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Introduction:

The U wave is the only remaining enigma of the ECG, and probably not for long. The origin of the U wave is still in question, although most authorities correlate the U wave with electrophysiologic events called "afterdepolarizations" in the ventricles. These afterdepolarizations can be the source of arrhythmias caused by "triggered automaticity" including torsade de pointes. The normal U wave has the same polarity as the T wave and is usually less than one-third the amplitude of the T wave. U waves are usually best seen in the right precordial leads especially V2 and V3. The normal U wave is asymmetric with the ascending limb moving more rapidly than the descending limb (just the opposite of the normal T wave).

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Differential Diagnosis of U Wave Abnormalities

 Prominent upright U waves

 Sinus bradycardia accentuates the U wave

 Hypokalemia (remember the triad of ST segment depression, low amplitude T waves, and prominent U waves)

 Quinidine and other type 1A antiarrhythmics

 CNS disease with long QT intervals (often the T and U fuse to form a giant "T-U fusion wave")

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(E.g., lead II, III, V4-6)

 LVH (right precordial leads with deep S waves)

 Mitral valve prolapse (some cases)

 Hyperthyroidism



 Negative or "inverted" U waves

 Ischemic heart disease (often indicating left main or LAD disease)

 Myocardial infarction (in leads with pathologic Q waves)

 During episode of acute ischemia (angina or exercise-induced ischemia)

 Post extrasystolic in patients with coronary heart disease

 During coronary artery spasm (Prinzmetal's angina)



 Nonischemic causes

 Some cases of LVH or RVH (usually in leads with prominent R waves)

 Some patients with LQTS (see below: Lead V6 shows giant negative TU fusion wave in patient with LQTS; a prominent upright U wave is seen in Lead V1)

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