IV - Single Dose

Give xx mg over yy min.

One major (but not only) use of this type of calculation, is giving a loading dose of a medication. There are a number of medications that require a minimum blood level before they are therapeutic. Two examples include Reopro (ABCIXIMAB) & Cardizem (diltiazem). I'll use Cardizem as an example.

Your monitored patient develops A-Fib with rapid ventricular response. They are currently asymptomatic (A & Ox3, stable B.P., pain free), but with a heart rate of 140-160 you recognize this as a dangerous rhythm that needs to be addressed.  You call the managing physician & they order a loading dose of 25mg of Cardizem, a calcium channel blocker, given over 60 minutes, following by a maintenance drip at 5mg/hr. Pharmacy sends the medication in a concentration of 125mg/125ml.  How do we give the loading dose?

We know:
Our ordered dose - 25mg over 1 hr.
Our available form - 125ml
Our available dose - 125mg

Given this information, what do we need to figure out to be able to give the loading dose? Depending on how we administer the dose, there are a couple things we'll be interested in. One, is how many cc's of the medication pharmacy sent correspond to the ordered loading dose.  The second is the actual IV rate we need to administer that dose over the specified time. We'll figure our dose first.

What we are actually going to give the patient is ml's. So our first step is going to be to calculate how many ml's are needed for the loading dose. Because Cardizem is in a 1:1 concentration, the math is trivial - but this kind of example makes it easy to verify that the process we are using works correctly.  Start with what we want to know :

 

 

Our first factor will be the thing we know that has what we want to know in it, oriented so it is in the same position.

 

 

And then multiply by our ordered dose oriented opposite that of the available dose weight unit :

 

 

We can see we are not going to have any units remaining except what we want to know, so factor & solve:

 

 

To give the loading dose, we will run the pump at 25ml/hr for one hour & then reduce the rate to the maintenance rate (we haven't calculated that). 

The above method gives us the volume of our loading dose. It doesn't actually give us information about administering the dose.  A second step calculates the actual flow rate for the loading dose. To do this, what do we want to know? We are still going to give ml's but now we also want to specify a time. We will give these ml's with a pump that is programmed with ml/hr. Start with what you want to know:

 

 

Again, our first factor will be the thing we know that has what we want to know in it, oriented so it is in the same position.

 

 

And multiply by the ordered dose as before but including the ordered time :

 

 

Looking at our factors, we see everything except what we want to know will cancel, so solve :

 

 

The usefulness of doing the problem this way becomes more obvious if we change the problem a little.  Suppose the order was to give the loading dose over 15 min. instead? The first two steps of setting up the problem will be unchanged. We are still going to start with ml/hr because that is what we use to program the IV pump.

 

 

Again, our first factor will be the thing we know that has what we want to know in it, oriented so it is in the same position.

 

 

And multiply by the ordered dose as before  including the ordered time as well:

 

 

Looking at our factors (above), we see we are not going to have matching units - we have hr on the left side of the equation and min. on the right. We need to add a conversion factor for hr<>min.:

 

 

Now all our units match, so cancel & solve:

 

 

We will run our pump at 100ml/hr for 15 minutes and then decrease the rate to the maintenance rate.

 

 

Safety Hints

In the scenario above, it would be easy to make a critical medication error. Running the pump at 100ml/hr, if you somehow "got busy" & failed to decrease the rate at the correct time it would not take long to give the patient too much medication - potentially enough to be fatal. To make sure this does not happen, always use the loading dose volume (never use the bag volume) when programming a pump for a loading dose (or bolus).

Most IV pumps provide for programming a Secondary Medication Infusion. A safer way of doing the bolus is to program the primary drip rate at the maintenance rate and putting the loading dose in as a Secondary Medication. This allows the pump to automatically switch to the maintenance rate after giving the bolus, greatly reducing the risk of over medicating the patient. Most likely, you will need to adjust the volume of the primary medication. It will need to be decreased by the volume of the loading dose since the secondary medication is not subtracted from that volume.

Using this method requires both of the calculations on this page. The first to calculate the volume of the Secondary infusion and the second to calculate the rate for the Secondary infusion.

When starting a medication such as IV Cardizem, the patient should be actively monitored and frequent vital signs should be taken while initiating the drug (q5min - q15min).  Be aware that co-morbid conditions, other medications and sensitivity to the drug can significantly influence the effect of the drug. Many facilities will have protocols for such medications and you should be aware of them and use them.

 

©1997-2006 Dale Sampson, RN