IV FLOW RATE CALCULATION

IV fluids are usually ordered to be administered at rates expressed in mL/hr. Examples: 3,000 mL in 24 hr, 1,000 mL in 8 hr. Small volumes of fluid are often used when the IV fluid contains medications such as antibiotics. Rates for IV fluids are usually expressed in drops per minute (gtt/min) when an infusion device is not used. When an infusion device is used, the rate must be expressed in mL/hr.

Calculating Flow Rates for Volumetric Pumps in mL/hr

When a client is using an electronic infuser such as a volumetric pump, the prescriber orders the volume, and the nurse is responsible for programming the pump to deliver the ordered volume. The prescriber may order the IV volume in mL/hr; however, if not, the nurse must calculate it and program the pump.

Remember that the pump delivers volume in mL/hr, so if a decimal fraction is obtained, it must be rounded to the nearest whole number.

Solution Using Formula Method

x mL/hr(whole number)=Amount of solutionTime in hours

Example 1:

Client with an infusion pump has an order for 3,000 mL D5W over 24 hours.

1. Think: pump is regulated in mL/hr.

2. Set up in formula:

x ml/hr=3,000 mL24 hrx= 125mL/hr

Answer:

The pump would be set to deliver 125 mL/hr.

Solution Using Ratio and Proportion

An alternative to the above formula would be to set up a ratio and proportion as follows:

3,000 mL:24 hr= x mL:1 hr24x24  =3,00024x= 125mL/hr

NOTE

The terms in the ratio and proportion must be in same sequence.

Remember, as stated in the chapter on ratio and proportion, that a ratio and proportion can be set up in several formats. This could have been set up with the desired time for the infusion (usually 1 hr) over the total time ordered in hours, and the other side would be the hourly amount in milliliters labeled “x” over the total volume to be infused in milliliters:

1 hr24 hr=x mL3,000  mL or 1 hr:24 hr=x mL:3,000 mL

Example 2:

A client with an infusion pump is to receive an antibiotic in 50 mL of 0.9% NS over 30 minutes.

1. Think: The pump infuses in mL/hr. When the infusion time is less than an hour, which often occurs when antibiotics are administered, use a ratio and proportion to determine mL/hr. Remember: 1 hr = 60 min.

2. Set up proportion:

Solution Using Ratio and Proportion

50 mL: 30 min = x mL:60 min30 x = 50×6030x30=3,00030x = 100 mL/hr

Answer:

The pump must be set to deliver 100 mL/hr for 50 mL to infuse within 30 minutes.

NOTE

The ratio and proportion could have been set up as follows:

50 mL30min=x mL60min

Calculating mL/hr Using Dimensional Analysis

Calculation of mL/hr using dimensional analysis is similar to the formula method.

Steps:

• Identify what you are looking for, and write it to the left of the equation in a fraction format.

• Write the starting fraction using the information from the problem.

Example 1:

Client with an infusion pump has an order for 3,000 mL of D5W over 24 hr.

Label factor: x mLhr=Starting fraction:3,000 mL24hrx mLhr=3,000 mL24 hr Note: No cancellation of units is required here.x=125mL/hr

Example 2:

A client with an infusion pump is to receive an antibiotic in 50 mL of 0.9% NS over 30 minutes.

Notice here that the time is less than an hour. The pump delivers mL/hr. Therefore the fraction 1 hr = 60 min is added to the equation.

Practice Problems

Calculate the flow rate in mL/hr.

1. 1,800 mL of D5W in 24 hr by infusion pump ____________________

2. 2,000 mL D5W in 24 hr by infusion pump ____________________

3. 500 mL RL in 12 hr by infusion pump ____________________

4. 100 mL 0.45% NS in 45 min by infusion pump ____________________

5. 1,500 mL D5RL in 24 hr by infusion pump ____________________

6. 750 mL D5W in 16 hr by infusion pump ____________________

7. 30 mL of antibiotic in 0.9% NS in 20 min by infusion pump ____________________

Answers on p. 654

CALCULATING IV FLOW RATES IN gtt/min

When an electronic infusion device is not used, the nurse manually regulates the IV rate. To manually regulate an IV, the nurse must determine the rate in gtt/min.

IV flow rates in gtt/min are determined by the type of IV administration tubing. The drop size is regulated by the size of the tubing. (The larger the tubing, the larger the drops.) The first step in calculating IV flow rate is to identify the type of tubing and its calibration. The calibration of the tubing is printed on each IV administration package (Figure 22-1).

