(Note: Originally posted with a typo, “800 mg” instead of the correct “400 mg” – but now corrected! Thanks everybody!)
Last week DTs began to brush up on med math, in preparation of precepting a rising medic with Major Transport Company.
“Brush up on medmath, DTs?” you say. “Shouldn’t you know this stuff?”
Well, yes. But as any medic will tell you, med math is one of those facets of EMS where everyone has their own method. My preceptee-to-be has already expressed an interest in learning more in this fascinating area.
So I was re-familiarizing meself with all the available methods, e.g. Ratio and Proportion method, Formula method, Cross-multiplication, Three-Step, Rule-of-Fours, and so-on, when I came to Dopamine, and the Neat Thing.
Dopamine: the bugaboo of 2-am drips. Here is a sample word problem: You have a patient who weighs 220 lbs, and the doctor orders you to start a 5 mcg/kg/min dopamine drip. You have 400mg dopamine and a 250ml bag of D5w. What is the drip rate?
Lessee here, this guy weighs 220lbs, which times 2.2 is 100kg, and we need 5 mikes/kg so that’s 100 kg * 5 is 500 micrograms per minute, and there’s 400 milligrams of medicine in 250 milliliters of D5w so that makes 1600 micrograms per milliliter…
At 2am medics have been known to run screaming into the night.
Now, dopamine is especially atrocious because different dosages seem to have different properties. For instance, 2mcg/kg/min is a “renal” dose, appropriate for maintaining renal function, while 5mcg/kg/min is considered an inotropic or “cardiac” dose, and 15+mcg/kg/min is the alpha agonist or “vasopressor” dosage, useful for maintaining blood pressure. So, yeah, somebody somewhere is at some point gonna make the medic start a dopamine drip.
So here’s the Neat Thing:
In all this reading, and now of course I can’t find exactly where, but props to the Brady company and Dr. Bryan Bledsoe – I’m pretty sure it was in one of their Tomes – DTs came across something called the Colorado Down and Dirty Dopamine Ditty. At least, I think that’s what it was called.
Easy-peasy dopamine calculation for when you want the cardiac dose of 5mcg/kg/min: When your concentration is 1600 micrograms/milliliter, take the patient’s weight IN POUNDS, divide by 10, and subtract 2. That’s your drip in milliliters per hour.
(220 POUNDS / 10) = 22, subtract 2 = 20 milliliters/hour.
As long, that is, as you want the 5 mcg/kg/min rate.
Now, as the “down and dirty” implies, this is not exactly right. For instance, a patient weighting 160 pounds gets, by the Colorado method, (160/10)-2 = 14 ml/hour. The actual calculation, where (milliliters per hour) = (weight kg) * (dose mcg/kg/min) * (60 min/hour) / (concentration mcg/ml)
would be ((160/2.2) * 5 * 60)/1600, or (72.73 * 300)/1600, or 13.64 ml/hr. The Colorado method is therefore 0.36 ml/hr off! This is a 2.67% error! BFD. At 2 am, this is great.
The DTs Cheat is even easier, and has a more consistant error rate.
The DTs Cheat: Weight (kilograms) / 5.
A 220 lb patient is (as we all know from endless classes) the Perfect Weight Drug Patient (not so much for lifting). 220 lb = 100 kg.
100 kg / 5 = 20 ml/hr.
“DTs, this is all fine,” you say, gently, “But you do realize that your simplistic, and simple-minded method, may not always apply? I mean, come on, it works for a 100 kg patient, but…”
That’s what I thought, too, so I built a model (simple spreadsheet did for it) and ran the formulas head-to-head from a 50 lb patient through a 380 lb patient. At 50 lbs, the Colorado method was 29.6% off; the DTs method was 6.67%; at 380 lbs, Colorado was 11.16% while DTs was 6.67%.
|Weight lbs||Weight kg
(lbs / 2.2)
ml/hr = (kg*dose*60)
( lbs / 10) – 2
So anyway, there’s that for what it’s worth. 400mg of dopamine in 250ml D5w, kgs/5, set your drip.
Of course, if you have a pump, by all means do it the long way – makes it much easier to modify during transport.
And, if anyone can figure out a neat little math trick to get rid of that constant 6.67% (I am certain there must be one!) please let me know, as that would be very cool indeed.