Lesson 4: Tonicity

Tonicity Comparison

  • Tonicity is a comparative measure of the osmolarity of a solution in relation to the osmolarity of plasma.

Definitions of Tonicity

  • Hypotonic Solutions:

    • Have a lower osmolarity than plasma or cells.

    • Examples: D5W (5% dextrose in water) and NaCl 0.45%.

  • Isotonic Solutions:

    • Have an osmolarity that is approximately equal to that of plasma or cells.

    • Examples: NaCl 0.9% and Lactated Ringer's (LR).

  • Hypertonic Solutions:

    • Have an osmolarity that exceeds that of plasma or cells.

    • Examples: NaCl 3% and D5LR (5% dextrose in lactated Ringer's).

Ranking of Tonicity

  • Rank the tonicity from highest (1) to lowest (4):

    1. NaCl 3% (Hypertonic)

    2. Albumin 5% (Colloid)

    3. D5 NaCl 0.45% (Hypotonic)

    4. D5W (Isotonic)

Detailed Analysis of Tonicity

Reference Point for Tonicity

  • Plasma serves as the standard for tonicity comparisons, which is isotonic to cells.

  • Understanding how IV fluids affect cellular volume is crucial for medical practice.

Classification of Solutions by Tonicity

Hypotonic Solutions

  • Definition: When a solution is hypotonic to a cell, it results in water entering the cell, causing it to swell.

  • Examples with osmolarity values:

    • NaCl 0.45%: 154 mOsm/L

    • D5W: 253 mOsm/L

  • Implications: These solutions generally provide free water that distributes throughout all body compartments, which is why they are inadequate for expanding intravascular volume and can be dangerous for patients with elevated intracranial pressure (ICP).

Isotonic Solutions

  • Definition: Isotonic solutions have osmolarity that maintains stability in plasma and intracellular fluid (ICF) volumes.

  • Implications: They increase extracellular fluid (ECF) volume while ICF remains stable.

  • Example: NaCl 0.9% has an osmolarity of 308 mOsm/L.

Hypertonic Solutions

  • Definition: Hypertonic solutions cause water to exit the cell, resulting in cell shrinkage.

  • Examples with osmolarity values:

    • NaCl 3%: 1026 mOsm/L

    • D5 NaCl 0.9%: 560 mOsm/L

  • Implications: Hypertonic solutions pull water from the ICF into the ECF, which may be beneficial in certain medical situations, such as treating cerebral edema or hyponatremia. However, rapid administration can risk conditions like central pontine myelinolysis (CPM).