Hyperkalemia is a common clinical problem. Potassium enters the body via oral intake or intravenous infusion, is largely stored in the cells, and is then excreted in the urine. The major causes of hyperkalemia are increased potassium release from the cells and, most often, reduced urinary potassium excretion (table 1).
This topic will review the causes and evaluation of hyperkalemia. The clinical manifestations, treatment, and prevention of hyperkalemia, as well as a detailed discussion of hypoaldosteronism (an important cause of hyperkalemia), are presented elsewhere. (See "Clinical manifestations of hyperkalemia in adults" and "Treatment and prevention of hyperkalemia in adults" and "Etiology, diagnosis, and treatment of hypoaldosteronism (type 4 RTA)".)
BRIEF REVIEW OF POTASSIUM PHYSIOLOGY
An understanding of potassium physiology is helpful when approaching patients with hyperkalemia. Total body potassium stores are approximately 3000 meq or more (50 to 75 meq/kg body weight) . In contrast to sodium, which is the major cation in the extracellular fluid and has a much lower concentration in the cells, potassium is primarily an intracellular cation, with the cells containing approximately 98 percent of body potassium. The intracellular potassium concentration is approximately 140 meq/L compared with 4 to 5 meq/L in the extracellular fluid. The difference in distribution of the two cations is maintained by the Na-K-ATPase pump in the cell membrane, which pumps sodium out of and potassium into the cell in a 3:2 ratio.
The ratio of the potassium concentrations in the cells and the extracellular fluid is the major determinant of the resting membrane potential across the cell membrane, which sets the stage for the generation of the action potential that is essential for normal neural and muscle function. Thus, both hyperkalemia and hypokalemia can cause muscle paralysis and potentially fatal cardiac arrhythmias. (See "Clinical manifestations of hyperkalemia in adults" and "Clinical manifestations and treatment of hypokalemia in adults", section on 'Manifestations of hypokalemia'.)
The causes of hyperkalemia can be best understood after a brief review of normal potassium homeostasis. The plasma potassium concentration is determined by the relationship among potassium intake, the distribution of potassium between the cells and the extracellular fluid, and urinary potassium excretion.