On completion of the chapter, the reader will be able to:
Identify the body compartments where potassium and magnesium are highly concentrated.
Discuss the metabolic processes that tightly regulate potassium and magnesium homeostasis.
Determine the causes of disorders of potassium and magnesium homeostasis, including those related to medications.
Identify the common clinical presentations associated with disorders of potassium and magnesium homeostasis.
Develop rational pharmacotherapy treatment and monitoring plans for hypo- and hyperkalemia and hypo- and hypermagnesemia.
Potassium and magnesium are electrolytes that are responsible for numerous metabolic activities. Disorders of these electrolytes are frequently seen in both the acute care and community ambulatory care settings. Therefore, clinicians need a firm understanding of the etiology, pathophysiology, symptoms, pharmacotherapy, and monitoring of these disorders. This chapter describes the homeostatic mechanisms that are responsible for the maintenance of normal potassium and magnesium serum concentrations. The clinical disorders responsible for the development of hyperkalemia, hypermagnesemia, hypokalemia, and hypomagnesemia are also reviewed.
Potassium is the most abundant cation in the body, with estimated total-body stores of 3,000 to 4,000 mEq (3,000 to 4,000 mmol).1 Ninety-eight percent of this amount is contained within the intracellular compartment, and the remaining 2% is distributed within the extracellular compartment. The sodium-potassium adenosine triphosphatase (Na+-K+-ATPase) pump located in the cell membrane is responsible for the compartmentalization of potassium. This pump is an active transport system that maintains increased intracellular stores of potassium by transporting sodium out of the cell and potassium into the cell at a ratio of 3:2. Consequently, the pump maintains a higher concentration of potassium inside the cell.
The normal serum concentration range for potassium is 3.5 to 5 mEq/L (3.5 to 5 mmol/L), whereas the intracellular potassium concentration is usually approximately 150 mEq/L (150 mmol/L).2 Approximately 75% of the intracellular potassium is located in skeletal muscle; the remaining 25% is located in the liver and red blood cells. Extracellular potassium is distributed throughout the serum and interstitial space. Potassium is dynamic in that it is constantly moving between the intracellular and extracellular compartments according to the body’s needs. Thus, the serum potassium concentration alone does not accurately reflect the total-body potassium content.
Potassium has many physiologic functions within cells, including protein and glycogen synthesis and cellular metabolism and growth. It is also a determinant of the electrical action potential across the cell ...