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136
DOI: 10.2174/1874434601913010136, 2019, 13, 136-138
The Open Nursing Journal
Content list available at: https://opennursingjournal.com
LETTER
A Comprehensive Formula for Calculating the Infusion Rate of Medications
Diluted with Infusion Solution
Mohammad Reza Soltani Zarandi1 and Hadi Khoshab2,*
1Inspection Unit, Police Hospital, Kerman, Iran.
2School of Nursing and Midwifery, Bam University of Medical Sciences, Bam, Iran.
Article History Received: April 18, 2019 Revised: June 19, 2019 Accepted: June 24, 2019
DEAR EDITOR,
Drug administration is an integral part of the nurse’s role
[1], and drug calculations are one of the most important
nursing care measures [2]. There are various calculating
formulas for different drug orders and the multiplicity of these
methods has led to the difficulty of various calculations [3] to
the extent that one of the most common nursing errors is
mistakes in the calculation of drug dosage especially, the
infusion rate of drug solutions [4]. A single formula that can
comprise all different aspects of the physician's order, drug
dose, and type of infusion set used and ultimately can express
infusion rate in terms of drops per minute (gtt/min) and
milliliter per hour (ml/hr) can partly reduce this problem.
Mistakes in calculating the infusion rate are more important in
Iran because of the limited financial resources for supplying
the infusion pumps and other automatic injecting devices that
have the ability of drug calculation and infusion rate. In this
research, researchers tried to innovate a simple, uniform and
applicable formula that can be used for a wide range of
infusions.
Two of the valid and widely used formulas and their
limitations and disadvantages are:
A) [5]
The most important limitations include:
(1) It is not able to calculate the infusion rate in gtt/min
* Address correspondence to this author at School of Nursing and Midwifery,
Bam University of Medical Sciences, Bam, Iran; Tel: 00989132425700;
Fax: 003442510847; Email: hadikhoshab@gmail.com
(because the type of infusion set is not included).
(2) The concentration of the drug should be available.
Although the volume of drug solution and the amount of he
drug (diluted) are usually present, the calculation of the
concentration will prolong the process and the probability of
errors increases.
(3) The dose of the drug should be in mcg, mg or a
specified unit, while the drugs may be used in different units,
such as gram.
(4) The units of drug dose and concentration should be
identical, while this formula does not have the ability to make
them identical.
B) [6]
The most important limitations include:
(1) It is not able to calculate the infusion rate in gtt/min
(because the type of infusion set is not included).
(2) The number 1000 is used for converting mg to mcg.
Whereas, if the drug unit is in gram and the order is in mcg,
then this formula is not applicable.
The benefits and advantages of our formula:
(1) The infusion rate is calculated in gtt/min, while having
the ability to calculate it in mL/hr too (Tips for using this
formula No 5).
(2) There is no need to calculate drug concentration.
(3) It is applicable for any type of drug unit.
(4) It is able to make different dose units and drug units
identical.
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A Comprehensive Formula for Calculating The Open Nursing Journal, 2019, Volume 13 137
(5) Unlike the two previous formulas, the type of infusion
set is included.
The method of obtaining the formula is as follows:
As we know, the infusion rate (when the infusion time is
known) is obtained from the following method:
On the other hand, if we do not have the total infusion
time, we can get it from the following formula using the total
dosage of the drug and the physician's order:
By replacing the second formula in the first, considering
and applying other conditions (Drug orders based on the
patient's weight, the difference of the drug unit with the
physician's order, and finally the request for the administration
of the infusion of the drug in terms of hours or minutes), the
formula for calculating the infusion rate will be as follows:
Tips for using this formula:
(1) Regarding the use of this formula, the volume of the
serum (or solution) should be in mL, and the physician's order
and the total dose of the drug inside the serum (or solution)
should be available.
(2) All physician's orders expressed as dose/time, or
dose/weight /time, can be calculated and executed using this
formula.
(3) The drug dosage can have any unit, but since the unit
converter in the denominator is to equalize the unit of the drug
dosage with the physician's order, it is only used when the drug
unit in solution is not the same as the unit of the physician's
order. For example, the drug is in mg, but the physician's order
is in mcg (mcg/min, mcg/kg/min, etc.), in which case, the unit
converter is 1000 (mcg = 1000mg). So if the dosage of the drug
inside the serum (or solution) is the same as the unit of the
physician's order (both mg, U, etc.), we will not use the unit
converter in the denominator of the fraction.
(4) The drop factor is used in the numerator of the fraction
to convert the serum (or solution) volume (ml) to the drop (gtt).
In microset, this number is 60 (60 drops = 1 ml) and in the
infusion set, it is 10, 15 or 20 (according to the infusion set
type).
(5) To use the syringe pump for infusion, you must
calculate the infusion rate as milliliter per hour (ml/hr). Also, in
some infusion pumps, even using regular infusion set, it is
necessary to set the infusion rate as ml/hr. On the other hand,
we know that the infusion rate is in microset in gtt/min = ml/hr
(1cc =60 gtts and 1h=60min). Therefore, through the microset
formula (i.e, taking the number 60 as the drop factor) and
taking into account, the solution volume (according to the
volume of the syringe, serum, etc.) we reach the answer in gtt
/min and set the same number as ml/hr on the infusion pump.
(6) The patient's weight in the numerator of the fraction
should be used only for drugs prescribed by the weight
(Example: mcg/kg/min). Note that the unit of the physician's
order must be the same as the unit of the patient's weight (kg,
lb…).
(7) Since the number 60 in the denominator is to convert hr
to min, we only use it when the physician's order is expressed
in terms of hours (mg/hr, u/hr, mcg/kg/hr, etc.). So, if the
physician's order is in minutes (mg/min, mcg, mcg/kg/min,
etc.), we will not use the number 60 in the denominator.
(8) Consider that if the dose of the drug in the solution
changes (half, twice, etc.), we will use the same modified dose.
For example, if 4 ampules of 100mg Lidocaine are poured in
the serum (400mg), we use the number 400 in the denominator.
(9) It is recommended that we use this formula for
infusions in which the total infusion time is not determined.
Applicable Example
Q: We want to infuse Midazolam according to the
physician's order as 30mcg/kg/hr for a patient weighing 70kg.
If we pour three 5mg ampules of this drug in a 20 ml syringe
and dilute it to the volume of 20 ml, calculate the infusion rate:
A: As already mentioned, the infusion rate in the syringe
pumps is in ml/hr, whereas the final infusion rate of the above
formula is in gtt/min. But it has already been determined that in
microsets, the infusion rate in gtt/min would be equal to ml/hr.
Therefore, the drop factor here is 60.
=2.8 gtt/min =2.8ml/hr
(1) As the drug is prescribed by weight, then the patient's
weight is used in the numerator.
(2) Since the dosage is in mg, but the physician's order is in
mcg, the number 1000 is used in the denominator as the unit
converter (mcg = mg × 1000).
(3) Since the physician's order is hour (mcg/kg/hr), the
number 60 is used in the denominator to convert the unit into
minutes.
(4) Because three 5mg ampules are used, the total dosage
of the drug in the syringe is considered 15mg
FUNDING
None.
CONFLICT OF INTEREST
The authors declare no conflict of interest, financial or
otherwise.
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138 The Open Nursing Journal, 2019, Volume 13 Zarandi and Khoshab
ACKNOWLEDGEMENTS
Declared none.
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© 2019 Zarandi et al.
This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is
available at: (https://creativecommons.org/licenses/by/4.0/legalcode). This license permits unrestricted use, distribution, and reproduction in any medium, provided
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