Need help with drip rate calculation

This is a multi-step problem that appears much more complicated than it actually is. When you are presented this type of problem you need to determine what you are actually being asked to solve. Once you have done that, take a few minutes, carefully read the information, and identify the individual steps required.

What are you being asked to determine? And what are the individual steps that you need to identify to be able to solve this?

A pt weighing 133 lbs is to receive 22 units/kg/day, to be delivered over 2 hours. The solution is 500 units/66ml and the drop rate is 15 gtts/ml. How many gtts/min should the pt receive?

Begin by recognizing that, while you have mixed units, you only have a few physical quantities to deal with:

mass/weight (lbs, kg)

time (days, mins, hrs)

drug quantity (units)

volume (mL)

Make sure that you're comfortable converting from one to another.

Next, focus on the order... 22 units/kg/day given over 2 hours.

Now focus on the patient... 133 lb

Now you can determine how many units the patient will get by converting the pt's weight in lb to weight in kg.

Now that you know how much the pt is to receive, you need to figure out what volume of the medication contains that quantity of units.

Utilizing 500 units per 66 mL and the units required, you can calculate the volume.

Now that you know the volume, you need to determine the rate... remember, it's ordered over 2 hours.

Now you have a volumetric rate in mL/hr... and in the real world, you'd be done... you'd simply program your pump with the volume and the rate, push start, and check that it's flowing.

In your problem, though, you're looking for a minute-rate, not an hour-rate so convert the mL/hr to mL/min.

Now, you have the volumetric rate per minute but that doesn't tell you how many drops per minute. For that, you need to drip factor which just quantifies how large are the drops... the common drip factors are 10, 15, 20, and 60... in each case it simply says how many drops are required to make 1 mL.

Your problem states 15 drops/mL so now you can convert your volumetric rate per minute to a drip rate per minute.

Once you have worked through this, you can spot check your answers below.

~~~~~

Amount required: 1330 units per day

Volume: 175.6 mL

Volumetric rate: 87.8 mL/hr

Drip rate: 22 drops per minute

You can also do it all in one step. Since the entire day's dose will be delivered over 2 h, for the calculation you can write: 22 units/kg/2 h. This is mathematically equivalent to 22 units/kg*2 h.

133 lb(1 kg/2.2 lb)(22 units/kg*2 h)(66 mL/500 units)(15 gtts/mL)(1 h/60 min)= 22 gtts/min

You can also do it all in one step. Since the entire day's dose will be delivered over 2 h, for the calculation you can write: 22 units/kg/2 h. This is mathematically equivalent to 22 units/kg*2 h.

133 lb(1 kg/2.2 lb)(22 units/kg*2 h)(66 mL/500 units)(15 gtts/mL)(1 h/60 min)= 22 gtts/min

This goes in line with what bjwojcik said about solving it all in one step. You use dimensional analysis - if you see any of my previous posts helping people with dosage calculation problems you will see I LOVE DIMENSIONAL ANALYSIS! Haha! I just love that you can start with what you have (what is ordered) and go along the path, canceling out what you don't need, using conversion factors to get to what you need, all the way down until you get to what you need (what is available). It helps me to be able to follow that path.

You can see in the equation that I started with the pts weight. You could start with 22 units/kg/2 hr, then put the pts weight, then you put the conversion factor 1 kg/2.2 lb, that cancels out the kg in the 22 units/kg/2 hr and the kg in the conversion factor. Then, you put in the solution which is 500 units/66 mL - you put it so the units are on the bottom, which cancels out the units in 22 units/kg/2 hr (important note here is that you can only cancel out diagonally). Then, you put 15 gtts/mL, which cancels out the mL from the 500 units/66 mL and the mL in the 15 gtts/mL. Now you have to do the conversion factor of 1 hr/60 min, to cancel out the hr in 22 units/kg/2 hr, and cancel out the hr in 1 hr/60 min. Now, all you are left with is gtts/min.

Hi KR,

Thanks for quoting me and getting the word out on how easy DA is compared to learning a long list of formulas. I have spent most of my time teaching pharmacy technicians but thought I would see what I could do for the nursing community. I am up to 1 like so far!

Just one little thing I would say about your set up, and I hope you don't take offense, but math is math and we all want to be mathematically correct.

Your hours don't cancel out. You would end up with 88 gtt*h^2/min. You can either use 22 units/kg over 2 h or 22 units/kg*2 h.

-Brad