How the ONE formula works with IVs

I can see the puzzled expression - how does this "one formula" work with IV drips ? As with the rest of our factor label problems, we need to ask the question, what exactly is it we want to know ?  What we are going to administer is drops (gtt) over some unit time. For IV drips, drops are usually expressed per minute - mostly because that's a practical unit to use. It combines not having to spend forever watching the drip with a long enough period to get a good estimate of the rate of flow.

The first part of our formula is

 

 

The ordered part is straight forward - we're given that by the MD as some volume per unit time, e.g. 50cc/hr (or some variation of that anyway).   Our formula so far is (gtt & ml are both units of volume!):

 

 

Now how about the available part? That is, what do we have on hand to give these drops with? The answer to that is the IV tubing we use. Tubing as you remember, comes in different "sizes" - those sizes are expressed as drop factors. Just as with needle gauge size, the smaller the number, the "bigger" the tubing. The tubing we pick will determine the available form we use in our formula. Common IV tubing includes:

 

 

 

 

There are other sizes - these are the three I most frequently encounter.

This completes our formula :

 

 

 

Lets put some numbers in here to see how it works. And how do we know which way to orient or second factor? Remember as we add factors, they are always added with the unit  OPPOSITE an existing like unit :

 

 

And go ahead and cancel & "solve" the equation:

 

 

 

We went ahead and solved this so you can see how difficult it is to get a "wrong" answer.  If you have an incorrect factor or are missing a factor, you answer tells you what is wrong!  As you can see, our unit of time is not correct & we need a conversion factor to complete this. Since we have "min" on the left (what we want to know) & "hr" in our answer, we need to convert hr <> min. Write the conversion factor so hour is in the opposite position:

 

 

 

Now go through and cancel and solve:

 

 

Since the only units remaining are the same as "what we want to know", we can be confident we have correctly worked through the problem.

Which tubing you choose to use makes a difference.  As a general rule, the faster the ordered IV rate, the larger the tubing we will need - meaning the smaller the drop factor. The following examples will help to demonstrate this ....

 

©1997-2006 Dale Sampson, RN