# How do I not kill my future patients?

March 2, 2012 2:17 PM Subscribe

I'm learning how to do dosage calculations in class - I'm terrible at math/dimensional analysis, and I am afraid I am going to overdose/underdose/kill all of my future patients! How do I quell my panic and embrace this aspect of my job? Advice from animal and human nurses especially appreciated.

I’m in the second semester of my third year at my four-year veterinary technology program. I took pharmacology last semester where we covered basic dosage calculations. It took my four times as long as everyone else to get the basics, but I did and I felt pretty comfortable with them. Recently I had to calculate a dosage during a practical – and everything I learned last semester flew out of my head. It was horrible, but I reviewed at home and was able to freshen up.

I have a pretty traumatic history with math. I dropped out of high school in my junior year because I couldn’t pass freshman math and got my GED instead. For three years I had the same sadist horrible man for a math teacher who took pleasure in publicly humiliating me. I got at A- in College Algebra, but I had to study five times as long as everyone else and barfed before every test.

Anyway, my surgical nursing class is now covering constant rate infusions, and I am having a mental block. I just can’t get it. I know that it will take 8 to 10 hours of extra studying to be able to accurately do these problems. My professor covered it at length today, and I feel just as lost as ever. I really enjoy pharmacology and surgical nursing, but it takes me months to learn this stuff when it should just take me days! And you know, not overdosing/underdosing/killing you patients, is like, uh, ESSENTIAL.

I know my classmates aren’t struggling with CRIs and other dosage calculation as much as I am because I talk to them, and often ask them for help.

So: How do you accurately do dosage calculations under pressure? How to overcome this math panic? How can I learn to love dosage calculations the way I love the other aspects of this field?

Advice and personal anecdotes from anyone else whose over come math panic is very much appreciated.

I'm not interested in advice on how to be a better student, nor am I looking "math hacks" in general. The math we use isn't very complex. Hence feeling like a total idiot.

I’m in the second semester of my third year at my four-year veterinary technology program. I took pharmacology last semester where we covered basic dosage calculations. It took my four times as long as everyone else to get the basics, but I did and I felt pretty comfortable with them. Recently I had to calculate a dosage during a practical – and everything I learned last semester flew out of my head. It was horrible, but I reviewed at home and was able to freshen up.

I have a pretty traumatic history with math. I dropped out of high school in my junior year because I couldn’t pass freshman math and got my GED instead. For three years I had the same sadist horrible man for a math teacher who took pleasure in publicly humiliating me. I got at A- in College Algebra, but I had to study five times as long as everyone else and barfed before every test.

Anyway, my surgical nursing class is now covering constant rate infusions, and I am having a mental block. I just can’t get it. I know that it will take 8 to 10 hours of extra studying to be able to accurately do these problems. My professor covered it at length today, and I feel just as lost as ever. I really enjoy pharmacology and surgical nursing, but it takes me months to learn this stuff when it should just take me days! And you know, not overdosing/underdosing/killing you patients, is like, uh, ESSENTIAL.

I know my classmates aren’t struggling with CRIs and other dosage calculation as much as I am because I talk to them, and often ask them for help.

So: How do you accurately do dosage calculations under pressure? How to overcome this math panic? How can I learn to love dosage calculations the way I love the other aspects of this field?

Advice and personal anecdotes from anyone else whose over come math panic is very much appreciated.

I'm not interested in advice on how to be a better student, nor am I looking "math hacks" in general. The math we use isn't very complex. Hence feeling like a total idiot.

Bring it all back around to the pharmacology that you love and reframe the calculations as gaining additional knowledge within the classifications of medications and the scenarios in which you use those medications.

So, within the context of a trauma, there are probably around 3 classifications of drugs that will be important because of the dynamics of what happens to patients in the majorities of traumas. Those three drugs will have their own dosage calculation "profile," that is, one will titrate up with the weight of your patient, one will have a very narrow therapeutic range and "never" go up over a certain dose, another will have a similar starting dose for just about every patient. Now, this isn't a hack, but a way to incorporate dosing into the pharmacokinetics/dynamics of a category of drugs (in this case, trauma drugs) that you are interested in learning about.

