9 minute read
Sodium is water
Serum sodium is a concentration, so its disorders are almost always water disorders: ADH deciding how much water the kidney keeps. That is why every hyponatremia workup runs through osmolality (is it real?), urine osmolality (is ADH on?), and volume with urine sodium (does ADH have an excuse?).
Potassium is a gradient
Ninety-eight percent of potassium is intracellular; the serum level is the visible two percent. Insulin, catecholamines, and pH shift it across membranes fast (hence DKA's paradox and the rebound after shifting therapy), while total-body balance belongs to aldosterone and the distal nephron on a slower clock. The ECG, not the serum value alone, reads the gradient the myocardium actually feels.
The company electrolytes keep
Hypokalemia resists correction while magnesium is low. Calcium reads falsely low with hypoalbuminemia. Phosphate crashes during refeeding and DKA treatment. Electrolytes travel in packs; order and correct them accordingly.
Pearls
- Correct sodium slowly when it fell slowly: the brain adapted, and fast correction is the injury.
- Refractory hypokalemia is hypomagnesemia until proven otherwise.
Connected