Calculations used in intravenous preparations

1. Calculate the volume of an injectable solution and the quantity of drug in an injectable solution.

2. Calculate powder volume for an injectable medication.

3. Calculate intravenous medications from a powder injectable.

4. Calculate intravenous flow rates for intravenous solutions.

Introduction

When preparing intravenous (IV) admixtures, technicians must not only be careful to observe aseptic technique procedures but also be extremely cautious when calculating amounts of medications. These calculations are performed by pharmacists and technicians when the product is being prepared, as well as by the nurses who administer them. Special consideration should be taken with IV calculations since this route of administration bypasses the alimentary canal and goes directly into the bloodstream. If an error occurs, there is no way to remove the medication from the blood and reverse the unwanted effects. In this chapter, we will discuss calculating medication dosages, as well as common units of measurements and IV flow rates.

Medications given intravenously require special calculations. An intramuscular (IM) injection is given into the skeletal muscle in an aqueous form, and drugs that are water-based are absorbed quickly. If given subcutaneously (Sub-Q), the medication goes into the fatty layers of tissue and is absorbed quickly. An IV medication is given into the vein. All of these routes of administration are most often ordered in milligrams, and these can be administered at home, in the hospital, or in the office.

DID YOU KNOW?

Due to the recent medication errors with heparin, new safety verification processes have been implemented, such as new product labeling, double checks for medication pulled from inventory by two pharmacy personnel, and verifications for dosages involving one or more technicians, pharmacists, or nurses.

a. Choose the stock vial with the concentration of medication as close as possible to the desired concentration ordered (Figure 5-1). Since there are 10,000 units in each milliliter of the stock vial of medication, this would be the most appropriate, because the order calls for just a little over 10,000 units of medication to be used.

FIGURE 5-1 Insulin vial and label. (Courtesy Eli Lilly and Company, Indianapolis, IN)

10,000U1mL=12,000UXmLXmL=1mL×12,000U10,000U=1.2mLofheparin

2. An order reads, “Add 44 mEq of sodium chloride (NaCl) to an IV bag.” How many milliliters of sodium chloride will be needed for this order?

4mEq1mL=44mEqXmLXmL=44mEq×1mL4mEq=1mL

Special considerations for parenteral medications

Vials of medication can contain either a solution or a powder (Figure 5-2). If it is in powder form, it is made in a freeze-dried state and then placed in a sterile container. In order to create a solution from this and be able to draw up the contents in a syringe, the powder must be reconstituted by adding a solution, such as bacteriostatic water, sterile water, or saline, known as a diluent. The correct type and amount of diluent can be found in drug references, such as the package insert or the Handbook on Injectable Drugs by Trissell.⁹ It is very important to use the manufacturer’s recommended solution to dilute the powder because each drug and patient is different.