A number of hypotheses have been advanced to explain the genesis of the U wave. Foremost among them is the relationship to late repolarization of the Purkinje system. A criticism of this hypothesis is that the conducting system does not have sufficient mass to generate a large deflection at the body surface. The recent identification of another population of (M) cells between epicardium and endocardium may provide the necessary mass to produce not only U waves but also the J (or Osborn) wave characteristic of hypothermia.44 What sometimes appears to be a U wave merging with a T wave simple may be a notched T wave whose ascending or descending limbs are interrupted by differences in the end of the composite action potential of epicardial and M cells.14 The normal U wave, most prominent in leads V2 and V3, has the same polarity as the T wave and is approximately 10 percent of its amplitude. A large positive U wave may be due to hypokalemia and multiple antiarrhythmic drugs. In orthodox ECG interpretation, merging of T and U is still considered a stage in hypokalemia but can result from such drugs as quinidine and sotalol.14 According to Antzelevitch, repolarization of the His-Purkinje system was first suggested by Watanabe as the most likely cause of the "real" U wave.14 Causes of negative U waves are ischemia, hypertension, and occasionally, right ventricular enlargement.45


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