Another category with limited classifications of drugs might be antimicrobials. Again, each antimicrobial will have a dosing profile unique to that drug that tells you more about that drug's personality and use. Really internalizing the dosing profile of a drug within these categories of clinical applications can give you a new pathway to access a calculation when you are on the spot providing patient care or covering orders, even if the orders are STAT.

Also, I think a lot of the "freeze" that comes with dosing calculation is that split-second moment of doubt that you've arrived at the right calculation. But if you know the dosing personality and profile of different drugs in different clinical settings (and a certain amount of studying this way will make you aware of their common ranges), you'll already know what roughed-in dose amount is "right" and your calculation will simply be a confirmation, removing the tendency to second-guess and freeze.

So, in your studying, my recommendation is to go back to the pharm, organizing according to clinical application, and then learning dosing profiles so that you have better general access to what I know you

posted by rumposinc at 2:41 PM on March 2, 2012 [3 favorites]

So, within the context of a trauma, there are probably around 3 classifications of drugs that will be important because of the dynamics of what happens to patients in the majorities of traumas. Those three drugs will have their own dosage calculation "profile," that is, one will titrate up with the weight of your patient, one will have a very narrow therapeutic range and "never" go up over a certain dose, another will have a similar starting dose for just about every patient. Now, this isn't a hack, but a way to incorporate dosing into the pharmacokinetics/dynamics of a category of drugs (in this case, trauma drugs) that you are interested in learning about.

Another category with limited classifications of drugs might be antimicrobials. Again, each antimicrobial will have a dosing profile unique to that drug that tells you more about that drug's personality and use. Really internalizing the dosing profile of a drug within these categories of clinical applications can give you a new pathway to access a calculation when you are on the spot providing patient care or covering orders, even if the orders are STAT.

Also, I think a lot of the "freeze" that comes with dosing calculation is that split-second moment of doubt that you've arrived at the right calculation. But if you know the dosing personality and profile of different drugs in different clinical settings (and a certain amount of studying this way will make you aware of their common ranges), you'll already know what roughed-in dose amount is "right" and your calculation will simply be a confirmation, removing the tendency to second-guess and freeze.

So, in your studying, my recommendation is to go back to the pharm, organizing according to clinical application, and then learning dosing profiles so that you have better general access to what I know you

*already know*about calculation.posted by rumposinc at 2:41 PM on March 2, 2012 [3 favorites]

Well, I will give you my favorite math hack first. The formula that gets me through everything but vasopressor drips is:

So if you want to give 1000 mg and your bottle has a bunch of 150 mg/ 5ml:

its 1000 / 150 =6.666. Then 6.666 x 5 = 33.33 Or, about an ounce, depending on how accurate you need to be with that particular drug.

And it you apply it to drips and break everything down, it does work for that, too.

Calculations for drips scare

For panic control: Develop a system for your self that starts with a basic written-down formula. Try making a tear sheet with the formjulas you'll need at the top of each page and room to calculate underneath. Do all your calculations in writing, every time. Use a new page for every new drug or dose that you have to calculate, even the simplest. Eventually you will find yourself gaining confidence, knowing the simple answers before you start writing. And at some point the more complex calculations will be less intimidating because you know that you have a system you can rely on.

Best of luck to you. I know you'll get past this hurdle!

posted by SLC Mom at 2:48 PM on March 2, 2012 [1 favorite]

*Dose desired*divided by*dose on hand*times*volume of the dose on hand.*So if you want to give 1000 mg and your bottle has a bunch of 150 mg/ 5ml:

its 1000 / 150 =6.666. Then 6.666 x 5 = 33.33 Or, about an ounce, depending on how accurate you need to be with that particular drug.

And it you apply it to drips and break everything down, it does work for that, too.

Calculations for drips scare

*everyone*, even your professor. And if someone acts like he's not intimidated by them, he's shamming. He has only conquered his fear by grabbing it by the throat and shaking it to death. Honestly, when I worked in PICU, we first got the rate from the pharmacist, then 2 nurses calculated it separately and then compared all 3 answers to ensure accuracy. So don't try to rely on memory or think that you should be able to do it in your head.For panic control: Develop a system for your self that starts with a basic written-down formula. Try making a tear sheet with the formjulas you'll need at the top of each page and room to calculate underneath. Do all your calculations in writing, every time. Use a new page for every new drug or dose that you have to calculate, even the simplest. Eventually you will find yourself gaining confidence, knowing the simple answers before you start writing. And at some point the more complex calculations will be less intimidating because you know that you have a system you can rely on.

