Infusion Pump Rates: mL per Hour CBT Practice Questions

Pumps run almost every drug infusion on a UK ward, and they’re programmed in one unit only: millilitres per hour. Every pump question, however dressed up, ends with you producing that number.

For plain fluids the formula is a single division:

mL per hour = total volume ÷ time in hours.

1,000 mL over 8 hours: 1,000 ÷ 8 = 125 mL/h. Done.

The exam builds difficulty in two ways from there.

Times that aren’t whole hours

A 48 mL antibiotic bag over 30 minutes. The pump still wants mL per hour, so scale: 30 minutes is half an hour, and 48 mL in half an hour means 96 mL in a full one. Set 96 mL/h and the pump delivers the bag in 30 minutes.

The real-world version of this error, setting the volume as the rate, runs the infusion at half speed. In the exam, the volume itself always sits among the options looking innocent. If the time was under an hour, the rate must be bigger than the volume.

Dose-calibrated infusions

Heparin, insulin and some other drugs are prescribed as a dose per hour, not a volume. One extra step converts dose to volume:

• Concentration = drug in the bag ÷ bag volume.
• Rate = prescribed dose per hour ÷ concentration.

Worked example. Heparin 25,000 units in 50 mL, prescribed at 1,000 units per hour.

• Concentration: 25,000 ÷ 50 = 500 units/mL.
• Rate: 1,000 ÷ 500 = 2 mL/h.

Small numbers, big stakes. Lose a zero in the concentration and the answer becomes 20 mL/h, a ten-fold heparin overdose. Syringe-driver drugs run at rates between about 0.5 and 10 mL/h, so a dose-calibrated answer of 20 or 50 mL/h deserves a second look before you commit.

If the prescription is per minute (some emergency drugs are), convert to per hour before anything else. 5 mg/min is 300 mg/h. Then proceed as normal.

Keeping the two IV formulas apart

The drip rate chapter and this one describe the same bags of fluid through different equipment. The exam exploits that: a 1,000 mL over 8 hours question offers both 42 (dpm, gravity) and 125 (mL/h, pump) as options. The last line of the question names the device. Read it before you calculate, not after.

Five pump questions below. The next chapter moves to weight-based dosing, where the patient’s size joins the arithmetic.