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 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:
- Raised intracranial pressure
- Left ventricular hypertrophy
- Hypertrophic cardiomyopathy
The following drugs may cause prominent U waves:
- 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
U waves associated with hypokalaemia
- Prominent U waves in a patient with a K+ of 1.9
U waves associated with digoxin use
- Prominent U waves in a patient taking digoxin
U waves associated with quinidine use
- 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
- Valvular heart disease
- Congenital heart disease
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 in a patient with unstable angina.
- Image reproduced from Girish et al.
- Inverted U waves in a patient with Prinzmetal’s angina.
- Image reproduced from Pérez Riera et al.
- 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.
- 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. Review. PMID: 18810715
- Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography
- Hampton, JR. The ECG In Practice, 6e
- Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
- Wagner, GS. Marriott’s Practical Electrocardiography 12e
- Chan, TC. ECG in Emergency Medicine and Acute Care
- Wang, K. Atlas of Electrocardiography
- Mattu, A. ECG’s for the Emergency Physician
LITFL Further Reading
- ECG BASICS — Waves, Intervals, Segments and Clinical Interpretation
- ECG A to Z by diagnosis –alphabetical diagnostic approach to the ECG
- ECG CLINICAL CASES — ECG’s placed in clinical context with a challenging Q&A approach
- 100 ECG Quiz — Self-assessment tool for examination practice
- ECG Reference SITES and BOOKS — the best of the rest
- LITFL ECG IMAGE Database — Searchable database of LITFL ECG’s
- ECG and Cardiology Eponymous Syndromes — Cheats guide to eponymous emancipation
- ECG Exam Template — a framework for answering ECG exam questions.