How is dehydration categorized?

Updated: Dec 07, 2018
  • Author: Lennox H Huang, MD, FAAP; Chief Editor: Timothy E Corden, MD  more...
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Answer

Dehydration can be categorized according to osmolarity and severity. Serum sodium is a good surrogate marker of osmolarity assuming the patient has a normal serum glucose. (Osmolarity = [2 × sodium] + [glucose/18] + [blood urea nitrogen/2.8]) Dehydration may be isonatremic (130-150 mEq/L), hyponatremic (< 130 mEq/L), or hypernatremic (>150 mEq/L). Isonatremic dehydration is the most common (80%). Hypernatremic and hyponatremic dehydration each comprise 5-10% of cases. Variations in serum sodium reflect the composition of the fluids lost and have different pathophysiologic effects, as follows:

  • Isonatremic (isotonic) dehydration occurs when the lost fluid is similar in sodium concentration to the blood. Sodium and water losses are of the same relative magnitude in both the intravascular and extravascular fluid compartments.

  • Hyponatremic (hypotonic) dehydration occurs when the lost fluid contains more sodium than the blood (loss of hypertonic fluid). Relatively more sodium than water is lost. Because the serum sodium is low, intravascular water shifts to the extravascular space, exaggerating intravascular volume depletion for a given amount of total body water loss. [1, 2]

  • Hypernatremic (hypertonic) dehydration occurs when the lost fluid contains less sodium than the blood (loss of hypotonic fluid). Relatively less sodium than water is lost. Because the serum sodium is high, extravascular water shifts to the intravascular space, minimizing intravascular volume depletion for a given amount of total body water loss. [2, 3, 4]


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