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Mathematics for Medication Administration

Recalculating Intravenous Flow Rates

Recalculating Intravenous Flow Rates Introduction When using a traditional, manual IV set, the flow rate quite often deviates from the intended rate of infusion. It is the nurse’s duty to take corrective action to ensure that the prescription is satisfied. This module will introduce you to the fundamentals of recalculating flow rates to keep an IV running on time. This will draw upon skills developed in previous modules and expand your knowledge of IV therapy.

Overview x x x

IV Medication Orders Recalculating IV Flow Rates Calculating Variance between Flow Rates

Module Outcomes x x x x

Identify components of an IV medication order, and calculate additional information. Understand the mechanism of manual IV sets and identify factors that cause deviation from intended flow rates. Recalculate the flow rate for an IV set infusing at an incorrect rate. Determine the variance of a recalculated IV flow rate, and recommend a course of action.

IV Fluid Administration

x

The overarching goal for IV fluid administration, with respect to flow rates, is to keep the IV running on time. Manual IV sets, those without an electronic infusion pump, are driven by gravity alone. It is almost as though the IV fluid is being poured from the IV bag, into the patient’s vascular system. For this reason, it is essential that the

Mathematics for Medication Administration

x x

x

Recalculating Intravenous Flow Rates

IV bag be placed above the level of the patient’s heart, the blood pump of the vascular system. The higher an IV bag is hung, the more quickly it will infuse. Conversely, infusion will slow down if the distance between the IV bag and the patient’s heart is too small. If the IV bag sits too low, then blood will begin to flow into the IV tubing. Of course, once the IV equipment is in place, the roller clamp is used to set the IV at the desired rate.

As a consequence of this effect, flow rates can also deviate when a patient moves, especially after standing up or lying down. Safe and accurate administration of IV fluids requires nurses to be vigilant, monitoring infusion at regular intervals of no more than 60 minutes. If it is found that an IV has infused more or less volume than expected, then the flow rate can be adjusted to compensate. For instance, if it is determined that 200 mL should infuse over a given time, but a patient actually receives 300 mL, then infusion can be slowed down for the remainder of the prescription. These adjustments compensate for any deviation from the prescribed flow rate, but are safe only within allowable limits.

IV Medication Orders IV fluid prescriptions take on several different forms, depending on prescriber preference. Consider the following IV medication order. x

Administer 800 mL RL over 8 hr.

Given: mL and hr

Mathematics for Medication Administration

Recalculating Intravenous Flow Rates

Here, the infusion volume and the infusion time are given. With this information, one can determine the prescribed flow rate, even though this is not explicitly stated in the order.

This triangle can be used with any IV order to deduce information beyond what is described by the IV order. In this case, the prescribed flow rate is: 800 mL 1

1

× 8 hr = 100

mL hr

.”

When administering IV fluids, the total volume and total time for infusion cannot change. In this example, the nurse must ensure that the client receives exactly 800 mL of RL within exactly 8 hr; however, the flow rate, 100 mL/hr, can be adjusted within reason to maintain this infusion volume and infusion time. The following IV medication orders take on different forms from the prescription above. Use the triangle to determine an additional value from each order. 1. Administer D10W at 60 mL/hr for 6 hr. (Given: mL/hr and hr) Answer: 360 mL 2. Administer 300 mL ½N/S @ 45 mL/hr. (Given: mL and mL/hr) Answer: 6 hr, 40 min 3. Administer 2/3 & 1/3 at 75 mL/hr, LOS. (Given: mL/hr only) Answer: 600 mL (Assume that the shift is 8 hours long.) x

Remember that LOS stands for length of stay (in hospital).

In every case, an additional value can be calculated from the IV medication order. However, it is always the infusion volume and infusion time that must remain constant. The flow rate may be adjusted within safe limits to ensure that the client receives the prescribed volume within the prescribed time.

Mathematics for Medication Administration

Recalculating Intravenous Flow Rates

Recalculating IV Flow Rates

x

x

It is standard practice for a nurse to monitor patients undergoing IV therapy every 30 to 60 minutes. One piece of information that is easily observable is the volume of fluid remaining in the IV bag. This volume can be used to determine if the IV is running on time, and also to recalculate the flow rate, if necessary. The acronym TBA (to be administered) is sometimes used to refer to the volume remaining in the IV bag; however, this is not necessarily the same as the volume prescribed. The volume TBA is what physically remains, and may be more or less than what the patient requires. For instance, if a patient has been prescribed 350 mL of IV fluid, a 500 mL IV bag would most likely be used, since IV bags are not manufactured in 350 mL volumes.

