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:

• Bradycardia
• Severe hypokalaemia.

Prominent U waves may be present with:

• Hypocalcaemia
• Hypomagnesaemia
• Hypothermia
• Raised intracranial pressure
• Left ventricular hypertrophy
• Hypertrophic cardiomyopathy

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 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

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.

ECG Library Basics

References

• 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

Online

• Wiesbauer F, Kühn P. ECG Mastery: Yellow Belt online course. Understand ECG basics. Medmastery
• Wiesbauer F, Kühn P. ECG Mastery: Blue Belt online course: Become an ECG expert. Medmastery
• Kühn P, Houghton A. ECG Mastery: Black Belt Workshop. Advanced ECG interpretation. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.
• Wiesbauer F. Little Black Book of ECG Secrets. Medmastery PDF

Textbooks

• Zimmerman FH. ECG Core Curriculum. 2023
• Mattu A, Berberian J, Brady WJ. Emergency ECGs: Case-Based Review and Interpretations, 2022
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Hampton J, Adlam D. The ECG Made Practical 7e, 2019
• Kühn P, Lang C, Wiesbauer F. ECG Mastery: The Simplest Way to Learn the ECG. 2015
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest