U Wave

The U wave is a small (0.5 mm) deflection immediately following the T wave, usually in the same direction as the T wave. It is best seen in leads V2 and V3.

waves of the ecg

Normal U Wave

Normal U Wave

Source of the U wave

The source of the U wave is unknown. Three common theories regarding its origin are:

  • Delayed repolarisation of Purkinje fibres
  • Prolonged repolarisation of mid-myocardial “M-cells”
  • After-potentials resulting from mechanical forces in the ventricular wall

Features of Normal U waves

  • The U wave normally goes in the same direction as the T wave
  • U -wave size is inversely proportional to heart rate: the U wave grows bigger as the heart rate slows down
  • U waves generally become visible when the heart rate falls below 65 bpm
  • The voltage of the U wave is normally < 25% of the T-wave voltage: disproportionally large U waves are abnormal
  • Maximum normal amplitude of the U wave is 1-2 mm

Abnormalities of the U wave

  • Prominent U waves
  • Inverted U waves

Prominent U waves

  • U waves are prominent if > 1-2mm or 25% of the height of the T wave.
  • The most common cause of prominent U waves is bradycardia.
  • Abnormally prominent U waves are characteristically seen in severe hypokalaemia.

Prominent U waves may also be seen with:

The following drugs may cause prominent U waves:

  • Digoxin
  • Phenothiazines (thioridazine)
  • Class Ia antiarrhythmics (quinidine, procainamide)
  • Class III antiarrhythmics (sotalol, amiodarone)

Note that many of the conditions causing prominent U waves will also cause a long QT.

Prominent U waves due to sinus bradycardia

Prominent U waves in a patient with marked sinus bradycardia due to anorexia nervosa

Prominent U waves due to hypokalaemia

Prominent U wave hypokalaemia

Prominent U waves in a patient with a K+ of 1.9

Prominent U waves due to digoxin

Prominent U waves due to digoxin

Prominent U waves in a patient taking digoxin

Prominent U waves due to quinidine

Prominent U waves due to quinidine

Prominent U waves in a patient receiving quinidine

Inverted U waves

  • U-wave inversion is abnormal (in leads with upright T waves)
  • A negative U wave is highly specific for the presence of heart disease

The main causes of inverted U waves are:

  • Coronary artery disease
  • Hypertension
  • Valvular heart disease
  • Congenital heart disease
  • Cardiomyopathy
  • Hyperthyroidism

In patients presenting with chest pain, inverted U waves:

  • Are a very specific sign of myocardial ischaemia
  • May be the earliest marker of unstable angina and evolving myocardial infarction
  • Have been shown to predict a ≥ 75% stenosis of the LAD / LMCA and the presence of left ventricular dysfunction

Inverted U waves due to unstable angina

Inverted U waves due to ischaemia

Inverted U waves in a patient with unstable angina. Image reproduced from Girish et al. (click for source)

Inverted U waves due to Prinzmetal’s angina

Inverted U waves in a patient with Prinzmetal's angina

Inverted U waves in a patient with Prinzmetal’s angina. Image reproduced from Pérez Riera et al. (click for source)

Inverted U waves due to NSTEMI

Note the subtle U-wave inversion in the lateral leads (I, V5 and V6) in this patient with a NSTEMI; these were the only abnormal findings on his ECG.

Inverted U waves in a patient with a NSTEMI

Inverted U waves in a patient with a NSTEMI

Further Reading

Author Credits

References

  • Gerson MC, McHenry PL. Resting U wave inversion as a marker of stenosis of the left anterior descending coronary artery. Am J Med. 1980 Oct;69(4):545-50. PMID: 7424944
  • Girish MP, Gupta MD, Mukhopadhyay S, Yusuf J, Sunil Roy TN, Trehan V. U wave: an important noninvasive electrocardiographic diagnostic marker. Indian Pacing Electrophysiol J. 2005 Jan 1;5(1):63-5. PMID: 16943944
  • Pérez Riera AR, Ferreira C, Filho CF, Ferreira M, Meneghini A, Uchida AH, Schapachnik E, Dubner S, Zhang L. The enigmatic sixth wave of the electrocardiogram: the U wave. Cardiol J. 2008;15(5):408-21. Review. PMID: 18810715
  • Sovari AA, Farokhi F, Kocheril AG. Inverted U wave, a specific
    electrocardiographic sign of cardiac ischemia. Am J Emerg Med. 2007
    Feb;25(2):235-7. PMID: 17276833
  • Zimmerman FH. Clinical Electrocardiography: Pre-Test Self-Assessment and Review, McGraw-Hill 1994.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
  • Wikipedia. The U Wave. http://en.wikipedia.org/wiki/U_wave
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