The Cancun congress on EM had a great lecture available for download on electrolyte emergencies. I had wanted to look at these when I was on medicine, but they had a, well, different approach that took forever. This is much more EM specific.
Hyperkalemia is the most dangerous abnormality; the most common cause is 'not', that is, a lab draw error, hemolysis with the draw, and so on. An EKG guides how we treat the patient, so the first step in a work-up is re-draw and get an EKG.
Causes: Not; renal failure with acidosis; drugs over weeks or months (ACEI and ARBs, NSAIDs; and cell death (burns, tumor lysis). There are other causes, but these are the big ones.
EKG changes: tall T, loss of P, QRS widening, PR lengthening, sine wave (um, bad).
Treatment: calcium is only used in an emergency, defined by a widened QRS on EKG. Calcium for QRS (calcium chloride, it's faster), 2 amps of d50 and 10 U insulin, bicarb if acidotic but only if acidotic, albuterol nebs, fluid if hypovolemic.
hypokalemia=prolonged QT with U waves.
hypokalemia=hypomagnesemia, and K always goes with mag. We must replete both. If you don't give mag the K will stay intravascular and be excreted. The deficit is always worse than you think.
So, if you give K, give mag. Treat with EKG changes.
Hyponatremic emergencies manifest as altered mental status. Causes include runners drinking too much, women more than men, X use with dancing all night--water intoxication and hyponatremia. The big worry with hyponatremia is seizures and the AMS; the big worry with repletion is central pontine myelinolysis, or the so called 'locked-in syndrome', which is truly awful. Never correct a patient with symptomatic hyponatremia faster than 0.5 an hour or 10-12 a day. Seizures with Na below 120, like 100 or 110; intial 3% NaCl MAX 200 cc total, start with 100 cc bolus over 10 minutes. Must be previously healthy. So, the gorked out nursing home resident on diuretics and who knows what else who comes in 'just not themselves' with a sodium of 118 should not get 3% NaCl. The 19-year-old clubbing person with an empty water bottle seizing at 5 am with a sodium of 105 should, in two boluses, as above.
See? Much easier than the old hypovolemic-euvolemic-hypervolemic triad just for sodium and so on for each electrolyte...see this post.