What is the U wave?
Small 0.5mm deflection following T wave: and best seen in V2 and V3
Usually in the same direction as T wave
Usually better visible at slow heart rates < 65 bpm
Grows bigger as HR decreases
Usually < ¼ of the T wave voltage, if much bigger (or >2mm)- its abnormal
Where is it coming from?
May be Purkinje fibers repolarization
May be some kind of after-potential
No one really knows
Abnormal U waves:
Prominent U wave - >1-2mm or 25% of the height of the T wave
Common causes:
Bradycardia
Severe hypokalemia.
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)
Hypokalemia
U waves associated with left ventricular hypertrophy
U waves associated with digoxin use
Inverted U waves:
A negative U wave is highly specific for the presence of heart disease
Common causes of inverted U waves
Early MI
Coronary artery disease
Hypertension
Valvular heart disease
Congenital heart disease
Cardiomyopathy
Hyperthyroidism
Inverted U waves in a patient with NSTEMI
References: UpToDate. LITFL, ecg.utah.edu, ecg weekly