Best of luck to you. I know you'll get past this hurdle!

posted by SLC Mom at 2:48 PM on March 2, 2012 [1 favorite]

Take a full sheet of paper to do your calculation. Write down all pertinent information - the patient's weight (in kg, convert from lbs), the concentration of the medication, the flow rate of the CRI, the dose.

Include units in all of your calculations. Make sure they cancel out. If you expect to get mL, and your answer contains mL/min. or mg/mL, you will know your calculation is correct.

Do practice problems. Do them over and over. Get an idea of what ballpark your answer will be in.

Example, for a Fentanyl CRI:

Dose = 3-10 mcg/kg/hr (we usually start at 3mcg/kg/hr dose and increase from there)

Concentration = 50mcg/mL (for example)

10kg patient

1. Start by calculating your patient's fentanyl dose

Patient kg x 3mcg/kg/hr = fentanyl/hour

10kg dog x 3mcg/kg/hr = 30mcg/hr

2. Calculate how many mL of 50mcg/mL fentanyl your patient will need in an hour

Fentanyl dose in mcg / 50mcg/mL = mL/hour

30mcg fentanyl / 50mcg/mL = 0.6mL/hour

3. Calculate how many mL of fentanyl your patient will need. A good rule of thumb is to multiply the patient's volume per hour by how many hours (12-14 hours or so) the patient will be on the CRI

0.6mL/hour x 14 hours = 8.4mL (plus some volume to fill the IV lines).

Example, Reglan CRI

1. Start by calculating your patient's reglan dose in mg/day

Patient's weight in kg x 2mg/kg/day = reglan dose per day (mg/day)

10kg dog x 2mg/kg/day = 20mg reglan/day

2. Now calculate how many mg of reglan your patient will need per hour

Patient's reglan dose in mg / 24 hours = mg/hour

20mg Reglan / 24 = 0.83mg/hour

3. Next find out how many hours you will get out of your fluid bag using the rate of infusion (in this example, 25mL/hour)

1000mL x 1L / 25mL/hour = 40 hours / 1L bag of fluid

4. Now determine how many mg of reglan need to be added to your bag of fluids

0.83mg Reglan / hour x 40 hours / 1L bag of fluid - 33.2 mg of reglan

posted by Seppaku at 2:53 PM on March 2, 2012 [3 favorites]

Include units in all of your calculations. Make sure they cancel out. If you expect to get mL, and your answer contains mL/min. or mg/mL, you will know your calculation is correct.

Do practice problems. Do them over and over. Get an idea of what ballpark your answer will be in.

Example, for a Fentanyl CRI:

Dose = 3-10 mcg/kg/hr (we usually start at 3mcg/kg/hr dose and increase from there)

Concentration = 50mcg/mL (for example)

10kg patient

1. Start by calculating your patient's fentanyl dose

Patient kg x 3mcg/kg/hr = fentanyl/hour

10kg dog x 3mcg/kg/hr = 30mcg/hr

2. Calculate how many mL of 50mcg/mL fentanyl your patient will need in an hour

Fentanyl dose in mcg / 50mcg/mL = mL/hour

30mcg fentanyl / 50mcg/mL = 0.6mL/hour

3. Calculate how many mL of fentanyl your patient will need. A good rule of thumb is to multiply the patient's volume per hour by how many hours (12-14 hours or so) the patient will be on the CRI

0.6mL/hour x 14 hours = 8.4mL (plus some volume to fill the IV lines).

