Disorders of potassium homeostasis: Hypokalemia and hyperkalemia
Section snippets
Normal homeostasis
Potassium intake is unregulated and varies according to diet, from as little as 40 to over 100 mEq/d. Normally, excretion matches intake on a day-to-day basis, and little attention is paid to the nature of the diet. Ingested K+ is absorbed rapidly and must be moved quickly into cells to prevent hyperkalemia. Fig. 1 illustrates the homeostatic mechanisms that exist to achieve transcellular and total body K+ balance. As ingested K+ is absorbed and enters the portal circulation, it stimulates
Hypokalemia
Hypokalemia is defined at various values, but most commonly is defined as a serum [K+] < 3.6 mEq/L. Hypokalemia reflects a disruption in normal homeostasis, with one rare exception: In some patients with leukemia and markedly elevated white cell counts, K+ is taken up by the abnormal cells in the test tube, causing factitious hypokalemia [28]. More commonly, these abnormal white cells release K+, causing factitious hyperkalemia (see later in article).
Hyperkalemia
Hyperkalemia, defined as a serum [K+] greater than 5.0 mEq/L, is less common than hypokalemia and can be life threatening when severe (serum [K+] >6.5 mEq/L). An elevated serum [K+] is indicative of disordered K+ homeostasis, with the exception of several situations in which K+ is released in the blood sample (factitious hyperkalemia; Table 4). These settings include thrombocytosis, hemolysis, and extremely high white cell counts. In all three settings, death or lysis of cells in the test tube
Acute management
Regardless of the cause, therapy to lower serum [K+] should be initiated immediately when it is greater than 6.5 mEq/L or if the ECG shows signs of conduction abnormalities (see Fig. 3). If the duration of hyperkalemia is unknown or is likely to be short, therapy should be initiated at a Serum [K+] greater than 6.0 mEq/L. Table 6 presents the approach to treatment. Intravenous calcium rapidly can reverse cardiac conduction abnormalities when they are present and should be used without delay in
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