Potassium Disorders: Hypo- and Hyperkalemia

Potassium is the main intracellular cation in all cells and is distributed unevenly between the intracellular fluid (98%) and extracellular fluid (2%). This large disparity is necessary for maintaining the resting membrane potential of cells, and explains why K+ balance is tightly regulated. The GI tract secretes 5%–10% of the absorbed K+ daily; however, the kidneys are responsible for 90%–95% of the overall K+ regulation. While most of the K+ is reabsorbed in the proximal tubules, the majority of regulation occurs in the principal and α-intercalated cells of the collecting ducts. The most important regulatory mechanisms include aldosterone, plasma K+ concentration, distal urinary flow rate, and the distal delivery of Na+ and water. Hyperkalemia and hypokalemia can result when K+ regulation is abnormal.

Hypokalemia is defined as plasma potassium (K+) concentration < 3.5 mEq/L. Homeostatic mechanisms maintain plasma concentration between 3.5–5.2 mEq/L despite marked variation in dietary intake. Hypokalemia can be due to renal losses, GI losses, transcellular shifts, or poor dietary intake. If minor in severity, hypokalemia is usually asymptomatic. However, acute reductions in K+ level or severe hypokalemia can lead to cardiac arrhythmias, muscle weakness, rhabdomyolysis, paralysis, and respiratory failure. Diagnosis is by clinical history and lab testing. Management is guided by severity and includes treating urgent symptoms, replacing the K+ deficit, and treating the underlying cause.

Hyperkalemia is defined as a serum potassium (K+) concentration > 5.2 mEq/L. Homeostatic mechanisms maintain the serum K+ concentration between 3.5 and 5.2 mEq/L, despite marked variation in dietary intake. Hyperkalemia can be due to a variety of causes, which include transcellular shifts, tissue breakdown, inadequate renal excretion, and drugs. Hyperkalemia is usually asymptomatic if minor in severity; however, acute elevations or severe hyperkalemia can lead to potentially fatal cardiac arrhythmias. Management is guided by severity and includes measures to stabilize the myocardial membrane potential, transiently shifting K+ intracellularly, removing K+ from the body, and treating the underlying predisposing conditions.