Review of Dosage Calculations

Chapter 5


Review of Dosage Calculations



Learning Objectives


Upon successful completion of this chapter, the student will be able to:


• Identify the weight and volume measures of the metric system.


• Calculate the basic dosages related to the administration of IV medications, including percentages of solvents in standard IV fluids.


• Interpret the meaning of abbreviated labels for common IV fluids.


• Calculate drip rates of IV fluids.


• Calculate flow rates of IV fluids.


• Calculate infusion times of IV fluids.


• Identify factors that might influence the flow rate of IV fluids.



Key Terms



As health care professionals the calculation of medications is not a new concept and probably is one that is used daily in your professional life. In this chapter we will review the basic concepts of weight and volume of the metric system because these are used with intravenous (IV) therapy. The basic calculations using the formula method and ratio/proportion will be included. After the physician has ordered the fluids for the patient, the correct flow rate (FR) to be used for administering the fluids during a specific time, such as a minute or hour, must be calculated. This calculation ensures that the order is accurate and that it will be safe for the patient. This chapter is intended to facilitate the calculations that may have been used in times past but have not been recently practiced. The hope is that the review will be a means of recalling knowledge previously gained.


The preparation of the fluids to be administered may be done by the person performing the administration, or these fluids may be prepared in a pharmacy prior to administration. When the pharmacy is involved in preparation, the triangulation between the health professionals–the physician who orders the fluids, the pharmacist who prepares the fluids, and the health professional administering the fluids–provides an added degree of patient safety. Through the entire process the verification of the order and the correct patient as well as the administration using the correct dose, time, rate, and route are necessary prior to administering the fluids.



CALCULATING DOSAGE USING THE METRIC SYSTEM


The metric system is the universally used system of measurement and the system most often used in the medical field. Because medications that are administered intravenously are immediately in the bloodstream for absorption, the exact dosage of a medication as ordered by the physician must be calculated and administered. Therefore, most medications are found in metric dosages and the metric system will be used often when administering any medications, including those given by infusion.


The metric system provides many of the commonly used dosage indications in both weight and volume measurements. The weight measurements in the metric system are determined using the base gram (g or gm). The increments are in magnitudes of 10 from kilograms (kg) being 1000 grams to micrograms (mcg or μg) being 1,000,000th of a gram. The main metric weights used for dosage calculations are gram, milligram (mg), microgram, and occasionally kilograms. Volume measurements in the metric system are in increments of 10 with a base of liter (l or L). The most commonly used volume measurements for liquid medications, including IV fluids, are milliliters (ml or mL) and liters, with liters containing 1000 mL. Remember that a milliliter may also be called a cubic centimeter (cc) because these both occupy the same amount of liquid space. The length measurements of centimeter (cm) or millimeter (mm) are seldom used with calculations for IV fluids.


Because the metric system is based on units of 10, decimals are used to show and calculate fractional amounts of medication. To convert a dosage amount to the next smaller unit, the decimal is moved one place to the right. Conversely, when moving from a smaller unit to the next larger unit of the metric system, the decimal is moved one place to the left. In the medical field, most conversions are made in 1000 units rather than in 10 units, so the decimal will be moved three places in most equations. For example, a gram is larger than a milligram so to change 1 gram to milligrams, the decimal is moved three places to the right and three zeros are added to show that 1 gram equals 1000 milligrams.


1 gram = 1 g or to move to milligrams 1 g = 1000 mg


Likewise, 1 milligram = 1 mg or to move to micrograms, 1 mg = 1000 mcg.


To convert from 1 liter to milliliters, the same movement to the left is necessary or


1 L=1000 mL


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However, to move from 1 g to kilograms, the movement is to the left and the decimal is moved behind the kilogram. 1 g = 0.001 kg


The metric system is shown in decimals because it is based on units of 10 (Table 5-1). To prevent medication errors when using the decimal system, always be sure that if a medication dose is a number less than one designation, a “0” should always precede that decimal point to prevent miscalculation of the dose to be given. Never allow a decimal point to be the first unit in a dosage indication (i.e., .025 mg should be shown as 0.025 mg or 25 mcg).




Learning Note


Always remember that the metric system is based on measurements of 10 between each designation. A kilogram to gram is a proportion of 1000 and a gram to a milligram is another 1000 proportion as is milligrams to micrograms. The proportion is the same for milliliter to liter.




