At 0300 hours, a 2 L of normal saline is set running through an infusion pump at 85 mL/h. After 8 hours, the rate is increased to 120 mL/h. At what time will the infusion be completed?

At 0300 hours, a 2 L of normal saline is set running through an infusion pump at 85 mL/h. After 8 hours, the rate is increased to 120 mL/h. At what time will the infusion be completed?

• A. 1800 h
• B. 2100 h
• C. 2300 h
• D. 2200 h

Answer: (D)

When an infusion rate changes partway through, split the total volume into portions delivered at each rate and add the times. The patient must deliver 2000 mL. In the first 8 hours at 85 mL/h, that portion equals 8 × 85 = 680 mL. So 2000 − 680 = 1320 mL remains. At 120 mL/h, delivering 1320 mL takes 1320 ÷ 120 = 11 hours. The total time from the start is 8 + 11 = 19 hours. Starting at 0300, adding 19 hours brings us to 22:00, or 2200 hours.

When an infusion rate changes partway through, split the total volume into portions delivered at each rate and add the times. The patient must deliver 2000 mL. In the first 8 hours at 85 mL/h, that portion equals 8 × 85 = 680 mL. So 2000 − 680 = 1320 mL remains. At 120 mL/h, delivering 1320 mL takes 1320 ÷ 120 = 11 hours. The total time from the start is 8 + 11 = 19 hours. Starting at 0300, adding 19 hours brings us to 22:00, or 2200 hours.