A one litre IV bag of normal saline has been running for 6 hours at a rate of 75 mL/h. The doctor orders the remaining contents to be run through in the next five hours. Calculate the new flow rate.

• A. 90 mL/h
• B. 120 mL/h
• C. 105 mL/h
• D. 110 mL/h

Answer: (D)

Start with what’s left to deliver and the time allotted. The bag has 1000 mL total. In 6 hours at 75 mL/h you’ve delivered 75 × 6 = 450 mL, so 1000 − 450 = 550 mL remains. The doctor wants that remaining volume delivered in 5 hours, so the new flow rate is 550 mL ÷ 5 h = 110 mL/h. Delivering at 110 mL per hour will complete the infusion in the requested time.