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KEY CONCEPTS
Osmotic pressure is generally dependent only on the number of nondiffusible solute particles. This is because the average kinetic energy of particles in solution is similar regardless of their mass.
Potassium is the most important determinant of intracellular osmotic pressure, whereas sodium is the most important determinant of extracellular osmotic pressure.
Fluid exchange between the intracellular and interstitial spaces is governed by the osmotic forces created by differences in nondiffusible solute concentrations.
Serious manifestations of hyponatremia are generally associated with plasma sodium concentrations less than 120 mEq/L.
Excessively rapid correction of hyponatremia has been associated with demyelinating lesions in the pons (central pontine myelinolysis), and more generally in both pontine and extrapontine central nervous system structures (osmotic demyelination syndrome), resulting in both temporary and permanent neurological sequelae.
A major hazard of increased extracellular volume is impaired gas exchange due to pulmonary interstitial edema, alveolar edema, and/or large collections of pleural or ascitic fluid.
Intravenous replacement of potassium chloride is usually reserved for patients with, or at risk for, significant cardiac manifestations or severe muscle weakness.
Because of its lethal potential, hyperkalemia exceeding 6 mEq/L should always be corrected.
Symptomatic hypercalcemia requires rapid treatment. The most effective initial treatment is rehydration followed by a brisk diuresis (urinary output 200–300 mL/h) utilizing intravenous saline infusion and a loop diuretic to accelerate calcium excretion.
Symptomatic hypocalcemia is a medical emergency and should be treated immediately with intravenous calcium chloride (3–5 mL of a 10% solution) or calcium gluconate (10–20 mL of a 10% solution).
Some patients with severe hypophosphatemia may require mechanical ventilation postoperatively because of muscle weakness.
Severe hypermagnesemia can lead to respiratory and cardiac arrest.
Isolated hypomagnesemia should be corrected prior to elective procedures because of its potential for causing cardiac arrhythmias.
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Fluid and electrolyte disturbances associated with surgical and coexisting medical disease are extremely common in the perioperative period. Moreover, large volumes of intravenous fluids and blood components are frequently required to correct fluid deficits and compensate for blood loss during surgery. Major disturbances in fluid and electrolyte balance can rapidly alter cardiovascular, neurological, and neuromuscular functions, and anesthesia providers must have a clear understanding of normal water and electrolyte physiology. This chapter examines the body’s fluid compartments and common water and electrolyte derangements, their treatment, and anesthetic implications. Acid–base disorders and intravenous fluid and blood therapy are discussed in Chapters 50 and 51.
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NOMENCLATURE OF SOLUTIONS
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The system of international units (SI) has still not gained universal acceptance in clinical practice, and many older expressions of concentration remain in common use. Thus, for example, the quantity of a solute in a solution may be expressed in grams, moles, or equivalents. To complicate matters further, the concentration of a solution may be expressed either as quantity of solute per volume of solution or quantity of solute per weight of solvent.
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MOLARITY, MOLALITY, & EQUIVALENCY
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