What is the clinical significance of a U wave on an electrocardiogram (ECG)?

Clinical Significance of U Waves on ECG

U waves on an electrocardiogram (ECG) have significant clinical implications, particularly when abnormal, as they may indicate electrolyte disturbances, drug effects, or cardiac pathology that could impact morbidity and mortality.

Normal U Wave Characteristics

• U waves are mechanoelectric phenomena appearing as low-amplitude, low-frequency deflections after the T wave 1, 2
• Most evident in leads V2 and V3, frequently absent in limb leads 1, 2
• Normal amplitude is approximately 0.33 mV or 11% of the T wave amplitude 1, 2
• Heart rate dependent: rarely present at rates >95 bpm, enhanced during bradycardia (present in 90% of cases at heart rates <65 bpm) 1, 2

Pathological U Wave Changes

Abnormal U Wave Patterns

• Inverted U waves in leads V2 through V5 are considered abnormal 1, 2
• Abnormal U waves are often subtle and rarely an isolated ECG abnormality 1, 2
• Negative U waves are associated with increased all-cause mortality, cardiac mortality, and cardiac hospitalization, particularly in men (HR 1.60,1.74, and 1.67 respectively) 3

Electrolyte Disturbances

• Hypokalemia is classically associated with:
  • Increased U-wave amplitude 1, 2
  • ST segment depression 1, 2
  • Decreased T-wave amplitude 1, 2
• In severe hypokalemia (K+ <2.7 mmol/L), U-wave amplitude may exceed T-wave amplitude 1, 2
• Recent evidence suggests these changes may be due to fusion of the U wave with the T wave rather than true U-wave amplitude increase 1, 2

Drug Effects

• Cardioactive drugs with quinidine-like effects can cause increased U-wave amplitude 1, 2
• These drugs often also affect the QT interval, which has implications for risk of ventricular arrhythmias 1, 4

Cardiac Pathology

• Inverted U waves may appear transiently during:
  • Acute myocardial ischemia 1, 2
  • Hypertension 1, 2
• Fusion of U wave with T wave occurs in:
  • Increased sympathetic tone 1, 2
  • Markedly prolonged QT interval (congenital and acquired long-QT syndromes) 1, 2
• U-wave alternans has been observed in catecholaminergic polymorphic ventricular tachycardia (CPVT), suggesting a relationship with underlying arrhythmogenic mechanisms 5

Clinical Approach to U Wave Assessment

• According to AHA/ACCF/HRS guidelines, statements concerning the U wave should be included in ECG interpretation when 1, 2:
  • The U wave is inverted
  • The U wave is merged with the T wave
  • U-wave amplitude is greater than the T-wave amplitude

Pitfalls in U Wave Interpretation

• Abnormal U waves are often overlooked by ECG readers and automated systems 1, 2
• U waves can be confused with T waves, especially when fused, leading to inaccurate QT interval measurements 1, 2
• Physiological U waves (normal variant) must be distinguished from pathological U waves 6
• The presence of other ECG abnormalities may make U wave identification challenging 1, 2

Clinical Implications for Patient Management

• Presence of abnormal U waves should prompt evaluation for:
  • Electrolyte abnormalities, particularly hypokalemia 4
  • Cardiac ischemia when inverted U waves are present 1, 2
  • Medication effects, especially those affecting cardiac repolarization 4
• In the general population, negative U waves are independently associated with adverse cardiac outcomes and should prompt thorough cardiac evaluation, particularly in men 3