ECG Academy

ECG Basic – U Wave

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Appcardio ECG Academy · ECG Basics

U Wave on ECG: Recognition and Clinical Significance

The U wave is a small deflection that follows the T wave. Learning to identify its normal appearance—and to recognize prominent or inverted U waves—can add useful context when interpreting a 12-lead electrocardiogram.

Foundational–Intermediate Approx. 7-minute read ECG waveform interpretation
Schematic ECG showing the U wave A stylized ECG waveform labels the QRS complex, T wave, and small U wave that follows the T wave. QRST waveU wave Schematic waveform — not a diagnostic tracing

Learning Objectives

  • Identify the U wave and distinguish it from the T wave.
  • Describe the usual polarity, amplitude, and lead distribution of a normal U wave.
  • Recognize prominent and inverted U-wave patterns.
  • Recall clinical settings and medications associated with U-wave abnormalities.
  • Interpret U-wave changes in context rather than in isolation.

1. What Is the U Wave?

The U wave is a small deflection that appears after the T wave and before the next P wave. It is often subtle and may be difficult to distinguish when the heart rate is relatively fast or when the preceding T wave is broad.

Labeled schematic of the ECG U wave A schematic ECG cycle labels the P wave, QRS complex, T wave, and U wave, with the U wave positioned after the T wave. PQRSTU Conceptual illustration; waveform shape and amplitude are not to scale.

The U wave follows the T wave. Its small size and close proximity to the T wave can make it easy to overlook or mislabel.

In the supplied ECG reference, the U wave is described as approximately 0.5 mm in a typical example, usually directed in the same direction as the T wave, and most readily seen in the anterior precordial leads V2 and V3.

Practical recognition: Look just after the end of the T wave. Confirm that the deflection is a separate, small wave rather than part of a notched or biphasic T wave.

2. Proposed Origin of the U Wave

The precise electrophysiological origin of the U wave remains uncertain. The supplied reference outlines three commonly discussed hypotheses:

Delayed Purkinje repolarization

One proposed explanation is delayed repolarization of Purkinje fibres after the main ventricular repolarization represented by the T wave.

Mid-myocardial M-cells

Another hypothesis attributes the deflection to prolonged repolarization in mid-myocardial cells, sometimes termed M-cells.

Mechanical after-potentials

A third proposal is that the U wave reflects after-potentials related to mechanical forces or stretch within the ventricular wall.

These are proposed mechanisms, not a settled explanation. The surface ECG alone cannot establish which mechanism is responsible for an individual U wave.

3. Features of Normal U Waves

FeatureTypical description in the source
TimingImmediately follows the T wave.
PolarityUsually points in the same direction as the T wave.
Lead distributionOften easiest to see in V2 and V3.
AmplitudeUsually less than 25% of the T-wave amplitude; the source gives a maximum normal amplitude of approximately 1–2 mm.
Heart rate relationshipU waves become more visible as the heart rate slows; the source notes they are generally visible below approximately 65 beats/min.
Remember the proportion: A U wave that is disproportionately large relative to the T wave should prompt a search for associated clinical or ECG abnormalities.

Amplitude thresholds are descriptive rather than stand-alone diagnostic criteria. Assess calibration, lead placement, baseline quality, heart rate, and the overall tracing before labeling a U wave abnormal.

4. Abnormal U Waves

The supplied reference focuses on two principal abnormalities: prominent U waves and inverted U waves.

Prominent U waves

A U wave may be considered prominent when its amplitude exceeds roughly 1–2 mm or reaches at least 25% of the T-wave height, as described in the source.

Inverted U waves

A negative U wave is abnormal when it occurs in a lead where the T wave is upright. It warrants attention to possible underlying cardiac disease, particularly in a patient with chest discomfort.

5. Prominent U Waves: Causes and Associations

Commonly emphasized associations

  • Bradycardia.
  • Severe hypokalaemia.

Other reported clinical settings

  • Hypocalcaemia and hypomagnesaemia.
  • Hypothermia.
  • Raised intracranial pressure.
  • Left ventricular hypertrophy.
  • Hypertrophic cardiomyopathy.

Medications associated with prominent U waves

Drug groupExamples listed in the source
Cardiac glycosideDigoxin
PhenothiazineThioridazine
Class Ia antiarrhythmicsQuinidine, procainamide
Class III antiarrhythmicsSotalol, amiodarone
Clinical context matters: Several conditions associated with prominent U waves may also produce QT prolongation. Review the QT interval, electrolytes, temperature, heart rate, and medication list; do not attribute the finding to a single cause from the ECG alone.