CALCULATING BASIC DOSAGE


Determining doses of medications is a basic task for medical professionals. This may be accomplished by using either ratio/proportion (DA [Dosage Available]: DF [Dosage Form]:: DO [Dose Ordered]: DG [Dose to be Given] or by using the formula method (DD [Dose desired]/DH [Dose on hand] × Qty [Quantity or form] = Dose to be given). With the ratio/proportion method, the means (the two numbers next to the equal signs or ::) should be multiplied by each other and the same for the extremes (the two numbers on the outside of the equation). When using ratio/proportion, the calculated answer should always be checked because following calculation the final answer of the multiplication of the means should equal the final answer of the multiplication of the extremes. The answer of the calculation will be in the designation of “x.” For example, if the equation mg: mL = mg: mL is used and the x is in the placement of the milliliter, the answer will be in milliliters. Always insert the “known” designations in the first ratio, being sure that all designations are within the same measurements systems. Also be sure that the like components in both ratios are in the same weight or liquid measure (i.e., mg in one ratio must be mg in the second ratio, not g or mcg). If the components are not the same, conversions within the system must be made prior to calculating the dosage.


EXAMPLE: A physician orders ampicillin 500 mg q6h to be infused to a patient with acute bronchitis. The medication is available as 1 g/4 mL following reconstitution. What volume of medication should be supplied to the patient every 6 hours?


Before the calculation can be made, the weight of the medication must be in the same metric measurement. One g is equal to 1000 mg. This should be changed to milligrams because this is the weight used in the order.





INTERPRETING IV FLUID LABELS


When interpreting the fluids that the patient is to receive, the commonly used infusates are available in percentage strengths in specific fluid amount. The first letter indicates the chemical that will be found in the fluid, the second number is the percentage strength of that chemical, and the third designation is the fluid in which the chemical is found. When a label reads 1000 mL NS it indicates that 0.9% sodium chloride (NaCl) or normal saline in 1000 mL containers. This designation actually means the parts per hundred of the solvent (NaCl) in the solute (water). In this example, the use of percentage states that 0.9 g of NaCl is found in each 100 mL of solute (usually water) or 9 g of NaCl are found in 1000 mL of solute or water. Another example of the use of percentage of solute in solvent is 5% dextrose in ½ NS (D-5-1/2 NS). In this case, 50 g of dextrose and 4.5 g of NaCl (½ NS indicates that the NaCl is 0.45% rather than the 0.9% found in NS) would be found in each 1000 mL of water. (Remember, the percentage amount is in 100 mL, NOT 1000 mL, so the percentage amount must be multiplied by the number of milliliters in the container.) Understanding the percentage of solute in a solvent is important for patient safety when administering fluids. When choosing the correct fluids for the patient, be sure that the percentages agree with the physician’s order.



Learning Note


1 mL of water used as a solvent weighs the same as 1 g of solute.



This information is just a reminder of the necessary background for administering medications and fluids. In most cases when drugs are added to IV fluids, this is accomplished by a pharmacist so the actual calculation and preparation of the medication dose has already been done. However, the person infusing the medication has the responsibility of being sure that an accurate dosage has been calculated and prepared. Therefore, all fluids administered with medications added should be rechecked for the accuracy of preparation and should be checked to the physician’s order prior to hanging the infusate. Again, the triangulation between health professionals provides quality assurance for patient safety.



CALCULATING SOLUTE WEIGHTS DURING INFUSION


The understanding of percentage of solutes in solvents is essential as the patient is monitored during the infusion. The total chemical content of infusates will affect how the patient reacts to the IV fluids, and early intervention of possible adverse reactions may be possible if the percentages of electrolytes and chemicals being used are known. Consider the person with diabetes mellitus who is receiving IV fluids with dextrose. The amount of dextrose will certainly be a factor in the patient’s response to the IV therapy. Remember that once IV fluids are injected into the vein, these medications cannot be retrieved, so care for understanding percentages of solutes in solvents is essential.


In some cases, a physician may ask that the amount of solute be calculated when the entire fluid amount is not infused. For example, a patient may have an order for 500 mL D-5-NS (5% dextrose in 0.9% sodium chloride). The fluids infiltrate after the patient has received 300 mL. How many grams of dextrose and how many grams of sodium chloride did the patient receive?


The way to calculate the weight of solutes is by using ratio/proportion:
















Known Unknown
25 g dextrose (5 g/100 mL): 500 mL::x g: 300 mL  
500 x = 7500 g
x = 15 g dextrose


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Aug 10, 2016 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Review of Dosage Calculations

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