Calculation of Medications Used Intravenously

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Chapter 12

Calculation of Medications Used Intravenously

Objectives

• Interpret orders for volume of medications to be administered as intravenous (IV) fluids

• Calculate dosages of IV solutes in IV fluids including those expressed as percentages

• Calculate IV flow rates in drops per minute (gtts/min)

• Calculate time needed to infuse an ordered volume of IV fluids

Key Words

Tech Note

The medications found in this chapter are capitalized because that is how IV medications are designated when used for IV medication therapy.

Pretest

Answer the following problems to determine your level of ability in calculating medication orders for IV administration of fluids. In final answers round to the nearest drop or nearest milliliter (if less than one, round to the nearest tenth). Measurements for total volume in containers (such as liters) should be rounded to the nearest hundredth. Weight measurements should be carried to the nearest hundredth as appropriate unless otherwise stated. Time measurements should be to the closest minute or hour as indicated. Show your work. When adding volumes of medication to fluids, the volume of medication(s) added should be included when calculating the total volume of fluids.

image 1 A physician orders a continuous infusion of 1000 mL D-5-W with 20 mEq KCl/L q8h.

image 2 A physician orders 500 mL D-5-1/2 NS q8h as a continuous infusion to be administered to a patient. The physician wants the pharmacy technician to calculate the total amount of DEXTROSE and SODIUM CHLORIDE the patient will receive.

 3. A physician orders 1 L of D-5-NS to be infused over 8 hours.

image 4 A physician orders 3000 mL LACTATED RINGER’S solution to infuse over 16 hours. The drop factor is 10 gtts/mL.

image 5 A physician orders ANCEF 1g in 100 mL D-5-W IVPB to be infused over 1 hour.

6. A container of fluids contains 550 mL, and the drop rate for the infusion set is 20 gtts/mL.

image 7 A physician orders 1500 mL 0.45% NaCl IV over 24 hours. The drop factor on the infusion set is 20 gtts/mL.

image 8 A physician orders 1000 mL of D-10-W to be administered over 6 hours. The infusion set supplies 10 gtts/mL.

9.image A physician orders LACTATED RINGER’S solution to be administered with a 20 gtts/min infusion set. The physician wants the pharmacy to decide the amount of fluid that should be necessary over a 24-hour period if the medication is administered at 2 mL/min.

10.image A physician orders D-5-NS q24h. The drop factor is 20 gtts/mL, and the flow rate is 50 mL/hr.

11.image An order is given for a patient to receive D-5-NS as a continuous infusion until further orders. The drop factor is 20 gtts/mL, and the flow rate is 40 mL/hr.

12.image How many grams of DEXTROSE are in the fluids for the label shown? ____________________

13.image How many grams of SODIUM CHLORIDE are in the fluids shown in the following label? ____________________

image

14.image One L of D-5-NS is to infuse at 8 mL/min, and the drop factor is 20 gtts/mL.

image 15 A physician orders KEFZOL 1 g IVPB; the KEFZOL is to be mixed in 100 mL D-5-W to infuse over 1 hour. The drop factor is 50 gtts/mL.

16.image Three L of D-10-W are to infuse over a 24-hour period.

17.image A liter of D-5-NS is infusing at the rate of 45 gtts/min. The drop factor is 15 gtts/mL.

image 18 A physician orders PEPCID (FAMOTIDINE) 20 mg IVPB q12h. The medication is available in 10 mg/mL vials. The medication is to be added to 100 mL LACTATED RINGER’S solution and is to infuse over 30 minutes.

image 19 A physician orders AMPHOTERICIN B 40 mg IV in 500 mL of D-5-W daily to be infused over 12 hours. After reconstitution, the medication contains 50 mg/10 mL. The drop factor is 25 gtts/mL.

image 20 A physician orders AMPICILLIN 2 g IVPB. After mixing in 500 mL of D-5-1/2 NS, it will be administered over 2 hours using a drop factor of 10 gtts/mL. The available medication is AMPICILLIN 1 g vials to be reconstituted with 2.5 mL NS in each vial.

Introduction

The definition of intravenous is to administer a medication and/or fluid directly into a vein. All medications administered intravenously must be in liquid form and must be sterile. IV fluids may be given in large volumes for continuous infusion in such basic fluids in 1000 mL amounts as sterile water or normal saline with dextrose added to meet caloric needs. Medications, such as electrolytes or antibiotics, may also be added to these containers to supply drugs or nutritional needs over a period of time. The physician’s order will include the type and total fluid volume to be administered over a total period of time. In other cases, medications may be added to small volumes of fluids for administration on an intermittent basis either through an injection port in the tubing or a secondary line called IV piggyback (IVPB).

