12. Nice Seeing "U" Again

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).

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")

  

• 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)

  

Test your knowledge on lesson 12

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