Clinical examples illustrated in the source

The source includes examples of prominent U waves in sinus bradycardia, severe hypokalaemia (including a patient with potassium reported as 1.9), left ventricular hypertrophy, and during digoxin or quinidine use. These examples illustrate associations; they do not establish a diagnosis without clinical and laboratory correlation.

6. Inverted U Waves and Ischaemia

U-wave inversion is described as abnormal when the T wave remains upright in the same lead. The supplied reference lists several associated conditions:

  • Coronary artery disease.
  • Hypertension.
  • Valvular heart disease.
  • Congenital heart disease.
  • Cardiomyopathy.
  • Hyperthyroidism.
Chest-pain context: In a patient presenting with chest pain, an inverted U wave—especially when new or dynamic—may support concern for myocardial ischaemia. The source discusses its potential association with unstable angina, evolving myocardial infarction, significant LAD/left-main disease, and left ventricular dysfunction.

The source also presents examples of U-wave inversion in unstable angina, Prinzmetal (vasospastic) angina, and NSTEMI. In the NSTEMI example, subtle inversion in lateral leads I, V5, and V6 was the only reported ECG abnormality.

Do not use in isolation: U-wave inversion is not, by itself, proof of acute coronary occlusion or a substitute for standard ACS assessment. Interpret it alongside symptoms, serial ECGs, cardiac biomarkers, haemodynamic status, and the complete ECG pattern. A normal-appearing U wave does not exclude ischaemia.

7. A Practical Approach to the U Wave

  1. Identify the T-wave endpoint. Look for a distinct small deflection after it.
  2. Check the lead. The U wave is often most apparent in V2–V3; compare adjacent leads.
  3. Assess polarity. Determine whether the U wave follows the T-wave direction or is inverted despite an upright T wave.
  4. Assess relative size. Compare U-wave amplitude with the T wave and note whether it appears disproportionately prominent.
  5. Review the context. Consider heart rate, potassium and other electrolytes, temperature, LVH/cardiomyopathy, intracranial pathology, and medications.
  6. Integrate with the whole ECG and clinical picture. Pay particular attention to QT measurement and possible ischaemic changes in patients with chest pain.
Knowledge check: a small wave follows an upright T wave in V2. What should you assess next?

First determine whether it is a discrete U wave and assess its amplitude and polarity. A small upright deflection may be a normal U wave. Then consider heart rate and the overall tracing; prominence or inversion should trigger a contextual review rather than an isolated diagnosis.

Knowledge check: a patient with chest pain has new U-wave inversion in lateral leads. How should this be interpreted?

It is a potentially important abnormal finding that may support myocardial ischaemia in the appropriate setting. Promptly integrate it with symptoms, serial ECGs, biomarkers, and the patient's clinical status; do not treat the U wave alone as diagnostic.

Key Takeaways

  1. The U wave is a small deflection after the T wave and is often best seen in V2–V3.
  2. It is usually concordant with the T wave and becomes more apparent at slower heart rates.
  3. A disproportionately large U wave is associated particularly with bradycardia and severe hypokalaemia, but has a broad differential.
  4. U-wave inversion in a lead with an upright T wave is abnormal and may be relevant to ischaemia in patients with chest pain.
  5. Always interpret U-wave changes with the full ECG, clinical presentation, electrolyte results, and medication history.

References

Burns E. U Wave. Life in the Fast Lane (LITFL) ECG Library. Published August 1, 2020. Primary educational source supplied for this lesson. https://litfl.com/u-wave-ecg-library/
Sovari AA, Farokhi F, Kocheril AG. Inverted U wave, a specific electrocardiographic sign of cardiac ischemia. American Journal of Emergency Medicine. 2007;25(2):235–237. PMID: 17276833. PubMed
Gerson MC, McHenry PL. Resting U wave inversion as a marker of stenosis of the left anterior descending coronary artery. American Journal of Medicine. 1980;69(4):545–550. PMID: 7424944. PubMed
Girish MP, Gupta MD, Mukhopadhyay S, et al. U wave: an important noninvasive electrocardiographic diagnostic marker. Indian Pacing and Electrophysiology Journal. 2005;5(1):63–65. PMID: 16943944. PubMed
Pérez Riera AR, Ferreira C, Filho CF, et al. The enigmatic sixth wave of the electrocardiogram: the U wave. Cardiology Journal. 2008;15(5):408–421. PMID: 18810715. PubMed
Medical education disclaimer: This page is intended for educational purposes only. It does not replace clinical assessment, professional judgement, local protocols, or current guideline-directed care. ECG findings must be interpreted in the context of the individual patient and the complete clinical picture.
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