Which is an example of an isotonic saline solution?

Which is an example of an isotonic saline solution?

• A. 0.9% saline
• B. 0.45% saline
• C. 5% dextrose in water
• D. Normal water

Answer: (A)

Isotonic solutions have an osmotic balance with plasma, so they expand the extracellular fluid without causing water to rush into or out of the cells. Normal saline at 0.9% NaCl has an osmolality close to that of blood plasma, making it the classic isotonic fluid. It increases circulating volume mainly in the extracellular space and doesn’t drive water into cells or pull it out. The 0.45% saline option is hypotonic compared with plasma, so water tends to move into cells, which can cause cell swelling. The 5% dextrose in water starts off as isotonic in the bag, but after infusion the dextrose is rapidly metabolized, leaving free water; this effectively becomes hypotonic and can also draw water into cells. Pure water is obviously hypotonic and would cause cells to swell as water moves into them. So the 0.9% saline choice best fits the definition of isotonic saline.

Isotonic solutions have an osmotic balance with plasma, so they expand the extracellular fluid without causing water to rush into or out of the cells. Normal saline at 0.9% NaCl has an osmolality close to that of blood plasma, making it the classic isotonic fluid. It increases circulating volume mainly in the extracellular space and doesn’t drive water into cells or pull it out.

The 0.45% saline option is hypotonic compared with plasma, so water tends to move into cells, which can cause cell swelling. The 5% dextrose in water starts off as isotonic in the bag, but after infusion the dextrose is rapidly metabolized, leaving free water; this effectively becomes hypotonic and can also draw water into cells. Pure water is obviously hypotonic and would cause cells to swell as water moves into them.

So the 0.9% saline choice best fits the definition of isotonic saline.