Two-step problems

At times, two steps are necessary to complete calculations in a problem. These will generally be identified for you as two-step problems, though perhaps not always. There are various forms two-step problems can take.

Reconstitution two-steps

You will find some of these problems in reconstitution problems, when you are told to add medication to a bag but the question in the problem asks you to determine what flow rate you will set. Looking at an example:

Example: The medication comes as 2 g of powder in a large vial. You are to add sterile water to create a concentration of 400 mg/ml, then add 1.2 g of the medication to a 100 ml bag and infuse it in 30 minutes. At what rate will you set the pump?

Step 1 – How much will I need to add to the 100 ml bag?

                ml = 1 ml/400 mg x 1200 mg = 3 ml

Note that it is not necessary to figure out how much fluid to add to the powder. All you need to know is the concentration of what you are going to put into the bag.

Step 2 – ml/hr = 103 ml/30 min x 60 min/hr = 206 ml/hr

Infusion order in something other than ml/hr

Another example of two-step problems can occur when the doctor orders an infusion in something other than ml/hr. Let’s check that one out with an example:

Example: The doctor has ordered 4.5 mg/Kg for your 142-pound patient. You are to add the medication to a 100 ml bag of NS and infuse it at 15 mg/hr.

Note that we could now turn this into a time problem: how long would it take to infuse? Or we could turn it into a flow rate problem: what would be the infusion rate for this medication? It will be more common to see the 2nd type, so let’s go with that.

Step 1 – How much has the doctor ordered for this specific patient?

mg = 4.5 mg/Kg x 1 Kg/2.2 lb x 142 lb = 639/2.2 = 290.5 mg

Step 2 – What’s the infusion rate?

ml/hr = 100 ml/290.5 mg x 15 mg/hr = 5.2 ml/hr

Complex problems

A third type of two-step problems are simply long and cumbersome problems. It is often easier for you to do these problems as two steps (or sometimes more). As an example of this, let’s look at a mcg/min/Kg problem.

Example: The medication is available as 500 mg in 100 ml and is infusing at 50 ml/hr. Your patient weighs 28.3 Kg. How many mcg/min/Kg is he receiving?

Step 1 – How many mcg/min is the patient receiving?

mcg/min = 1000 mcg/1 mg x 500 mg/100 ml x 50 ml/hr x 1 hr/60 min = 4166.7 mcg/min

Now step 2 – How many mcg/min/Kg would that be?

mcg/min/Kg = 4166.7 mcg/min divided by 28.3 Kg = 147.2 mcg/min/Kg

You are unlikely to see any problems this complex during Med/Surg I, but they will be a reality during Med/Surg II.

Because these two-step problems are of different types, there are no separate problems and answers given to you here. Watch for them in the other types of problems you are doing. They will usually be identified as two-step problems.