Recalculating IV Flow Rates: Example Consider the following example. A patient has been prescribed 500 mL of ½N/S to run over 10 hours. A 500 mL IV bag is used for infusion. After 5 hours, you observe 200 mL remaining in the IV bag. The first thing that you should recognize is that a new value can be inferred from the prescription. A volume and a time are given, so it is possible to determine the prescribed flow rate, as well.

Mathematics for Medication Administration 500 mL 1

1

× 10 hr = 50

Recalculating Intravenous Flow Rates

mL . hr

As always, the total volume (500 mL) and the total time (10 hr) cannot change. The flow rate, however, can be adjusted to maintain these parameters. In order to calculate a mL

new flow rate ( ), we need a volume and a time. Here, 200 mL remains in the hr

prescription, and 5 hours are left for infusion. Therefore, the new flow rate is: 200 mL 1

1

× 5 hr = 40

mL hr

.

This appears to be a reasonable answer. After 5 hours, the patient should have received 50

mL

× 5 hr = 250 mL of ½N/S. Instead, a volume of 500 mL − 200 mL = 300 mL infused. Since the IV is running ahead of schedule, the recalculated flow rate should be less than the originally prescribed flow rate. You can verify that the new flow rate is appropriate and will result in a total infusion time of 10 hours for 500 mL of fluid. hr

Recalculating IV Flow Rates: Activity 1. An IV prescription requires RL to infuse at 125 mL/hr over 8 hr. The IV drop factor is 30 gtt/mL. x What is the prescribed drip rate for this order? Answer: 63 gtt/min x

What is the prescribed volume for this order?

Answer: 1000 mL 2. After 2 hours, the patient has received 200 mL of RL. x What is the new flow rate? Answer: 133 mL/hr x

What is the new drip rate?

Answer: 67 gtt/min

Calculating Variance In the previous example, IV fluid was prescribed to run at 125 mL/hr (63 gtt/min). It was then discovered that the IV was running behind schedule, so the rate was recalculated as 133 mL/hr (67 gtt/min). This requires that the patient receive RL at 8 mL/hr above the prescribed flow rate. This is a minor adjustment and is generally considered safe. However, it begs the question: How much is too much? mL

Rather than expressing the change in flow rate as 133 hr − 125

mL hr

=8

mL

hr

, it is more

informative to describe it with respect to the original flow rate. In other words, we can

Mathematics for Medication Administration

Recalculating Intravenous Flow Rates

compare this difference to the IV order by expressing it as a percentage of the prescribed rate. What percentage of 125 mL/hr is 8 mL/hr? 8 mL/hr

125 mL/hr

≈ 0.06 = 6%”

This value, 6%, is called variance. It implies that the flow rate needs to increase by 6% of the originally prescribed rate. Variance is always expressed as a percentage of the original rate. It can be summarized by the formula below. Variance =

New Rate − Original Rate Original Rate

When the new rate is less than the original rate, this formula will yield a negative value. The negative sign should be retained because it provides additional information about the recalculated rate. A negative variance suggests that infusion is slowing down; a positive variance means that it’s speeding up.

UNSAFE

+25%

SAFE

0%

–25%

UNSAFE

Mathematics for Medication Administration

Recalculating Intravenous Flow Rates

A variance of less than 25% (or more than –25%) is generally considered safe, although each medical unit has its own policies regarding acceptable changes in flow rate. For the purposes of this course, 25% is considered to be the dividing line. x x x

A variance of +25% or more is considered unsafe. A variance less than +25%, but more than –25% is considered safe. A variance of –25% or less is considered unsafe.

