Isotonic FVD can result from:
A. GI fluid loss through diarrhea.
B. Insensible water loss during prolonged fever.
C. Inadequate ingestion of fluids and electrolytes.
D. Impaired thirst regulation.
Correct Answer: C. Inadequate ingestion of fluids and electrolytes.
Isotonic FVD may result from inadequate intake of fluids and electrolytes that can occur secondary to an inability to ingest orally. Isotonic dehydration is a condition in which both water and sodium are lost proportionally and the serum sodium concentration maintains normal serum osmolality. Serum osmolality determines the movement of fluids and electrolytes across membranes. The normal serum osmolality is 285–295 mOsm/kg.
Option A: GI fluid loss through diarrhea is an etiology of hypotonic FVD. Hypotonic dehydration occurs when sodium loss is greater than water loss, resulting in a decrease in serum osmolality. This causes a shift of water from the extracellular space into the intracellular space. The cells swell and cerebral edema may occur.
Option B: Insensible water loss during prolonged fever is a cause of hypertonic FVD. Fever will increase the respiratory rate and therefore, water loss. Sweating also increases to lower the body temperature. Water intake is commonly decreased during a fever which will aggravate dehydration.
Option D: Impaired thirst regulation is a cause of hypertonic FVD. Hypertonic dehydration occurs when water excretion from the body exceeds that of sodium excretion, resulting in increased sodium concentration in the extracellular fluid (hypernatremia). Blood osmolality is increased, causing water to shift from the intracellular to the extracellular space.
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