Quite an assignment I have: “Write about drug math conversationally. Keep it light. Don’t make it sound like a lecture.”
Light? Conversational? What was I thinking when I suggested that? How informally can I present such a dry, technical topic that many paramedics find scarier than breech births?
If you’re nervous about milligrams and kilograms, you’re not alone. Four studies published since 2000 portray paramedics’ drug math proficiency as frighteningly low.1,2 During one experiment in a simulated high-stress environment, “advanced paramedics” scored only 61% on a series of calculations.1 That’s the good news; less experienced medics averaged not quite 40%.
It gets worse. Only 3 of 47 paramedics from two U.S. sites were able to solve more than 7 of 10 drug problems correctly.2 The mean was 3 of 10. In North Carolina, 109 medics averaged 51% on a 10-question math test—with calculators and unlimited time.1 Researchers Kathryn Eastwood, Malcolm Boyle and Brett Williams speculated that “pressure” was a “major factor” in the above results. However, none of those studies involved actual patients. Practical exams are stressful, but more than real calls? I don’t think so.
I blame technology and our school systems for two generations of practitioners with mediocre math skills. I think we started to devalue manual number crunching in the early 1970s, as first calculators, then computers, made mental arithmetic—a major component of my primary education—seem as outdated as iceboxes. Most paramedics know that even high-tech devices don’t tell us how to frame drug math problems.
My job is to help you make sense of that process by beginning with a review of terminology, adding arithmetic, then focusing on the simplest ways I know to handle drug math in the field. What I won’t be doing is preaching gimmicks that work only for very small subsets of real-world problems. For example, I saw a trick for dopamine drips that reads something like, “Take your waist size, divide it by the number of minutes to defrost a strip steak, add 37 if the patient has red hair, etc.” What good are such shortcuts if you have to remember a different one for each med? I’d rather help you polish skills you can use for a variety of calculations.
Memo to the engineers and physicists among you (including you, Dad): I’m going to bend the definitions of some terms and compromise the precision of certain calculations to achieve a balance between simplicity and accuracy that I believe is acceptable when treating anyone except manikins. If it bothers you to see me call the kilogram a unit of weight instead of mass, please keep two things in mind: (1) I’m assuming all patients are treated on Earth, even if they weren’t born here, and (2) I still suffer post-traumatic stress from early-morning physics classes. If you send me e-mails quoting Newton or Einstein about anything more complicated than growing tomatoes, you might push me over the edge.
Let’s begin with some basics:
Weight is the amount of drug we’re giving. Forget about pounds and ounces; almost all doses are in metric units—grams, milligrams and micrograms. Kilograms are used, too—for patient weights. A kilogram equals a thousand grams, a gram is a thousand milligrams, and a milligram has a thousand micrograms—field-friendly multiples of 10 that make our “English” system of grains, ounces, pounds and tons seem primitive. You disagree? Okay, how many grains in an ounce? I’ll give you a hint: It’s not a round number.
The only conversion factor you might need between English and metric weights for drug math is 2.2 pounds per kilogram. Usually we start with the weight of our patient in pounds, then divide by 2.2 to get kilos. Since we’re often guessing people’s weights, and weight-related drug doses are estimates, also, we could drop the decimal point and just divide by two (except during exams!), which is the same as half the pounds. My approximate weight in kilograms is 150 lbs/2, or 75. (If I were cooking for myself, it would be 120 lbs/2.)