U Wave

U wave Overview

The U wave is a small (0.5 mm) deflection immediately following the T wave

  • U wave is usually in the same direction as the T wave.
  • U wave is best seen in leads V2 and V3.

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 described as prominent if they are

  • >1-2mm or 25% of the height of the T wave.

Causes of prominent U waves

Prominent U waves most commonly found with:

Prominent U waves may be present with:

Drugs associated with prominent U waves:

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

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

Prominent U waves due to sinus bradycardia

U waves associated with hypokalaemia
U waves associated with hypokalaemia potassium 1.9
Prominent U waves in a patient with a K+ of 1.9

U waves associated with left ventricular hypertrophy

U waves associated with digoxin use

U waves associated with quinidine use

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

Common causes of inverted U waves

  • 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

Unstable angina
  • Inverted U waves in a patient with unstable angina. Reproduced from Girish et al.

Inverted U waves in Prinzmetal angina

  • 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.

ECG Library Basics

  • 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
  • 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. PMID: 18810715

Advanced Reading



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Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

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