Hypotonic and Hypertonic Crystalloids

Mechanism

Hypertonic Saline (NaCl 3% and 7.5%)

3% saline contains 513 mEq/L of sodium and chlorine, with an osmolarity of 1026 mOsm/L. Its main mechanism of action consists of the immediate increase in the osmolarity of the extracellular space (ECL). This generates an osmotic pressure gradient that forces the exit of free water from the cellular intracellular compartment (LIC) to the vascular extracellular compartment.

Use of Hypertonic Solution in Cerebral Edema and Herniation

In the edematous brain parenchyma, hypertonic saline acutely decreases the volume of astrocytes and interstitial space, decreasing intracranial pressure (ICP) and restoring cerebral perfusion. In addition, it exerts a hemodynamic rheological effect by reducing apical leukocyte volume and restoring arteriolar endothelial reflex vasoreactivity.

Emergency Dose in Active Brain Herniation Syndrome: Administer an intravenous bolus of 100 to 150 mL of 3% NaCl through a central venous line over a period of 10 to 20 minutes. A second dose may be repeated if neurological deterioration or signs of uncal herniation persist, closely monitoring that plasma sodium does not rise above 155 mEq/L or that serum osmolarity exceeds 320 mOsm/kg.

5%, 10% and 50% Dextrose Whey

5% glucose whey (SG 5%) contains 50 grams of dextrose monohydrate per liter of solution, providing a total of 170 kcal/L. Although its initial osmolarity in vitro is isotonic (≈ 278 mOsm/L), once infused, glucose is immediately taken up and internalized by cells through GLUT transporters stimulated by endogenous insulin and completely metabolized by cellular hexokinase to carbon dioxide and water.

Pharmacokinetics

Physiological Distribution of Glucose-Free Water

Upon completion of intracellular glucose metabolism, the solution behaves for all practical purposes like an infusion of solute-free water. Water is distributed evenly throughout all biological compartments, following the laws of fractional distribution of total body water (ACT):

Approximate distribution of 1000 mL of 5% dextrose after glucose metabolism: 667 mL in intracellular fluid and 333 mL in extracellular fluid (250 mL interstitial and 83 mL intravascular).

Therefore, 5% dextrose solution is completely ineffective as an expander of plasma volume (only 8.3% of the total volume remains in the intravascular space) and its exclusive use is limited to the correction of states of dehydrating hypernatremia through the programmed contribution of cellular free water.

Hypotonic Saline Solution (NaCl 0.45% or "Physiological Medium")

Contains 77 mEq/L each of sodium and chloride, with an osmolarity of 154 mOsm/L. It is indicated for severe hyperosmolar dehydration with concurrent loss of sodium and free water, or for hypernatremic patients with clinical contraindications to glucose loading (diabetic ketoacidosis or refractory hyperosmolar coma).