Randomized titratable-drip problems with the premixed bags on the label: dopamine 400 or 800 mg in 250 mL, dobutamine 250, 500 or 1,000 mg in 250 mL, norepinephrine 4 or 8 mg in 250 mL, nitroglycerin 25 or 50 mg in 250 mL. Standard asks for the pump rate. Hard adds patient weights in pounds and reverse problems: what dose is this drip giving?
Convert the bag to mcg/mL, multiply the dose by the weight and by 60 minutes, then divide by the concentration. Dopamine 5 mcg/kg/min, 80 kg, 400 mg in 250 mL (1,600 mcg/mL): 5 × 80 × 60 ÷ 1,600 = 15 mL/hr.
Three conversions hide in every one of these problems: mg to mcg in the bag, minutes to hours in the rate, and the weight, unless the drug is dosed per minute only. Miss any one and the answer is off by a factor of 60 or 1,000, which is why these questions are on critical care exams.
| Order | Setup | Answer |
|---|---|---|
| Dopamine 5 mcg/kg/min; 80 kg; 400 mg/250 mL | 1,600 mcg/mL; 5 × 80 × 60 ÷ 1,600 | 15 mL/hr |
| Dobutamine 5 mcg/kg/min; 70 kg; 500 mg/250 mL | 2,000 mcg/mL; 5 × 70 × 60 ÷ 2,000 | 10.5 mL/hr |
| Norepinephrine 8 mcg/min; 4 mg/250 mL | 16 mcg/mL; 8 × 60 ÷ 16 | 30 mL/hr |
| Nitroglycerin 10 mcg/min; 50 mg/250 mL | 200 mcg/mL; 10 × 60 ÷ 200 | 3 mL/hr |
| Dopamine 400 mg/250 mL at 21 mL/hr; 70 kg | 21 × 1,600 ÷ 60 ÷ 70 | 8 mcg/kg/min |
Dopamine and dobutamine are labeled in mcg/kg/min, so the weight goes into the calculation. Norepinephrine and nitroglycerin are labeled in mcg/min, with no weight. Some units dose norepinephrine per kilogram anyway, which is why the order, not memory, decides which formula to use. If the order says mcg/min and you multiply by weight, the answer is 70 or 80 times too big.
At the bedside you are as likely to read a drip as to start one. The pump says 21 mL/hr; what dose is that? Multiply by the concentration to get mcg/hr, divide by 60 for mcg/min, and divide by the weight if the drug is dosed per kilogram. Hard mode mixes reverse problems in, because they are where students forget the ÷ 60.
A quick sense check for both directions: with a common premix, most starting rates land between about 1 and 40 mL/hr. An answer of 900 mL/hr or 0.03 mL/hr means a conversion went the wrong way.
Vasoactive drips run through smart pumps with drug libraries that do this math for you once the drug, concentration and weight are entered, and they flag doses outside the library limits. Those checks only work if the inputs are right: the correct bag concentration and the patient's actual weight in kilograms. Titration orders, target blood pressures and maximum doses come from the prescriber and the facility protocol. These problems teach the calculation, not the titration decision.
Multiply the dose in mcg/kg/min by the weight in kg and by 60, then divide by the bag concentration in mcg/mL. Dopamine 5 mcg/kg/min for 80 kg from 400 mg in 250 mL (1,600 mcg/mL) is 5 × 80 × 60 ÷ 1,600 = 15 mL/hr.
Multiply the mg in the bag by 1,000 to get mcg, then divide by the bag volume in mL. 400 mg in 250 mL is 400,000 ÷ 250 = 1,600 mcg/mL.
Multiply mL/hr by the concentration in mcg/mL to get mcg/hr, divide by 60 to get mcg/min, then divide by the weight in kg. 21 mL/hr of 1,600 mcg/mL for 70 kg is 8 mcg/kg/min.
The US label doses it in mcg/min, without weight. Some units use mcg/kg/min instead, so always calculate from the units written in the order.