IV TUBING

IV tubing has a drop chamber. The nurse determines the flow rate by adjusting the clamp and observing the drop chamber to count the drops per minute (Figure 22-2). The size of the drop depends on the type of IV tubing used. The calibration of IV tubing in gtt/mL is known as the drop factor and is indicated on the box in which the IV tubing is packaged. This calibration, which is necessary to calculate flow rates, is shown on the packaging of IV administration sets (see Figure 22-1).

The two common types of tubing used to administer IV fluids are as follows:

Macrodrop Tubing

Macrodrop is the standard type of tubing used for general IV administration. This type of tubing delivers a certain number of gtt/mL, as specified by the manufacturer. Macrodrop tubing delivers 10, 15, or 20 gtt equal to 1 mL. Macrodrops are large drops; therefore large amounts of fluid are administered in macrodrops (Figure 22-3, A).

Microdrop Tubing

Microdrop tubing delivers tiny drops, which can be inferred from the prefix micro. It is used when small amounts and more exact measurements are needed, for example, in pediatrics, for the elderly, and in critical care settings. Microdrop tubing delivers 60 gtt equal to 1 mL. Because there are 60 minutes in an hour, the number of microdrops per minute is equal to the number of mL/hr. For example, if clients are receiving 100 mL/hr, they are receiving 100 microdrops/min (see Figure 22-3, B).

Figure 22-4 shows a comparison of calibrated drops.

Tips for Clinical Practice

The nurse must be aware of the drop factor to accurately administer IV fluids to a client.

Practice Problems

Identify the drop factor and type of tubing for the IV tubing pictured below.

8. ____________________

9. ____________________

10. ____________________

11. ____________________

Answers on p. 654

Points to Remember

• Knowing the drop factor is the FIRST step in accurate administration of IV fluids.

• The drop factor always appears on the package of the IV tubing.

• Macrodrops are large and deliver 10, 15, or 20 gtt/mL.

• Microdrops are small and deliver 60 gtt/mL.

• Drop factor = gtt/mL.

• To determine flow rates for an electronic infusion device (pump or controller), determine mL/hr by using the following formula:

x mL/hr (rounded to the nearest whole number)=Total mL orderedTotal hr ordered

CALCULATING FLOW RATES IN DROPS PER MINUTE USING A FORMULA

The calculation of IV flow rate in gtt/min can be done by using a formula method or dimensional analysis. Several formulas can be used; this text will focus on the most popular formula used. Calculation of the rate in gtt/min by using dimensional analysis will also be demonstrated in this chapter. The most common calculation necessary when an IV is manually regulated or an infusion device is not used involves solving to determine the rate in gtt/min.

To calculate the flow rate at which an IV is to infuse, regardless of the method used (formula or dimensional analysis), the nurse needs to know the following:

Formula Method

The information is placed into a formula. Let’s examine a formula that might be used. This formula is the most popular when calculating flow rate when the rate can be expressed as 60 minutes or less.

x gtt/min=Amount of solition (mL) × Drop factorTime(min)

Before calculating, let’s review some basic principles:

1. Drops per minute are always expressed in whole numbers. You cannot regulate something at a half of a drop. Because drops are expressed in whole numbers, principles of rounding off are applied; for example, 19.5 gtt = 20 gtt.

2. Carry division of the problem one decimal place to round to a whole number of drops.

3. Answers must be labeled. The label is usually drops per minute unless otherwise specified. Examples: 100 gtt/min or 17 gtt/min. To reinforce the differences in drop factor, the type of tubing is sometimes included as part of the label. Examples: 100 microgtt/min or 17 macrogtt/min.

Let’s look at some sample problems and a step-by-step method of using the formula to obtain answers.

Example 1:

Order: D5W to infuse at 100 mL/hr. Drop factor: 10 gtt/mL. At what rate in gtt/min should the IV be regulated?

1. Set up the problem, placing the information given in the correct position in the formula.

x gtt/min=100 mL×10 gtt/mL60 min

2. Reduce where possible to make numbers smaller and easier to manage. Note that the labels are dropped when starting to perform mathematical steps.

3. Divide 100/6 to obtain rate in gtt/min. Carry division one decimal place and round off to the nearest whole number.

x=1006=16.6x=17gtt/min

Answer:

x = 17 gtt/min; 17 macrogtt/min

To deliver 100 mL/hr with a drop factor of 10 gtt/mL, the IV rate should be adjusted to 17 gtt/min. This answer can also be expressed with the type of tubing as part of the label, for example, 17 macrogtt/min.

