McKenna's Pharmacology, 2e

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P A R T 1  Introduction to nursing pharmacology

give for the ordered dose. In order to calculate this, we use the following formula:

Strength required Strength in stock

× volume of stock solution

Strength required Strength in stock

125 250

625 250

Number of tablets or capsules =

× 5 =

= 2.5 mL

=

Try an example: An order is written for 0.05 g Aldactone (spironolactone) to be given orally (PO). The Aldactone is available in 25 mg tablets. How many tablets would you have to give? First, you will need to convert the grams to milligrams: So 0.05 g of Aldactone is equal to 50 mg of Aldactone . The order has been converted to the same measure- ment as the available tablets. Now solve for the number of tablets that you will need: Sometimes the desired dose will be a fraction of a tablet or capsule, 1 / 2 or 1 / 4 . Some tablets come with scored markings that allow them to be cut. Pill cutters are readily available in most pharmacies to help people cut tablets appropriately. However, one must use caution when advising a person to cut a tablet. Many tablets come in a matrix system that allows for slow and steady release of the active drug. These drugs cannot be cut, crushed or chewed. Always consult a drug reference before cutting a tablet. However, as a quick reference, any tablet that is designated as having delayed or sus- tained release may very well be one that cannot be cut. Capsules can be very difficult to divide precisely, and some of them also come with warnings that they cannot be cut, crushed or chewed. If the only way to deliver the correct dose to a person is by cutting one of these preparations, a different formulation of the drug, a dif- ferent drug or a different approach to treating the person should be tried. Other oral drugs come in liquid preparations. Many of the drugs used in paediatrics and for adults who might have difficulty swallowing a pill or tablet are prepared in a liquid form. Some drugs that do not come in a standard liquid form can be prepared as a liquid by the pharmacist. If the person is not able to swallow a tablet or capsule, check for other available forms and consult with the pharmacist about the possibility of preparing the drug in a liquid as a suspension or a solution. The formula for calculating liquid volumes is as follows: Strength required Strength in stock = 50 mg 25 mg = 2 tablets

Even if you are working in an institution that provides unit-dose medications, practise your calcu- lation skills to keep them sharp. Power can be lost, computers can go down and the ability to determine cal- culations is a skill that anyone who administers drugs should have in reserve. Periodically throughout this text you will find a Focus on Calculations box to help you refresh your dose calculation skills as they apply to the drugs being discussed. Parenteral drugs All drugs administered parenterally must be adminis- tered in liquid form. The person administering the drug needs to calculate the volume of the liquid that must be given to administer the prescribed dose. The same formula can be used for this determination that was used for determining the dose of an oral liquid drug:

Strength required Strength in stock

× volume of stock solution

= amount of drug to administer Try this example: An order has been written for 75 mg of pethidine to be given intramuscularly (IM). The vial states that it contains pethidine 50 mg in 1.0 mL. Set up the equation just as before:

75 mg 50 mg

× 1 mL

= 75 / 50 = 1.5 mL to be administered

Intravenous solutions Intravenous (IV) solutions are used to deliver a pre- scribed amount of fluid, electrolytes, vitamins, nutrients or drugs directly into the bloodstream. For infusions, most institutions now use electronically monitored delivery systems. However, it is still important to be able to determine the amount of an IV solution that should be given, using standard calculations. Most IV delivery systems come with a standard control, by which each millilitre delivered contains 20 drops. Microdrip systems, which usually deliver 60 drops/mL, are also available; they are usually used in paediatric settings. Always check the packaging of the IV tubing to see how many drops/mL are delivered by that par- ticular device if you have any doubts or are unfamiliar with the system.

Strength required Strength in stock

× volume of stock solution

Try this example: An order has been written for 125 mg of phenytoin. The bottle states that the solution contains 250 mg/5 mL. How much of the liquid should you give?