A 28-year-old woman receives 1 liter of 3% NaCl IV (hypertonic saline) for severe hyponatremia. Regarding fluid compartment redistribution, which change is most accurately predicted?

• A ECF volume increases and ICF volume increases equally
• B Only plasma volume increases; no redistribution between ICF and ECF occurs
• C ECF osmolality increases, creating an osmotic gradient that draws water out of cells; ECF volume increases while ICF volume decreases ✓
• D ICF volume increases due to osmotic buffering by intracellular proteins

Explanation

Administration of hypertonic saline increases ECF osmolality. Because cell membranes are freely permeable to water, this creates a transient osmotic gradient driving water from ICF (lower osmolality) to ECF (higher osmolality), continuing until osmotic equilibrium is re-established. The net effect is expansion of ECF volume and contraction of ICF volume. This is the therapeutic rationale in severe symptomatic hyponatremia: brain cell volume (which had expanded due to hyponatremia) decreases as water moves from ICF to ECF in response to raised plasma osmolality. The rate of correction must be controlled (<8-12 mEq/L per 24h) to avoid osmotic demyelination syndrome.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.