Example, Reglan CRI

1. Start by calculating your patient's reglan dose in mg/day

Patient's weight in kg x 2mg/kg/day = reglan dose per day (mg/day)

10kg dog x 2mg/kg/day = 20mg reglan/day

2. Now calculate how many mg of reglan your patient will need per hour

Patient's reglan dose in mg / 24 hours = mg/hour

20mg Reglan / 24 = 0.83mg/hour

3. Next find out how many hours you will get out of your fluid bag using the rate of infusion (in this example, 25mL/hour)

1000mL x 1L / 25mL/hour = 40 hours / 1L bag of fluid

4. Now determine how many mg of reglan need to be added to your bag of fluids

0.83mg Reglan / hour x 40 hours / 1L bag of fluid - 33.2 mg of reglan

posted by Seppaku at 2:53 PM on March 2, 2012 [3 favorites]

I do these calculations over and over and over again, so I know that, for example, if I'm setting up a fentanyl CRI and my calculated rate is less than 0.1mL/hour or greater than 10mL/hour, I know I've done something wrong. Or if I'm calculating a reglan CRI and I calculate that I need to add 3mg of reglan or 300mg of reglan to a bag, I know I've done something wrong.

I went to grad school with a bunch of math wizards at Caltech. I was a mere biochemist and my math skillz were pathetic compared to the theoretical physicists and engineers. It was intimidating. I thought I wasn't good at math. But I eventually learned that wasn't it - I just didn't use it as much as they did. It's like learning a language.

So when I started doing veterinary medicine, I practiced. I wrote everything down. And I practiced some more. Don't be afraid to ask for help, particularly of those classmates that seem to know what they're doing. You're in school to learn, it's not expected that you will automatically know how to do this without any practice.

Good luck!

posted by Seppaku at 3:09 PM on March 2, 2012 [1 favorite]

I went to grad school with a bunch of math wizards at Caltech. I was a mere biochemist and my math skillz were pathetic compared to the theoretical physicists and engineers. It was intimidating. I thought I wasn't good at math. But I eventually learned that wasn't it - I just didn't use it as much as they did. It's like learning a language.

So when I started doing veterinary medicine, I practiced. I wrote everything down. And I practiced some more. Don't be afraid to ask for help, particularly of those classmates that seem to know what they're doing. You're in school to learn, it's not expected that you will automatically know how to do this without any practice.

Good luck!

posted by Seppaku at 3:09 PM on March 2, 2012 [1 favorite]

It's been a while since I read it, but my recollection of the paper "Proportional Reasoning in Nursing Practice" is that insurance companies were very worried about how often nurses make errors in calculating dosages, and the take-home message (if I remember the paper correctly) is that they aren't really thinking mathematically at all when they do this. Instead, they follow a memorized algorithm that most of them have down pat, and an error in calculating dosages is very rare. I suspect that how well or poorly you do in an exam will have little to do with how competently you carry out this algorithm for reals -- you'll get lots of practice, and when you start out someone will be looking over your shoulder.

posted by Killick at 3:23 PM on March 2, 2012 [1 favorite]

posted by Killick at 3:23 PM on March 2, 2012 [1 favorite]

This doesn't apply directly to dosage calculations, but in my college algebra class we were assigned "The Math Anxiety Workbook" and it was one of those books that I was like, "I'm so not buying that!," but it ended up being great. It's a thin book that you could easily read in a couple hours. I don't remember specifics except that I found it helpful.

I was in the dummy algrebra class that met every weekday even though it was a 3 hour class, as an English major, after not having math of any kind for a couple of years. I didn't realize that I didn't like math because of my anxiety about it until I read that book.

Also, a lot of my anxiety was based on the fact that I didn't understand what math was all about. I could memorize the steps, but I never understood what it could do or even what a graph really meant. Rumposinc above has the excellent advice that you are tying your math to a specific, real, purpose.

Good luck to you!

posted by shortyJBot at 3:37 PM on March 2, 2012

I was in the dummy algrebra class that met every weekday even though it was a 3 hour class, as an English major, after not having math of any kind for a couple of years. I didn't realize that I didn't like math because of my anxiety about it until I read that book.

Also, a lot of my anxiety was based on the fact that I didn't understand what math was all about. I could memorize the steps, but I never understood what it could do or even what a graph really meant. Rumposinc above has the excellent advice that you are tying your math to a specific, real, purpose.

