U-Wave on Electrocardiogram: Association with Electrolyte Abnormalities
The appearance of a prominent U-wave on electrocardiogram is most commonly associated with hypokalemia (option b). 1
Characteristics of U-Waves in Hypokalemia
Hypokalemia produces several distinctive ECG changes, with the prominent U-wave being the most significant and characteristic finding:
- Prominent U-waves typically measure approximately 0.33 mV or 11% of the T-wave amplitude
- Most evident in precordial leads V2 and V3 1
- In advanced hypokalemia (serum potassium <2.5 mEq/L), U-wave amplitude may exceed T-wave amplitude 1, 2
- At very low potassium levels (e.g., 1.7 mEq/L), the T-wave may become merely a notch on the upstroke of a giant U-wave 1, 2
- Often accompanied by sagging ST-segments and low T-waves 2
Physiological Factors Affecting U-Wave Appearance
- Bradycardia enhances U-wave amplitude, with U-waves present in 90% of cases with heart rates <65 bpm
- U-waves are rarely present at heart rates >95 bpm 1
- Normal U-waves are thought to be due to delayed repolarization of the Purkinje system 3
Differentiating from Other Electrolyte Abnormalities
Hyperkalemia (option a)
- Does NOT typically produce U-waves
- Characterized by tall, peaked T-waves with narrow base at levels of 5.5-7.0 mmol/L
- At higher levels (>10.0 mmol/L), can cause sinus arrest, conduction delays, and ventricular arrhythmias 4
Calcium Abnormalities (options d and e)
- Hypocalcemia results in prolonged ST segment and QT interval
- Hypercalcemia causes shortening of ST segment and QT interval
- Neither is characteristically associated with prominent U-waves 4
Magnesium Abnormalities (options c and d)
- The direct ECG effects of isolated magnesium disorders are controversial
- Often occur with other electrolyte abnormalities, making specific ECG changes difficult to attribute solely to magnesium 4
- No specific association with prominent U-waves in the evidence provided
Clinical Significance of Abnormal U-Waves
- Inverted U-waves in leads V2-V5 are abnormal and may appear during acute ischemia or hypertension 1
- Negative U-waves are highly specific for heart disease and associated with:
- Higher blood pressure and history of hypertension
- Higher heart rate
- Left ventricular hypertrophy
- Increased risk of mortality and cardiac hospitalization (particularly in men) 5
Potential Pitfalls in U-Wave Interpretation
- The giant U-wave of severe hypokalemia can be mistaken for ST-segment depression and long QT interval of myocardial ischemia 2
- U-wave variability can occur in certain clinical scenarios, such as heart failure with electrolyte disturbances 6
- The origin of the U-wave remains debated, with multiple theories including late depolarization, delayed repolarization, electro-mechanical stretch, and intrinsic potential differences 5