ECG

 

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

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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

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Abnormalities of the U wave

• Prominent U waves
• Inverted U waves

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Prominent U waves

U waves are described as prominent if they are

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

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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)

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

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Prominent U waves due to sinus bradycardia

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U waves associated with hypokalaemia

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

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U waves associated with left ventricular hypertrophy

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U waves associated with digoxin use

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U waves associated with quinidine use

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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

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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

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Unstable angina

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

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Inverted U waves in Prinzmetal angina

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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.