Is a U wave significant?

Clinical Significance of U Waves on ECG

Abnormal U waves are clinically significant and should be included in ECG interpretation when they are inverted, merged with the T wave, or when their amplitude exceeds that of the T wave, as they may indicate underlying cardiac pathology. 1

Normal U Wave Characteristics

• U waves are low-amplitude, low-frequency deflections that occur after the T wave, representing a mechanoelectric phenomenon after ventricular repolarization 1, 2
• Normal U waves are most evident in leads V2 and V3, with amplitude approximately 0.33 mV or 11% of the T wave 1, 2
• U waves are heart-rate dependent - rarely present at rates >95 bpm and enhanced during bradycardia (present in 90% of cases at heart rates <65 bpm) 1, 2
• Normal U waves are typically upright in the same leads where the T wave is upright 2

Abnormal U Wave Patterns and Their Clinical Significance

Inverted U Waves

• Inverted U waves in leads V2 through V5 are abnormal and clinically significant 1, 2
• Inverted U waves may appear transiently during acute myocardial ischemia or in the presence of hypertension 1, 3
• Negative U waves are highly specific for heart disease, with over 90% of patients showing other ECG abnormalities 3
• The three most common conditions associated with negative U waves are:
  • Systemic hypertension
  • Valvular heart disease (aortic and mitral regurgitation)
  • Ischemic heart disease 3

Prominent U Waves

• Increased U-wave amplitude, often with ST depression and decreased T-wave amplitude, is associated with:
  • Hypokalemia (especially when K+ <2.7 mmol/L, where U-wave amplitude may exceed T-wave amplitude)
  • Cardioactive drugs with quinidine-like effects 1, 2
• Recent evidence suggests some cases of apparent increased U-wave amplitude may actually represent fusion of the U wave with the T wave 1

U Wave Fusion and QT Interval

• Fusion of the U wave with the T wave occurs in:
  • Increased sympathetic tone
  • Markedly prolonged QT interval (congenital and acquired long-QT syndromes) 1, 2
• This fusion can complicate accurate measurement of the QT interval 2

U Wave Alternans

• U-wave alternans has been observed in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) 4
• This phenomenon appears to be related to the underlying pathogenesis of CPVT and may have diagnostic significance 4

T-U Wave Relationship

• The significance of U waves should not be assessed in isolation but rather in relation to their respective T waves 5
• Discordant T and U waves (where one is positive and the other negative) have different clinical implications than concordant negative T and U waves 5
• Patients with both negative T and negative U waves have significantly higher rates of coronary disease (88%), hypertension (88%), and left ventricular hypertrophy (70%) compared to those with discordant T and U waves 5

Clinical Approach to U Wave Assessment

• Abnormal U waves are often subtle and rarely an isolated ECG abnormality, making them easy to overlook 1, 2
• When evaluating U waves, consider:
  • Polarity (normal vs. inverted)
  • Amplitude (normal vs. prominent)
  • Relationship to T wave (separate vs. fused)
  • Electrolyte status, particularly potassium levels 1, 2
• Improvement in the underlying condition (e.g., blood pressure reduction, renal transplantation, valve replacement, or coronary bypass) may convert negative U waves to upright 3