XII. Nice Seeing "U" Again
Frank G. Yanowitz, MD
Professor of Medicine
University of Utah School of Medicine
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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