Example 2:

Order: IV medication in 50 mL NS in 20 minutes. Drop factor: microdrop (60 gtt/mL). At what rate in gtt/min should the IV be regulated?

Answer:

x = 150 gtt/min; 150 microgtt/min

To deliver 50 mL in 20 minutes with a drop factor of 60 gtt/mL, the IV should be adjusted to 150 gtt/min. This may sound like a lot; however, remember the type of tubing used is a microdrop.

Dimensional Analysis Method

Let’s look at calculating rate in gtt/min using the process of dimensional analysis. Remember that IV fluids are ordered in small volumes of fluid that usually contain medication or in large volumes to infuse over several hours. Let’s look at the previous examples by using dimensional analysis.

Example 1:

Order: D5W to infuse at 100 mL/hr. Drop factor: 10 gtt/mL. At what rate in gtt/min should the IV be regulated?

1. You are calculating gtt/min, so write gtt/min to the left of the equation, followed by the equals sign (=), and label gtt/min x, since that is what you’re looking for:

xgttmin=

2. Extract the information that contains gtt from the problem; the drop factor is 10 gtt/1 mL.

    Write this factor into the equation, placing gtt in the numerator.

x gttmin=10gtt1mL

3. The next fraction is written so that the denominator matches the previous fraction (what you are looking for). Go back to the problem and you will see that the order is to infuse 100 mL in 1 hr. Enter the 1 hr as 60 min in the denominator because you are calculating gtt/min (100 mL/60 min).

x gttmin=10 gtt1 mL×100 mL60 min

4. Now that you have the completed equation, cancel the units and notice that you are left with the desired gtt/min.

Answer:

x = 17 gtt/min; 17 macrogtt/min

Note: Example 1 could have been done without changing the hourly rate to 60 min, but it would have required the addition of a 1 hr = 60 min conversion factor to the equation.

x gttmin=10 gtt1 mL×100 mL1 hr×1 hr60 min

The next step would be to cancel the denominator/numerator mL and hr, leaving the desired gtt and min.

Remember that reducing can make numbers smaller.

Example 2:

Order: An IV medication of 50 mL NS in 20 min. Drop factor: microdrop (60 gtt/mL). At what rate in gtt/min should the IV be regulated?

1. You are calculating gtt/min, so write gtt/min to the left of the equation, followed by the equals sign (=), and label gtt/min x, since that is what you’re looking for:

x gttmin=

2. Extract the information that contains gtt from the problem; the drop factor is 60 gtt/1 mL.

    Write this factor into the equation, placing gtt in the numerator.

x gttmin=60 gtt1 mL

3. The next fraction is written so that the denominator matches the previous fraction (what you are looking for). Go back to the problem and you will see that the order is to infuse 50 mL in 20 minutes. Enter the third fraction so that 50 mL is in the numerator and 20 minutes is in the denominator.

x gttmin=60 gtt1 mL×50 mL20 min

4. Now that you have the completed equation, cancel the units and notice that you are left with the desired gtt/min.

Answer:

150 gtt/min; 150 microdrops/min

Calculating Drops per Minute With Large Volumes of Fluid

Remember that IV fluids can be ordered in large volumes to infuse over several hours, for example, 1,000 mL over x hr; or the large volume can be ordered by total volume to infuse and the mL/hr rate of infusion (125 mL/hr). Example: 1,000 mL D5W at a rate of 125 mL/hr. Remember that when a large volume to infuse over several hours is ordered, a preliminary step can be done to change it to mL/hr. Example: 1,000 mL D5W to infuse in 8 hr. Divide the total volume by the number of hours to get mL/hr. In this case, 1,000 mL ÷ 8 hr = 125 mL/hr. Then proceed to calculate gtt/min.

x mL/hr=Amount of solution (mL)Time (hr)

The formula method or dimensional analysis may be used to calculate gtt/min for a volume of fluid to be administered in more than 1 hour. Now let’s look at some examples where a large volume of fluid will infuse over more than 1 hour.

Example 1:

Order: 1,000 mL D5W to infuse in 8 hr. Drop factor: 20 gtt/mL. At what rate gtt/min should IV be regulated?

Solution Using the Formula Method

1. Calculate the mL/hr.

    

x mL/hr=1,000 mL8  hrx=125 mL/hr

2. Calculate the gtt/min.

x gtt/min=125 mL×20 gtt/mL60 min

3. Reduce.

Answer:

x = 42 gtt/min; 42 macrogtt/min

Solution Using Dimensional Analysis

Order: 1,000 mL D5W to infuse in 8 hr. Drop factor: 20 gtt/mL. At what rate in gtt/min should IV be regulated?

