Cerebrovascular Effects of Hypertonic Resuscitation

Abstract

Intravenous fluids vary in oncotic pressure, osmolarity, and tonicity. Osmotically active particles attract water across semipermeable membranes until equilibrium is attained. The osmolarityof a solution, which quantifies the forces determining the distribution of water, refers to the number of osmotically active particles per liter of solution. In contrast, osmolalityis a measurement of the number of osmotically active particles per kilogramof solvent. The osmolality of body fluids can be estimated as follows:

$${\rm{osmolality = ([N}}{{\rm{a}}^ + }]\,\, \times \,\,2)\,\, + \,({\rm{glucose}}\, \div \,18)\, + \,({\rm{BUN}}\, \div \,2.3)$$

where [Na ⁺ ] is expressed in mEq 1⁻¹, serum glucose is expressed in mg-dl⁻¹, and BUN is expressed in mg-dl⁻¹ respectively. Urea, sugars, alcohols, and radiographic dyes that increase osmolality may increase the measured value, generating an increased “osmolal gap” between the calculated and measured values. Osmotic activity can also be expressed in terms of osmotic pressure, a measure of the attraction for water exerted across a semipermeable membrane. Osmotic pressure is approximated from the equation [1]:

$${\rm{Osmotic}}\,{\rm{pressure}}\,{\rm{(mm}}\,{\rm{Hg)}}\,{\rm{ = }}\,{\rm{19}}{\rm{.3}}\,{\rm{ \times }}\;{\rm{Osmolality}}\;{\rm{(mOsm/kg)}}$$

A hyperosmolar state occurs whenever the concentration of osmotically active particles is high. Thus, uremia (increased BUN) and hypernatremia (increased serum sodium) increase both serum osmolality and osmolarity. However, because urea distributes throughout total body water, an increase in BUN, unlike an increase in [Na ⁺ ], does not cause hypertonicity, i.e. osmotically mediated redistribution of water from intracellular volume (ICV) to extracellular volume (ECV). The term “tonicity” is also used colloquially to compare the osmotic pressure of a solution to that of plasma. A fluid in which the osmotic pressure is similar to that of plasma; is termed isotonic. Hypotonie solutions exert lower osmotic pressures than plasma; hypertonic solutions exert higher osmotic pressures (Table 1).