Critical Care Calculations

 

Dosages of certain IV medications must be titrated to administer a specific dosage per unit of time, for example, mg per minute, mcg per minute, or units per hour.

Dimensional analysis can be used to calculate flow rate, or when the flow rate is known, to assess the  dosage being administered per unit of time. 

The start factor to calculate flow rate is either mL per hour or drops per minute, depending on the type of infusion set being used.  

Example:     

Order: Heparin 25,000 units in 250 mL D5W to infuse at 500 units per hour on an infusion pump.

mL/ hour  = 250 mL  X 500 U = 5 mL/ hour
  25,000 U 1 hour  

Sometimes the dosage is ordered in units or mg per minute.  It is convenient to multiply this dosage by 60, to convert it to units or mg per hour, then use dimensional analysis as above.


To calculate the dosage being administered see the following examples.

Example:

    The client is receiving Isuprel at 30 mL/hr.  Solution is available in 2 mg Isuprel in 250 mL D5W. 
     Calculate how many mcg/ min the client is receiving. 

mcg/ min =  1000 mcg  X 2 mg   X 30 mL  X 1 hour  = 4 mcg/ min
  1 mg 250 mL 1 hour 60 min  

Example:     

Order: Drug XYZ 100mg mixed in 250 mL of fluid with drop factor of 60 gtt/mL and infusing at 10 gtts/min. What is the mcg/min being administered?

mcg/min = 1000 mcg X 100 mg X 1 mL X 10 gtt = 66.67 mcg/min
  1 mg 250 mL 60 gtt 1 min  

 

The dosage may be titrated according to the patient's weight each day.  In that case, the dose per minute or hour must be multiplied by the patient's weight in kg, then perform the calculations as above.

Example:   

1.  Drug A premixed 50,000 units/600 mL

Order: 70 units/kg/h

Client weighs 269 lbs

a)      How many units of Drug A are in each mL?

units/mL =  50,000 units/    83.333 83.3 units/mL 
  600 mL    

                   b)      How many mL/h are required to get the required units?

 

mL/hr =  600 mL/ 70 units  X kg/ X 269 lbs  11,298 102.709 mL/hr    = 102.7 mL/hour
  50,000 units kg/hour 2.2 lbs 1 (person) 110    

 

  1. Drug B contains 275 mg in 450 mL NS

Order is Drug B 8.5 mcg/kg/min

Client weighs 25.5 lbs

a)      How much Drug B will client get per minute?

mcg/min =  8.5 mcg/ kg/ X 25.5 lbs  216.75 98.522 mcg/min or 98.5
  kg/hour 2.2 lbs 1 (person) 2.2  

b)      How many mL/min would you set the pump for?

 

mL/min =  450 mL/ 1 mg  X 8.5 mcg/ kg/ X 25.5 lbs   97537.5  0.161 mL/min  = 0.16 mL/min
  275 mg 1000 mcg kg/min 2.2 lbs 1 (person) 605,000    

Thank you to Rod Kebicz RN BN MN from Winnipeg, Canada for suggesting the addition of these two examples :-)

Assess your skills in critical care IV calculations in Quiz 8

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