1. Begin by determining mL/hr, converting the time in hours to minutes. 1 hr = 60 min. Now proceed to set up the problem in the dimensional analysis equation.

2. Enter the gtt/min being calculated first, followed by the equals sign (=).

x gttmin=

3. Enter the drop factor (20 gtt/mL) with gtt in the numerator.

x gttmin=20 gtt1 mL

4. Take from the problem the amount to be administered over 1 hr (60 min) and write the fraction so that it matches the denominator of the fraction immediately before it.

x gttmin=20 gtt1 mL×125 mL60 min

5. Now that you have completed the equation, cancel the units and proceed with the mathematical process to obtain the answer.

Note: The above example could have been done without changing 1 hr to 60 min; however, if it is left as 1 hr, you will need to add the additional fraction (1 hr = 60 min). With the same example, the equation would be stated as follows:

x gttmin=20 gtt1 mL×125 mL1 hr×1 hr60 min

Notice that the conversion factor 1 hr = 60 min is written so the numerator matches the denominator of the fraction immediately before it. To solve the equation, cancel units and proceed as follows:

Remember that determining mL/hr before calculating gtt/min helps to keep the numbers smaller. However, using dimensional analysis also allows you to determine gtt/min without this step by writing one equation; but you would need the conversion factor 1 hr = 60 min. The equation would be stated as follows:

x gttmin=20 gtt1 mL×1,000 mL8 hr×1 hr60 min

Notice that the volume ordered is written to match the denominator of the fraction immediately before it and that the conversion factor 1 hr = 60 min is written so the numerator matches the denominator before it.

Example 2:

Order: 1,500 mL 0.9% NS in 10 hr. Drop factor: 15 gtt/mL. At what rate in gtt/min should the IV infuse?

Solution Using the Formula Method

1. Calculate mL/hr.

    

x mL/hr=1,500 mL10 hrx=150 mL/hr

2. Calculate gtt/min.

x gtt/min=150 mL×15gtt/mL60 min

3. 

4. 

x=1504=37.5

Answer:

x = 38 gtt/min; 38 macrogtt/min

Solution Using Dimensional Analysis

Order: 1,500 mL 0.9% NS in 10 hr. Drop factor: 15 gtt/mL. At what rate in gtt/min should the IV infuse?

1. Calculate mL/hr (1,500 mL ÷ 10 hr = 150 mL/hr); convert the time in hours to minutes. Proceed using the steps outlined in Example 1.

    

    or

    Set up the equation without changing 1 hr to 60 min.

    or

    Set up the equation without finding mL/hr first.

Practice Problems

Calculate the flow rate in gtt/min using the formula method or dimensional analysis.

12. Administer D5RL at 75 mL/hr. The drop factor is 10 gtt/mL.____________________

13. Administer D5 ½ NS at 30 mL/hr. The drop factor is a microdrop.____________________

14. Administer RL at 125 mL/hr. The drop factor is 15 gtt/mL.____________________

15. Administer 1,000 mL D5 0.33% NS in 6 hr. The drop factor is 15 gtt/mL.____________________

16. An IV medication in 60 mL of 0.9% NS is to be administered in 45 min. The drop factor is a microdrop.____________________

17. 1,000 mL of Ringer lactate solution (RL) is to infuse in 16 hr. The drop factor is 15 gtt/mL. ____________________

18. Infuse 150 mL of D5W in 2 hr. The drop factor is 20 gtt/mL.____________________

19. Administer 3,000 mL D5 and ½ NS in 24 hr. The drop factor is 10 gtt/mL.____________________

20. Infuse 2,000 mL D5W in 12 hr. The drop factor is 15 gtt/mL. ____________________

21. An IV medication in 60 mL D5W is to be administered in 30 minutes. The drop factor is a microdrop.____________________

Answers on pp. 654–655

Calculation of IV Flow Rates Using a Shortcut Method

This shortcut method can be used only in settings where the IV sets have the same drop factor. Example: an institution where all the macrodrop sets deliver 10 gtt/mL. This method can also be used with microdrop sets (60 gtt/mL). It is important to note that this method can be used only if the rate of the IV infusion is expressed in mL/hr (mL/60 min). It is imperative that nurses become very familiar with the administration equipment at the institution where they work.

