Hypokalemia

Low potassium is a frequent occurrence among in-patients.  Common causes include low intake compounded by GI losses through diarrhea and renal losses from thiazides, loop-diuretics, and other medications. A favorite rare cause would be hypokalemic periodic paralysis.  When managing hypokalemia always check for hypomagnesemia, as they frequently go together and adequate K repletion requires Mg (remember ATP - check the image).

Low K may manifest as muscle weakness and mental status changes.  The ECG may show U waves, flat or inverted T waves and QT prolongation.  Be careful with patients on digoxin as dig cardiac toxicity is potentiated by hypokalemia.

Serum hypokalemia will usually only manifest once significant total body losses have occurred, as K+ is mainly an intracellular cation.  The ICU book  has an interesting chart showing that by the time serum K is 2.0 the total body K deficit is 600-900 mEq and may be more if there is acidosis.  Of course we don't replace all of the deficit at once, but remember to supplement enough over a few days (preferably orally or if you must via a central line iv in severe hypokalemia).

 To complement this quick overview I highly recommend this article by Drs Kamel and Halperin.