Sodium chloride in local anesthetic solutions helps achieve what property?

• A. Isotonicity
• B. Vasoconstriction
• C. pH balance
• D. Solubility

Answer: (A)

The main idea is tonicity: sodium chloride is added to local anesthetic solutions to make them isotonic with body fluids. Matching the osmotic pressure of extracellular fluid (roughly 0.9% saline, about 290 mOsm/kg) helps prevent cells in the tissue from swelling or shrinking when the solution is injected. This makes the injection more comfortable for the patient and reduces tissue irritation, helping the anesthetic diffuse through the tissues more predictably.

Vasoconstriction is caused by epinephrine or other vasoconstrictors, not by sodium chloride. pH balance is adjusted separately, often with buffering agents like sodium bicarbonate, and solubility is influenced by the anesthetic form and solvents rather than by NaCl’s primary function.