Critically ill patients have disturbances in multiple organ systems and hence are at risk for serious imbalances in electrolyte and water handling. Electrolyte disorders are extremely prevalent in the ICU: Hyponatremia can be found in 18% of ICU patients,1 hypokalemia in 21%,2 hypomagnesemia in more than 50%,3 hypocalcemia in 20%,4 and hypophosphatemia in 28%.5 This chapter will outline these disturbances with an emphasis on pathophysiology and treatment of electrolyte problems more commonly seen in the intensive care unit.
Plasma sodium (Na) concentration is the principal determinant of the relative volume of extracellular fluid.6 It is the major extracellular solute, whereas potassium is the major intracellular solute. Total solute activity, also called osmotic activity and expressed in osmoles (osm), is the sum of the individual osmotic activities of all the solute particles in the solution. For monovalent ions, the osmotic activity in milliosmoles (mOsm) per unit volume is equivalent to the concentration of the ions in milliequivalents (mEq) per unit volume.
Example: Osmotic activity in isotonic saline (0.9% NaCl)
0.9% NaCl = 154 mEq Na/L + 154 mEq Cl/L = 308 mOsm/L
Osmolarity is the osmotic activity per volume of solution, whereas osmolality is the osmotic activity per kilogram of water (mOsm/kg H2O). Osmolality is used to describe the osmotic activity of body fluids. Since the weight of body fluids is basically equal to the weight of water, there is little difference between osmolality and osmolarity of body fluids. The effective osmolality or osmotic activity in 2 solutions is called tonicity. Water passes from the solution of lower osmotic activity to the solution of higher osmotic activity. The tendency for water to move into and out of cells is determined by the relative tonicity of intracellular and extracellular fluids. Because of the ability of water to pass freely across membranes, the intracellular and extracellular space have equivalent osmolality, normally about 280 mOsm/kg (Fig. 2-1). However, the intracellular space comprises a larger percentage of total body water (about 60%).
The relative volumes of intracellular and extracellular compartments of a 70-kg man. TBW = total body water.
Urea is freely permeable across cell membranes. An increase in the urea concentration in extracellular fluid increases the osmolality of the extracellular fluid but does not increase the tonicity of the extracellular fluid or cause a net movement of water out of cells. Azotemia (increase in blood urea nitrogen [BUN]) is a hyperosmotic condition but not a hypertonic condition.
Major solutes in extracellular fluid include sodium, chloride, glucose, and urea. Plasma osmolality is estimated by the following equation:
Solutes other than sodium, chloride, glucose, and urea are ...