Potassium

Potassium is the most abundant cation in the body and has a much higher concentration in the intracellular space than in extracellular fluids. Normal potassium levels are maintained despite fluctuating potassium intake by adjustments in renal secretion of potassium. Hyperkalemia is defined as a serum potassium level greater than 5.1 mmol/L. Occasionally, hyperkalemia can be an artifact (pseudohyperkalemia) of phlebotomy, associated with thrombocytosis, leukocy-tosis, or hemolysis during phlebotomy. In a patient with hyperkalemia of no apparent cause, a plasma potassium level can eliminate these effects on the potassium measurement. Because the normal response to increased potassium intake is to increase excretion, hyperkalemia is not likely to be attributed to increased intake unless there is a deficiency in potassium excretion. Shifts of potassium from intracellu-lar to extracellular fluids, such as with acute metabolic aci-dosis, crush injury, burns, insulin deficiency, beta-adrenergic blockade, and hemolysis, can be associated with a transient hyperkalemia. Persistent hyperkalemia is usually associated with decreased potassium excretion. Potassium excretion by the kidney is flow dependent; therefore, oliguria and anuria are important causes of hyperkalemia. Because aldosterone deficiency is an important cause of decreased potassium excretion, hyperkalemia is seen with hyporeninism, hypoal-dosteronism, type 4 renal tubular acidosis, and drugs that inhibit aldosterone (Table 15-20).

Hypokalemia is associated with a serum potassium level of less than 3.5 mmol/L. Symptoms are nonspecific and include muscular weakness. Occasionally, hypokalemia is associated with sustained inadequate potassium intake, particularly in patients with alcohol abuse. Transient episodes of hypokale-mia are associated with increased extracellular to intracellu-lar potassium shifts and occur with catecholamine increase, hyperinsulinemia, and adrenergic drugs such as bronchodi-lators. More frequently, hypokalemia is a result of GI loss of potassium, such as with protracted vomiting, diarrhea, and laxative abuse. Other causes of hypokalemia include drugs, metabolic alkalosis, skin losses, and increased urinary losses. Hypomagnesemia is an important cause of refractory hypo-kalemia.

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