Intravenous medication prescriptions (intermittent, IVPB) specify the drug (solute), the dose to be given, and frequency of dosing but do not commonly state infusion rate or infusion time. The pharmacy determines the solvent, solvent volume, and time to be infused while the physician’s order provides the solute to be added. This information from both the physician’s order and the determinations made by the pharmacy is the information to be included on the label added to the parenteral fluid container for the specific person.

Fluids may be given as replacements for electrolytes or lost fluids such as with patients who have dehydration. Maintenance therapy provides the nutrients necessary to meet the daily needs for water, electrolytes, and glucose. With a deficit of fluids and electrolytes over a period of time, usually 48 hours, replacement therapy may be necessary to meet the fluid and electrolyte needs of the person. Finally, restorative therapy is a day-by-day restoration of vital fluids and electrolytes. With this therapy, the types of fluids being lost are those that are replaced. Often, several different types of fluids will be ordered for the patient on the same day.

Fluids may also be ordered as a means for transporting medications needing rapid absorption in the body. With the addition of drugs, the infusion is used as prophylaxis or therapeutically for disease processes.

Intravenous solutions are available in 50-mL to 1000-mL containers (commonly in flexible plastic bags) with each containing solutes and solvents prescribed for the patient’s specific needs. The reason for therapy dictates the type of fluid and rate of infusion ordered. Fluids given to keep a vein open are given slowly, whereas replacement fluids are usually given at a rate that will provide the necessary fluids while preventing an overload on the vascular system. The rate of flow depends on the patient’s physical condition. Most IV fluids are found in percentages of solutes (solids) in the total volume of solvent (solution) such as D-5-W, which is 5% dextrose in water. As learned in Chapter 11, this means that 5 g of dextrose are dissolved in every 100 mL of sterile water, or 50 g of dextrose are found in 1000 mL of water. While most IV fluids are infused by pump today, it is still important to be conscious of selecting the correct fluids for the IV infusion and ensuring an infusion is administered at the prescribed rate.

This chapter will focus on how to determine flow rates in drops per minute and determining the infusion time. You may need to determine the total time necessary for an amount of fluid to be administered as ordered, or you may need to calculate the flow rate when a total time of administration has been specified in the physician’s order.

Persons allowed to prepare and administer IV fluids are regulated by state law and the regulations of the facility where the fluids are to be administered. In some states, technicians may prepare the IV fluids for infusion, whereas in others this preparation must be done by the pharmacist. As a pharmacy technician, you must always be cognizant of the laws of your state of practice and regulations of the place of employment.

Pharmacy technicians who work in retail pharmacy may find that they do not use this chapter often, because IV infusion preparation is rarely performed in a community pharmacy. On the other hand, IV fluids are often used in the hospital setting. Understanding the calculations related to IV infusions is important for the hospital pharmacy technician but is also necessary for the retail pharmacy technician, who might one day practice in the hospital setting.

Interpreting Amounts of Solutes in Intravenous Fluids

To provide patients with continuous medications or for fluid replacement over a period of time, IV fluids are often administered to them rather than requiring them to undergo repeated injections by other parenteral routes. The medications found in fluids may be standard medications found in prepared IV fluids, such as dextrose or sodium chloride, or they may be added in the pharmacy to fill the physician’s orders, such as antibiotics or electrolyte replacements. In some instances medications may be prepared by manufacturers as commonly seen with premixed IV piggyback solutions containing antibiotics found in routinely used volumes such as 50-mL containers. The health care professional must have the ability to read the physician’s order and select the correct fluids and additives for the patient’s needs. Remember that once the fluids have been injected into the vein, the medications cannot be retrieved because the medication circulates throughout the body instantaneously. Therefore it is imperative for patient safety that the correct fluids with the correct additives (medications) are used at a safe flow rate depending on the patient’s physical condition and the medication being administered. Calculations for IV medications are performed using either ratio and proportion, formula (Dose desired/Dose on hand × Quantity = Dose to be given), or dimensional analysis, depending on method of choice.

In most cases the physician will use common abbreviations for ordering IV fluids. These are shown in Table 12-1. Notice fluids are labeled with a percentage of solute in the container of fluids or the amount of solute found in the solvent. D-5-W indicates that 5% dextrose is available in the solvent (sterile water). Remember that percentage is based on 100. So if the percentage is shown in 100 mL of solution, the 5% indicates that 5 g of dextrose is found in 100 mL of fluid. However, if the 5% is shown in 500 mL of a solvent such as water, then the total weight of the solute must be calculated. The easiest method for calculating the weight of solute is ratio and proportion (Known solute : Known volume :: Desired-to-know solute : Desired-to-know volume). To review percentage and ratio and proportion, see Chapter 2.

