sympathetic tone
- Markedly prolonged QT interval (congenital and acquired long-QT syndromes) 1
Clinical Recommendations
According to AHA/ACCF/HRS guidelines, statements concerning the U-wave should be included in ECG interpretation when:
- The U-wave is inverted
- The U-wave is merged with the T-wave
- U-wave amplitude exceeds T-wave amplitude 1
Common Pitfalls
- Mistaking prominent U-waves of severe hypokalemia for ST-segment depression and long QT interval of myocardial ischemia
- Overlooking U-waves, which are often subtle and may be missed by both automated systems and human readers
- Failing to recognize that multiple electrolyte disturbances occurring simultaneously may generate atypical ECG patterns 4, 3