Good luck to you!

posted by shortyJBot at 3:37 PM on March 2, 2012

Seconding Seppaku: write out every step in full, and KEEP TRACK OF YOUR UNITS. I don't calculate patient doses but I do a whole lot of concentration/dilution calculations and keeping track of units at every step is a huge sanity check for me. If I have an extra unit (like mg/ml where I expect only mg), I know I goofed somewhere.

I have colleagues who do similar calculations in a single step and they kind of snicker at me (by now, I should know this stuff cold, right?), but to hell with 'em. I want the extra confidence in my calculations that comes from writing everything out in detail - and if things go wrong, I can go back and find where the mistakes crept in. Just as important, sometimes it wasn't my fault and I didn't screw up the calculation and I can show it. So there.

posted by Quietgal at 6:07 PM on March 2, 2012 [1 favorite]

I have colleagues who do similar calculations in a single step and they kind of snicker at me (by now, I should know this stuff cold, right?), but to hell with 'em. I want the extra confidence in my calculations that comes from writing everything out in detail - and if things go wrong, I can go back and find where the mistakes crept in. Just as important, sometimes it wasn't my fault and I didn't screw up the calculation and I can show it. So there.

posted by Quietgal at 6:07 PM on March 2, 2012 [1 favorite]

My wife, who has been a tech for 15 years recommends this equation:

She's had people try to teach her CRIs a million different ways, and plugging numbers into this equation is the only thing that works for her. You can keep it on a cheat sheet for easy reference. She also suggests the Medical Calculations page in the "Notebook" on vspn.org in general, which you should be eligible to join as a student, and you should do anyway, if you haven't already.

posted by Rock Steady at 6:35 PM on March 2, 2012 [1 favorite]

fluid mL/hr total volume of fluid__drug mL/hr__=__x mL of drug__fluid mL/hr total volume of fluid

She's had people try to teach her CRIs a million different ways, and plugging numbers into this equation is the only thing that works for her. You can keep it on a cheat sheet for easy reference. She also suggests the Medical Calculations page in the "Notebook" on vspn.org in general, which you should be eligible to join as a student, and you should do anyway, if you haven't already.

posted by Rock Steady at 6:35 PM on March 2, 2012 [1 favorite]

I'm a math person, not a nurse of any variety. But, if I understand what you're doing correctly, it's 'just' factor-label/dimensional analysis. It takes practice not to get overwhelmed by the information (so I recall from high school chemistry), but once you get the hang of it, you're good to go and you won't kill anyone. Seriously. (This is probably the single most useful thing I learned in high school chemistry, coincidentally. Possibly the most useful thing I learned in high school science.)

Whenever this sort of calculation comes up, I do it in one giant line and cancel the units as I go. So for me Seppaku's fentanyl example would look something like:

10 kg/1 * 3 mcg /(kg *hr) * (1 mL)/(50 mcg) = (30 mL)/(50 hr) = .6 mL/hr.

Each time I write down a new term, I cancel what units I can. I know to write 1/50 mL/mcg rather than 50 mcg/mL since I've got a mcg on the top that I've got cancel since I want to end with mL/hr (and ending up with mcg^2 in the units isn't going to help).

If you're lucky, your local library with have a chemistry textbook that'll have some detailed examples. It's all about trusting the units. If you can do currency conversions or change feet to meters, you can do this.

For example, $1 is about 63p. £1 is €1.20. So how much is $2 in euros? Well,

2 USD /1 * .63 GBP/ 1 USD * 1.2 EUR / 1 GBP = €1.51, which is what Google says.

Or suppose I tell you that 1 inch is 2.54 cm and want you to tell me how many meters 2 feet is?

2 ft * 12 in / 1 ft * 2.54 cm/1 in * 1m /100 cm = .6096 m

If it's not too condescending a suggestion, try copying out those two examples and seeing how they work on paper (where the fractions look better and look like stuff actually cancels). It's exactly the same calculation as the dosages. Heck, it's the same calculation you do at the grocery store when you're trying to figure out if the bigger thing is really cheaper, just with more conversions.

posted by hoyland at 8:07 PM on March 2, 2012 [1 favorite]

Whenever this sort of calculation comes up, I do it in one giant line and cancel the units as I go. So for me Seppaku's fentanyl example would look something like:

10 kg/1 * 3 mcg /(kg *hr) * (1 mL)/(50 mcg) = (30 mL)/(50 hr) = .6 mL/hr.

