characters of normal U waves
It is usually recorded in the same direction as that of T waves
If it is opposite it should be given significance .(Ischemic U waves ?)
More prominent in bradycardia
Mechanism of U waves
There are Three hypothesis
- Repolarization of papillary muscle
- Late repolarization of M cells in Mid myocardium
- After potential to QRS ie a form of depolarization
Currently the third option appears closer to truth
How hypokalemia result in tall U waves ?
If Hyperkalemia elevates T waves it is natural to expect hypokalemia to blunt , flatten or Invert the T waves .Of course , this happens in many . In others with severe hypokalemia an intact T is accompanied by a tall upright U as well .This adds a mystery twist* to the exact genesis of this wave .
* During diastole cell membrane is closer to k + if K + is deficient it tends to drift . (Some where i heard this . . . but I do not understand it fully !)
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Reference
Dr.S.Venkatesan MD 
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