If the recalculated flow rate has a safe variance, you are permitted to freely adjust the flow rate. If the variance is unsafe, it may still be possible to adjust the flow rate, but two conditions must first be satisfied. 1) The prescriber must be notified of the required adjustment to the flow rate. 2) The patient must be able to tolerate the change in flow rate. This is essential for patients in critical condition, or for IV solutions containing potentially harmful drugs. In the previous example, the 6% variance was calculated using flow rates (mL/hr). Variance can also be calculated using drip rates (gtt/min). 67 gtt/min − 63 gtt/min 4 gtt/min ≈ 0.06 = 6% = 63 gtt/min 63 gtt/min Here, the variance works out to the same value as when using flow rates. Occasionally, there is some difference in these values due to rounding errors. Variance can be calculated using either flow rates or drip rates, but both cannot be used in the same calculation.

Recalculating IV Flow Rates Activities Recalculating IV Flow Rates Activity 1 A patient has been prescribed 250 mL of 2/3 & 1/3 at 50 mL/hr. A 250 mL IV bag is used for infusion, with a drop factor of 15 gtt/mL. x

What additional information can be gathered from this IV order? a) Volume b) Time c) Flow rate

Answer: b) Time x

What is the prescribed infusion time?

Answer: 5 hr

Mathematics for Medication Administration

Recalculating Intravenous Flow Rates

Solution: 250 mL 1

x

1 hr × 50 mL = 5 hr

What is the prescribed drip rate?

Answer: 13 gtt/min Solution: 1 hr 50 mL 15 gtt × × = 12.5 gtt/ min ≈ 13 gtt/min 1 hr 1 mL 60 min After 1 hr, 30 min, 150 mL is left TBA. x

What is the new flow rate?

Answer: 43 mL/hr Solution: 5 hr − 1.5 hr = 3.5 hr left for infusion 1 150 mL × = 42.  857142 mL⁄ hr ≈ 43 mL/hr 1 3.5 hr x

What is the new drip rate?

Answer: 11 gtt/min Solution: 43 mL 15 gtt 1 hr = 10.75 gtt/ min ≈ 11 gtt/min × × 60 min 1 hr 1 mL x

What is the variance?

Answer: 14% or 15% or 15.4% Solution: New Rate − Original Rate Original Rate =

11 gtt⁄min − 13 gtt/min 13 gtt/min

= −0.  15384615 ≈ −15.4% This question can also be solved using flow rates.

Mathematics for Medication Administration

=

Recalculating Intravenous Flow Rates

43 mL⁄ hr − 50 mL/hr 50 mL/hr

= −0.14 ≈ −14% x

Is it safe to adjust the flow rate?

Answer: Yes

Recalculating IV Flow Rates Activity 2 A patient has been prescribed 1 500 mL of D5W over a 24 hour period. On hand, you have a 1 500 mL IV bag of D5W. The IV tubing drop factor is 20 gtt/mL. x

What additional information can be gathered from this IV order? a) Volume b) Time c) Flow rate

Answer: c) Flow Rate x

What is the prescribed infusion time?

Answer: 63 mL/hr Solution: 1 1500 mL × = 62.5 mL⁄ hr ≈ 63 mL⁄ hr 1 24 mL x

What is the prescribed drip rate?

Answer: 21 gtt/min Solution: 63 mL 20 gtt 1 hr = 21 gtt/ min × × 60 min 1 hr 1 mL After 11 hr, 1 100 mL is left TBA. x

What is the new flow rate?

Answer: 85 mL/hr Solution: 24hr − 11 hr = 13 hr left for infusion 1100 mL 1 = 84.  615384 mL ⁄hr ≈ 85 mL/hr × 13 hr 1

Mathematics for Medication Administration x

Recalculating Intravenous Flow Rates

What is the new drip rate?

Answer: 28 gtt/min Solution: 85 mL 20 gtt 1 hr  gtt/ min ≈ 28 gtt/min = 28. 33333 × × 60 min 1 hr 1 mL x

What is the variance?

Answer: 35% or 34.9% or 33% or 33.3% Solution: New Rate − Original Rate Original Rate =

28 gtt⁄min − 21 gtt/min 21 gtt/min

= 0.  33333 ≈ 33.3% This question can also be solved using flow rates. =

85 mL⁄ hr − 63 mL/hr 63 mL/hr

= 0.349206 ≈ 34.9% x

Is it safe to adjust the flow rate?

Answer: No Since the variance is greater than 25%, you must check with the physician, and check the patient’s condition....