Each time I write down a new term, I cancel what units I can. I know to write 1/50 mL/mcg rather than 50 mcg/mL since I've got a mcg on the top that I've got cancel since I want to end with mL/hr (and ending up with mcg^2 in the units isn't going to help).

If you're lucky, your local library with have a chemistry textbook that'll have some detailed examples. It's all about trusting the units. If you can do currency conversions or change feet to meters, you can do this.

For example, $1 is about 63p. £1 is €1.20. So how much is $2 in euros? Well,

2 USD /1 * .63 GBP/ 1 USD * 1.2 EUR / 1 GBP = €1.51, which is what Google says.

Or suppose I tell you that 1 inch is 2.54 cm and want you to tell me how many meters 2 feet is?

2 ft * 12 in / 1 ft * 2.54 cm/1 in * 1m /100 cm = .6096 m

If it's not too condescending a suggestion, try copying out those two examples and seeing how they work on paper (where the fractions look better and look like stuff actually cancels). It's exactly the same calculation as the dosages. Heck, it's the same calculation you do at the grocery store when you're trying to figure out if the bigger thing is really cheaper, just with more conversions.

posted by hoyland at 8:07 PM on March 2, 2012 [1 favorite]

So, I was once you and now I teach this very class at a VT program.

Rock Steady's wife has it, as do Seppuku, really everyone

You may be a "formula" person if you want an answer under pressure, so find a formula for each problem type that you like and just apply

My experience teaching plug-in formulae is that they help greatly as long as everything is in the right kind of units

Dimensional analysis helps when formulae don't; I reccomend "getting" dimensional analysis as a student because then when you have a problem that a formula doesn't exactly handle smoothly you can convert a thing to another thing easily if you know the conversion factor, and these are easy to learn.

Setting up the problem in dimensional analysis is another skill. Not hard when it's one little fraction at a time.

I recommend practice practice practice, and playing around with fractions, drug and drip rates are all that those are, mcg/lb, mg/kg/min, etc.

also: asking another person in practice to check your math is not only ok, it's great, and done all the time!

You can memail me if you want, I have hundreds of problem sets I could send you or talk about this more with you and I really want all techs in the world to be medical math ninjas

posted by bebrave! at 11:50 PM on March 2, 2012 [1 favorite]

Rock Steady's wife has it, as do Seppuku, really everyone

You may be a "formula" person if you want an answer under pressure, so find a formula for each problem type that you like and just apply

My experience teaching plug-in formulae is that they help greatly as long as everything is in the right kind of units

Dimensional analysis helps when formulae don't; I reccomend "getting" dimensional analysis as a student because then when you have a problem that a formula doesn't exactly handle smoothly you can convert a thing to another thing easily if you know the conversion factor, and these are easy to learn.

Setting up the problem in dimensional analysis is another skill. Not hard when it's one little fraction at a time.

I recommend practice practice practice, and playing around with fractions, drug and drip rates are all that those are, mcg/lb, mg/kg/min, etc.

also: asking another person in practice to check your math is not only ok, it's great, and done all the time!

You can memail me if you want, I have hundreds of problem sets I could send you or talk about this more with you and I really want all techs in the world to be medical math ninjas

posted by bebrave! at 11:50 PM on March 2, 2012 [1 favorite]

This thread is closed to new comments.

How do you accurately do [insert action here] under pressure? How to overcome this [insert subject here] panic?This isn't the hack you are looking for, but I have found that the best answer to these questions is practice. It will be time consuming and frustrating at first, but make it a priority to do X amount of calculations per day. Take your time at first, and as you become more proficient, time yourself. Make a game out if if you have to. Give yourself a treat if you can correctly solve X problems in Y minutes.

Basically, practice dosage calculations so much that they become automatic and a non-issue. I'm sure your teacher will be happy to provide extra questions if you exhaust the material in your texts.

posted by Kevtaro at 2:41 PM on March 2, 2012 [1 favorite]