TABLE 12.1

Abbreviations for Common Intravenous Solutions

ABBREVIATION SOLUTION
NS Normal saline; 0.9% sodium chloride
image Half-normal saline; 0.45% sodium chloride; or image strength normal saline
D-5-W or 5% D/W Dextrose 5% in water
D-5-LR Dextrose 5% in lactated Ringer’s
RL or LR Ringer’s lactate solution or lactated Ringer’s
D-5-NS Dextrose 5% in 0.9% sodium chloride; dextrose 5% in normal saline
D-5-image Dextrose 5% in image normal saline or 0.45% sodium chloride
D-2.5-imaget 2.5% dextrose in 0.45% sodium chloride
D-2image-W image dextrose in water

image

Tech Note

Remember that when interpreting fluid solutes, 1 mL of water has a weight of 1 gram.

Example 12-1

A physician orders 500 mL 5% D/W to be given to a patient. What is the metric weight (in grams) of the dextrose in this bag of fluids? Remember, 5% means that 5 g (solute) of dextrose is found in 100 mL of water (solvent).

image

< ?xml:namespace prefix = "mml" />5 g:100::x:500

image

100x=5 g×500or2500

image
image

x=25 g of DEXTROSE in 500 mL of the above fluids

image

In some cases it is necessary to calculate the amount of medication a patient has received when the entire amount of ordered fluids is not infused.

Example 12-2

A 1000-mL container of D-5-W contains 80 mEq KCl. The patient has received 650 mL of the fluids when it is discontinued. How many mEq KCl has the patient received?

KnownUnknown80 mEq:1000 mL::x:650 mL

image
image

1000x=650×80 mEqor52,000 mEq

image
image

x=52 mEq of KCl were received by the patient

image

Practice Problems A

Calculate the solute volume found in the IV fluids. Use the method of choice for the calculations. Round answers to the nearest tenth. In some cases, the answer must be shown in hundredths; these will be noted with an asterisk. Supply answers in metric weight such as grams, unless otherwise noted. Show all of your work.

 1. What is the weight of SODIUM CHLORIDE in these fluids? ____________________

image

 2. What is the weight of DEXTROSE in these fluids? ____________________

image

 3. image A 50-mL IVPB contains PENICILLIN 1.5 million units. The patient has received 35 mL of the medication.

 4. What is the weight in grams of SODIUM CHLORIDE in the fluids shown in the following illustration? ____________________

image

 5. *What is the weight of SODIUM CHLORIDE in grams found in the fluids shown below? ____________________

image

 6. image A physician orders PENICILLIN G POTASSIUM 2.4 million units to be added to 1 L NS. You have on hand a vial of this medication that has been reconstituted to 1,000,000 units/mL.

 7. image One liter of D-5-NS contains the POTASSIUM CHLORIDE as shown on the following label using the entire vial. The vein infiltrates after 900 mL have been infused.

image

 8. A physician orders FUROSEMIDE 100 mg in 200-mL NS. The patient receives 150 mL of the medication. Available medication is FUROSEMIDE 10 mg/mL.

 9. image A physician orders FENTANYL CITRATE 50 mcg to be added to D-5-W 500 mL. The medication is available as 10 mcg/mL.

10. A physician orders 1 L of DEXTROSE 3% to be administered over 6 hours.

11. image A physician desires that a patient receive LIDOCAINE 150 mg in D-5-W 200 mL IVPB. The available medication is 1% LIDOCAINE for injection.

12. image A physician orders AMPICILLIN 2000 mcg/kg IV stat for a patient who weighs 165 lb. The medication is to be added to 50 mL of D-5-W to infuse for 20 minutes. The AMPICILLIN is reconstituted to 250 mg/5 mL.

13. image A physician orders METHYLPREDNISOLONE 125 mg IV in D-5-W 100 mL to run for an hour for a patient with asthma. The medication comes in a vial containing 250 mg/5 mL.

14. image A 50 mL container of D-10-NS contains PENICILLIN 1.5 million units. These fluids are to be infused over 30 minutes.

15. image A physician orders AMINOPHYLLINE as a loading dose of 5 mg/kg to be administered IVPB over 1 hour for a patient who weighs 154 lb. The available medication is AMINOPHYLLINE 250 mg/10 mL.

16. image A physician orders OXYTOCIN 2 units added to 1 L of D-5-W. The available medication is OXYTOCIN 10 units/mL.

17. A physician orders MAGNESIUM SULFATE 10 g to be added to LACTATED RINGER’S solution 1 L. The available medication is 50% MAGNESIUM SULFATE.

18. A physician orders 1 L of D-10-image to infuse over 10 hours.

How many grams of DEXTROSE would the patient receive per hour? ____________________

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