McKenna's Pharmacology, 2e

57

C H A P T E R 5  Mathematics and dosage calculations

Use the following formula to determine how many drops of fluid to administer per minute: drops per minute (rate) = volume × drop factor (drops/mL) time (hours) × 60 That is, the number of drops per minute, or the rate that you will set by adjusting the roller clamp on the IV tubing, is equal to the amount of solution that has been prescribed per hour times the number of drops delivered per millilitre (mL), divided by 60 minutes in an hour. Try this example: An order has been written for a person to receive 400 mL of 5% dextrose in water over a period of 4 hours in a standard system (i.e. 20 drops/mL). Calculate the correct setting (drops per minute):

to the effects of the drug also may vary because of imma- turity of the organs. Most of the time a child requires a smaller dose of a drug to achieve the comparable critical concentration. On rare occasions, a child may require a higher dose of a drug. For ethical reasons, drug research is not done on children. Over time, however, enough information can be accumulated from experience with the drug to have a recommended paediatric dose. The drug guide that you select to use in the clinical setting will have the paediatric dose listed if this information is available. Unfortu- nately, there may be times when no recommended dose for a child is available but that particular drug is needed. In these situations, established formulae can be used to estimate the appropriate dose. Determining a paediatric dose takes into consideration the child’s weight, or body surface. The nomogram that uses body surface area is more accurate for determining doses (see Figure 5.1). Regardless of the calculation method used for children, even a tiny dose error can be critical. When working in paediatrics, one needs to be familiar with at least one of these methods of determining the drug dose. Many institutions require that two nurses check critical

volume × drop factor (drops/mL) time (hours) × 60 =

400 4

20 60

×

rate =

Simplify:

volume × drop factor (drops/mL) time (hours) × 60 =

400 412

= 33.3

rate =

= 33 drops/min (Note: drops must be in whole numbers as it is not possible to deliver part of a drop) Now calculate the same order for an IV set that delivers 60 drops/mL:

Nomogram

Height cm in.

SA

Weight lb kg

m 2

volume × drop factor (drops/mL) time (hours) × 60 =

400 4

60 60

×

rate =

180 80 70 60 50 40 30 25 20 15 160 140 120 100 90 80 70 60 50 45 40 135

= 100 drops/min If a person has an order for an IV drug, the same principle can be used to calculate the speed of the delivery. For example, an order is written for a person to receive 60 mL of an antibiotic over 30 minutes. The IV set used dispenses 20 drops/mL, which allows greater control. Calculate how fast the delivery should be:

2.0

240 220 200 190 180 170 160 150 140 130 120 110 100

90 80 75 70 65 60 55 50 45 40 35 30 28 26 24 22 20 19 18 17 16 15 14 13 12

1.5

1.0 0.9 0.8 0.7 0.6 0.5 0.4

30 25 20 18 16 14 12 10

volume × drop factor (drops/mL) time (hours) × 60

rate =

10 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.5 2.0 1.5

90 80 70 60

60 0.5

20 60

1200 30

×

=

=

0.3

= 40 drops/min

9 8 7 6 5 4 3

Paediatric considerations For most drugs, children require doses different to those given to adults. The “standard” drug dose that is listed on package inserts and in many references refers to the dose that has been found to be most effective in a 70 kg adult male. An adult’s body handles drugs differ- ently and may respond to drugs differently to a child’s. A child’s body may handle a drug differently in all areas of pharmacokinetics—absorption, distribution, metab- olism and excretion. The responses of the child’s organs

0.2

50

40

0.1

1.0

30

FIGURE 5.1  The West nomogram for calculating body surface area (BSA). Draw a straight line connecting the child’s height (left scale) to the child’s weight (right scale). The BSA value, which is calculated in square metres, is found at the point where the line intersects the SA column.