Dose from IV Rate

Whenever we want to calculate a drug dose from an IV rate, the three things we have to know are the IV rate, the available form & available dose (i.e., concentration). What we are going to 'want to know' will depend on how the infusion was ordered because typically we will want to compare the dose the patient is getting to the ordered dose. Generally, this will be some form of weight/unit time such as ug/min or mg/hr.

Example 1 - Dosage ug/min

This example uses the data from the NTG problem Mg/Min Example 2. Our patient has nitroglycerine infusing at 10ml/hr and we want to know the actual dose they are getting.

We know:
Our IV rate - 3ml/hr
Our available form - 250ml
Our available dose - 50mg

NTG dosage is usually ordered as ug/min  so we will start with that:

 

 

Our first factor is something we know that has a unit ( or class of unit) in common with what we want to know, oriented with the common unit  in the same position:

 

 

And then multiply by our available drug, orienting units so they are opposite:

 

 

We can see we have ug/min on the left of the equation and mg/hr on the right, so we will need conversion factors for min<>hr and mg<>ug. Lets add those, remembering to orient our units opposite:

 

 

Canceling our units, we can see the only units remaining are what we want to know:

 

 

So we can finish factoring and solve :

 

 

 

Example 2 - Dosage ug/kg/min

This example uses the data from the dopamine problem Mg/Kg/Min example 1.  Our patient has a dopamine infusion running at 10ml/hr and we want to know the actual dose they are getting.  We want to know ug/min because we want to compare it to the calculated value of what the MD ordered.

 What do we know:
Our drip rate - 10ml/hr
Our available form - 250ml
Our available dose - 400mg
Our ordered dose - 260ug/min (calculated value to compare our answer to - pt. weight X ordered dose)
Our ordered dose - 4ug/kg/min
Our patient's weight - 65kg

Starting with what we want to know :

 

 

Our first factor will be something we know that has a unit (or a unit in the same class) in common  with what we want to know, orienting it so the unit is in the same position. It could be either our drip rate or our available drug - the important part is writing it with the correct orientation. We'll use the drip rate :

 

 

 

 

Multiply by our available drug orienting the units opposite :

 

 

We can see we have ug/min on the left of the equation and mg/hr on the right, so we will need conversion factors for min<>hr and mg<>ug. Lets add those:

 

 

Canceling our units, we can see the only units remaining are what we want to know:

 

 

So we can finish factoring and solve :

 

 

 

If we then divide 266ug by the patients 65kg we get 4.1ug/kg/min. 

One reason we might want to do this calculation is if there had been a change in the patients weight. When the infusion was started, the patient weighted 65kg. Today we weigh the patient & get 70kg and we wonder how this has affected the dosage.  Our calculation doesn't change because that is giving us the actual amount of drug infusing.  Instead of dividing 266ug by 65kg, divide by the new weight of 70kg. We get a dose of 3.8ug/kg/min with the new weight.

 

Example 3 - total dose (mg) infused

Sometimes we have a need to know how much drug a patient has had infused. Your patient with chronic respiratory problems was started on a theophylline infusion during the previous shift. When you assess the patient, they are anxious, flushed, and have a heart rate in the 120's. You know these are not necessarily uncommon side effects of this drug, but want to make sure the physician is aware the patient is experiencing them.  When you call, the physician asks, how much theophylline has the patient had so far?  How do we calculate this?

What do we know:
Our drip rate - 50ml/hr
Our available form - 500ml
Our available dose - 400mg
Volume remaining in bag - 200ml
Volume infused (Total volume - remaining volume) - 300ml

We want to know mg so we start with that:

 

 

Our next factor will be something we know that includes mg. The only thing we know that includes mg is the available drug, so add that orienting it so the units are in the same position:

 

 

 

Our next factor must have ml in it because the only unit we want remaining is mg. Since we want to know how much the patient has already got, we need to use the volume that has already been infused. Add that orienting the units so they are opposite.

 

 

We can see the only remaining unit is what we want to know, so cancel & solve:

 

 

We could also calculate how much drug remained to be infused. Instead of using the volume infused in this calculation, we would use the volume remaining.

 

©1997-2006 Dale Sampson, RN