To use this method you must know the drop factor constant for the administration set you are using. The drop factor constant is sometimes referred to as the division factor. To obtain the drop factor constant (division factor) for the IV administration set being used, divide 60 by the drop factor calibration. Box 22-1 shows the constant calculated based on the drop factor for the tubing.

BOX 22-1

Drop Factor of Tubing	Drop Factor Constant
10 gtt/mL	6010=6
15 gtt/mL	6015=4
20 gtt/mL	6020=3
60 gtt/mL	6060=1

Example:

The drop factor for an IV administration set is 15 gtt/mL. To obtain the drop factor constant:

6015=4 (drop factor constant=4)

RULE

After the drop factor constant is determined, the gtt/min can be calculated in one step:

x gtt/min=mL/hrgtt factor constant

Practice Problems

Calculate the drop factor constant for the following IV sets.

22. 20 gtt/mL____________________

23. 10 gtt/mL____________________

24. 60 gtt/mL____________________

Answers on p. 655

Now that you know how to determine the drop factor constant, let’s look at examples of using a shortcut method to calculate gtt/min.

Example 1:

Administer 0.9% NS at 100 mL/hr. The drop factor is 20 gtt/mL. The drop factor constant is 3.

Solution Using the Shortcut Method

Therefore this problem could be done by using the shortcut method once you know the drop factor constant (division factor).

x gtt/min=100 mL/hr3=33.3=33 gtt/minx=33gtt/min

Notice that the 100 mL/hr rate divided by the drop factor constant gives the same answer.

Answer:

33 gtt/min; 33 macrodrops/min

Solution Using Dimensional Analysis

Administer 0.9% NS at 100 mL/hr. The drop factor is 20 gtt/mL. The drop factor constant is 3.

Step 1:

State mL/hr as mL/60 min.

Note: In the equation, because time is stated as 60 min, the administration set calibration (20) will be divided into 60 (min) to obtain a constant number (3). 3 is the drop factor constant for 20 gtt/mL administration set. Using the drop factor constant, you can calculate gtt/min in one step (divide mL/hr by the drop factor constant).

Answer:

33 gtt/min; 33 macrogtt/min

Example 2:

Administer D5W at 125 mL/hr. The drop factor is 15 gtt/mL. The drop factor constant is 4.

NOTE

It is important to realize that with a microdrop, which delivers 60 gtt/mL, the gtt/min will be the same as mL/hr. This is because the set calibration is 60, and the drop factor constant is based on 60 minutes (1 hr); therefore the drop factor constant is 1.

Solution Using the Shortcut Method

Calculate the gtt/min.

x gtt/min=125 mL/hr4=31.2=31gtt/minx=31 gtt/min

Answer:

31 gtt/min; 31 macrogtt/min

Solution Using Dimensional Analysis

Administer D5W at 125 mL/hr. The drop factor is 15 gtt/mL. The drop factor constant is 4.

Answer:

31 gtt/min; 31 macrogtt/min

Example 3:

Administer 0.9% NS at 75 mL/hr. The drop factor is 60 gtt/mL. The drop factor constant is 1.

Solution Using the Shortcut Method

Calculate the gtt/min.

x gtt/min=75 mL/hr1=75=gtt/minx=75 gtt/min

Answer:

75 gtt/min; 75 microgtt/min

Solution Using Dimensional Analysis

Administer 0.9% NS at 75 mL/hr. The drop factor is 60 gtt/mL. The drop factor constant is 1.

Answer:

75 gtt/min; 75 microgtt/min

Practice Problems

Calculate the rate in gtt/min using the shortcut method.

25. Order: D5W 200 mL/hr.

    Drop factor: 10 gtt/mL____________________

26. Order: RL 50 mL/hr.

    Drop factor: 15 gtt/mL____________________

27. Order: 0.45% NS 80 mL/hr.

    Drop factor: 60 gtt/mL____________________

28. Order: 0.9% NS 140 mL/hr.

    Drop factor: 20 gtt/mL____________________

Answers on p. 656

Remember that the shortcut method discussed (using the drop factor constant) can be used to calculate the gtt/min for any volume of fluid that can be stated in mL/hr or mL/60 min.

RULE

The shortcut method can be used if the volume is large; however, an additional step of changing mL/hr first must be done. You can then proceed to calculate the gtt/min using the shortcut method.

Example 1:

Order: RL 1,500 mL in 12 hr. Drop factor: 15 gtt/mL. Drop factor constant: 4.

1,500 mL÷12=125mL/hr

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