Potassium is the most abundant intracellular cation, but only about 2% of total body potassium is extracellular. Because most intracellular potassium is contained within muscle cells, total body potassium is roughly proportional to lean body mass. An average 70-kg adult has about 3500 mEq (3500 mmol) of potassium.
Potassium is a major determinant of intracellular osmolality. The ratio between potassium concentration in the intracellular fluid (ICF) and concentration in the extracellular fluid (ECF) strongly influences cell membrane polarization, which in turn influences important cell processes, such as the conduction of nerve impulses and muscle (including myocardial) cell contraction. Thus, relatively small alterations in serum potassium concentration can have significant clinical manifestations. Total serum potassium concentration may be
Clinical manifestations of disorders of potassium concentration can involve muscle weakness and cardiac arrhythmias Overview of Arrhythmias The normal heart beats in a regular, coordinated way because electrical impulses generated and spread by myocytes with unique electrical properties trigger a sequence of organized myocardial... read more .
In the absence of factors that shift potassium in or out of cells, the serum potassium concentration correlates closely with total body potassium content. Once intracellular and extracellular concentrations are stable, a decrease in serum potassium concentration of about 1 mEq/L (1 mmol/L) indicates a total potassium deficit of about 200 to 400 mEq (200 to 400 mmol). Patients with stable potassium concentration < 3 mEq/L (< 3 mmol/L) typically have a significant potassium deficit.
Factors that shift potassium in or out of cells include the following:
Insulin moves potassium into cells; high concentrations of insulin thus lower serum potassium concentration. Low concentrations of insulin, as in diabetic ketoacidosis Diabetic Ketoacidosis (DKA) Diabetic ketoacidosis (DKA) is an acute metabolic complication of diabetes characterized by hyperglycemia, hyperketonemia, and metabolic acidosis. Hyperglycemia causes an osmotic diuresis with... read more , cause potassium to move out of cells, thus raising serum potassium, sometimes even in the presence of total body potassium deficiency.
Beta-adrenergic agonists, especially selective beta 2-agonists, move potassium into cells, whereas beta-blockade and alpha-agonists promote movement of potassium out of cells.
Acute metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3−), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly subnormal... read more causes potassium to move out of cells, whereas acute metabolic alkalosis Metabolic Alkalosis Metabolic alkalosis is primary increase in bicarbonate (HCO3−) with or without compensatory increase in carbon dioxide partial pressure (Pco2); pH may be high or nearly normal. Common causes... read more causes potassium to move into cells. However, changes in serum bicarbonate concentration may be more important than changes in pH; acidosis caused by accumulation of mineral acids (nonanion gap, hyperchloremic acidosis) is more likely to elevate serum potassium. In contrast, metabolic acidosis due to accumulation of organic acids (increased anion gap Calculation of the anion gap Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3−) that typically produce abnormal arterial pH values. Acidemia is serum pH 7... read more acidosis) does not cause hyperkalemia. Thus, the hyperkalemia common in diabetic ketoacidosis results more from insulin deficiency than from acidosis.
Acute respiratory acidosis Respiratory Acidosis Respiratory acidosis is primary increase in carbon dioxide partial pressure (Pco2) with or without compensatory increase in bicarbonate (HCO3−); pH is usually low but may be near normal. Cause... read more and respiratory alkalosis Respiratory Alkalosis Respiratory alkalosis is a primary decrease in carbon dioxide partial pressure (Pco2) with or without compensatory decrease in bicarbonate (HCO3−); pH may be high or near normal. Cause is an... read more affect serum potassium concentration less than metabolic acidosis and metabolic alkalosis. Nonetheless, serum potassium concentration should always be interpreted in the context of the serum pH (and bicarbonate concentration).
Dietary potassium intake normally varies between 40 and 150 mEq (40 and 150 mmol)/day. In the steady state, fecal losses are usually close to 10% of intake. The remaining 90% is excreted in the urine, so alternations in renal potassium secretion greatly affect potassium balance.
When potassium intake is > 150 mEq (> 150 mmol)/day, about 50% of the excess potassium appears in the urine over the next several hours. Most of the remainder is transferred into the intracellular compartment, thus minimizing the rise in serum potassium. When elevated potassium intake continues, aldosterone secretion is stimulated and thus renal potassium excretion rises. In addition, potassium absorption from stool appears to be under some regulation and may fall by 50% in chronic potassium excess.
When potassium intake falls, intracellular potassium again serves to buffer wide swings in serum potassium concentration. Renal potassium conservation develops relatively slowly in response to decreases in dietary potassium and is far less efficient than the kidneys’ ability to conserve sodium. Thus, potassium depletion is a frequent clinical problem. Urinary potassium excretion of 10 mEq (10 mmol) /day represents near-maximal renal potassium conservation and implies significant potassium depletion.
Acute acidosis impairs potassium excretion, whereas chronic acidosis and acute alkalosis can promote potassium excretion. Increased delivery of sodium to the distal nephrons, as occurs with high sodium intake or loop diuretic therapy, promotes potassium excretion.
Pseudohypokalemia, or falsely low serum potassium, occasionally is found when blood specimens from patients with chronic myeloid leukemia and a white blood cell count > 100,000/mcL (100 x 109/L) remain at room temperature before being processed because abnormal leukocytes in the sample take up serum potassium. It is prevented by prompt separation of plasma or serum in blood samples.
Pseudohyperkalemia, or falsely elevated serum potassium, is more common, typically occurring due to hemolysis and release of intracellular potassium. To prevent false results, phlebotomy personnel should not rapidly aspirate blood through a narrow-gauge needle or excessively agitate blood samples. Pseudohyperkalemia can also result from platelet count > 400,000/mcL (> 400 × 109/L) due to release of potassium from platelets during clotting; in these cases, the plasma potassium (unclotted blood), as opposed to serum potassium, is normal.