Calculating Osmolarity of an IV Admixture

Calculating Osmolarity of an IV Admixture

Introduction

There are any number of ways to calculate the osmolarity of an I.V. admixture, here is one method:

1. For each component of the admixture, multiply the volume in milliliters of that component times the value of mOsm per ml of the component.
2. Add the mOsm obtained in step [1] for each of the components in order to determine the total number of milliosmoles in the admixture.
3. Add together the volumes of each component in order to obtain the total volume of the admixture.
4. Divide the total number of milliosmoles from step [2] by the total volume from step [3], then multiply by 1,000 to obtain an estimate of the osmolarity of the admixture in units of milliosmoles per liter.

Example Calculation

Small Volume Parenterals

The following table lists the values of mOsm per ml for common IV admixture components:

Large Volume Parenterals

The following table lists the values of mOsm per ml for common IV solutions:

Considerations for IV admixture administration

Normal osmolarity of blood/serum is about 300-310 mOsm/L. The tonicity of an IV fluid dictates whether the solution should be delivered via the peripheral or central venous route. Hypotonic and hypertonic solutions may be infused in small volumes and into large vessels, where dilution and distribution are rapid.

Solutions differing greatly from the normal range may cause tissue irritation, pain on injection, and electrolyte shifts. When solutions with extremes of tonicity are infused, fluids shift into or out of cells, including endothelial cells of the tunica intima near the catheter tip and blood cells. The resulting changes in the cell size of the vein wall causes the inflammatory and clotting processes to occur, leading to phlebitis and thrombophlebitis.

The generally accepted upper limit for a peripheral IV is 900 mOsm/L. When the osmolarity exceeds 900 mOsm/L, the ability of the peripheral veins to dilute parenteral infusions sufficiently is compromised, and chemical irritation of the vein intima occurs. Admixtures greater than 600 to 900 mOsm/L are associated with a dramatic increase in phlebitis and should be administered via a central line.

In a brief review of the literature, I couldn't find a 'magic' number regarding the lowest acceptable osmolarity or tonicity of an IV solution. Most people have been taught 0.45% sodium chloride, at 154 mOsm/L, is the lowest osmolarity that should be used via any IV route. Very hypotonic IV solutions such as 1/4 NS cause red blood cells to swell and burst. If a sufficient number of RBC's are so affected, the patient may develop anemia. This condition is usually referred to as hemolytic anemia.

Definitions

1. Osmolarity
   The concentration of the solute in a solution per unit of solvent, usually mmol solute per Liter.
2. Tonicity
   Frequently used in place of osmotic pressure or tension, is related to the number of particles found in solution. Osmolarity is most often used when referring to blood, and tonicity is most often used when referring to IV fluid, but the terms may be used interchangeably.
3. Isotonic
   Of equal tension. Denoting a solution having the same tonicity as another solution with which it is compared.
4. Hypertonic
   Having a higher concentration of solute particles per unit volume than a comparison solution, regardless of kinds of particles. A solution in which cells shrink due to efflux of water.
5. Hypotonic
   Having a lower concentration of solute particles per unit volume than a comparison solution, regardless of kinds of particles. A solution in which cells expand due to influx of water.