ISOTONIC FLUIDS
Figure. How body flu...
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A solution is isotonic when the concentration of dissolved particles is similar to that of plasma. Isotonic solutions have an osmolality of 250 to 375 mOsm/L.7 With osmotic pressure constant both inside and outside the cells, the fluid in each compartment remains within its compartment (no shift occurs) and cells neither shrink nor swell. Because isotonic solutions have the same concentration of solutes as plasma, infused isotonic solution doesn't move into cells. Rather, it remains within the extracellular fluid compartment and is distributed between the intravascular and interstitial spaces, thus increasing intravascular volume.6 Types of isotonic solutions include 0.9% sodium chloride (0.9% NaCl), lactated Ringer's solution, 5% dextrose in water (D5W), and Ringer's solution.
A solution of 0.9% sodium chloride is simply salt water, and contains only water, sodium (154 mEq/L), and chloride (154 mEq/L). It's often called "normal saline solution" because the percentage of sodium chloride dissolved in the solution is similar to the usual concentration of sodium and chloride in the intravascular space.
Because water goes where sodium goes, 0.9% sodium chloride increases fluid volume in extracellular spaces. It's administered to treat low extracellular fluid, as in fluid volume deficit from hemorrhage, severe vomiting or diarrhea, and heavy drainage from GI suction, fistulas, or wounds. Conditions commonly treated with 0.9% sodium chloride include shock, mild hyponatremia, metabolic acidosis (such as diabetic ketoacidosis), and hypercalcemia; patients requiring a fluid challenge may also benefit from 0.9% sodium chloride solution. It's the fluid of choice for resuscitation efforts.2,8 In addition, it's the only fluid used with administration of blood products.
Remember that because 0.9% sodium chloride replaces extracellular fluid, it should be used cautiously in certain patients, such as those with cardiac or renal disease, because of the potential for fluid volume overload.