Infusion Rate and the Missed Hour
A patient is ordered 1,200 mg of Drug X in 500 mL of normal saline, to infuse over exactly 24 hours. Pharmacy supplies a 500 mL bag containing 1,200 mg of Drug X. The tubing drop factor is 15 gtt/mL, and the nurse is using a manual gravity set rather than a pump. Two hours into the infusion, the patient is taken to radiology and the infusion is paused for 45 minutes, as required by department policy. When the patient returns, the bag hangs with noticeably more fluid in it than the nurse expected, and the nurse considers adjusting the rate so the full 1,200 mg is delivered by the original 24-hour mark.
1. Calculate the ordered rate in mL/hr and the corresponding drip rate in gtt/min. Explain how you decided to round the drip rate.
Read the worked answer
Concentration first: 1,200 mg ÷ 500 mL = 2.4 mg/mL.
mL/hr: 500 mL ÷ 24 hr = 20.8 mL/hr (about 21 mL/hr on a manual set).
Cross-check by dose: 1,200 mg ÷ 24 hr = 50 mg/hr; 50 mg/hr ÷ 2.4 mg/mL = 20.8 mL/hr. Both routes agree, which is the check that catches a mis-set decimal.
gtt/min: 20.83 mL/hr × 15 gtt/mL ÷ 60 min/hr = 5.2 gtt/min → 5 gtt/min.
Rounding decision: a gravity set delivers whole drops, so 5.2 cannot be set exactly. Rounding up to 6 gtt/min would deliver about 576 mL in 24 hours - 15% over, and 15% more drug than ordered. Rounding to 5 gtt/min delivers about 480 mL (20 mL short, ~4%). The nurse counts drops over a full minute and tilts toward the lower whole number, then monitors the volume remaining against the clock rather than trusting a single count.
2. Should the nurse increase the rate to make up the 45 lost minutes? Justify your answer in terms of the order, the drug, and safe practice, and state what the nurse should do instead.
Read the worked answer
No - not without a new order.
Three reasons:
- The order is a rate, not just a total. "1,200 mg over 24 hours" authorizes an average of 50 mg/hr. Compressing the remaining dose into 23.25 hours requires a rate above the ordered rate, which is a new order, not a nursing adjustment.
- Concentration matters more than total at any instant. Catching up means a period of higher plasma concentration and a higher risk of rate-related adverse effects. The total dose being unchanged does not make the peak safe.
- The interrupted volume is small. At 20.8 mL/hr, 45 minutes is roughly 15.6 mL, or about 37 mg of drug - well under 4% of the total. The clinical exposure from leaving it undelivered is minor; the exposure from a bolus correction is not.
What to do instead: resume at the ordered 20.8 mL/hr, document the interruption and the volume remaining, note the expected finish time, and notify the prescriber or pharmacist if the full 24-hour dose is clinically required, so the rate change is ordered rather than assumed. This is the recurring exam pattern: a plausible arithmetic fix that is the wrong clinical action.