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A patient is prescribed 1 litre of fluid to run over 8 hours. (1) What rate in mL per hour should the pump be set to? (2) If no pump is available and a gravity giving set that delivers 20 drops per mL is used, how many drops per minute are needed? (3) After 3 hours, how much fluid should remain? You have 4 minutes.
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Unit conversion, rate calculations and practical checking.
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Convert units
1 litre = 1,000 mL.
mL per hour
1,000 ÷ 8 = 125 mL/h.
Drops per minute
125 mL/h × 20 drops/mL = 2,500 drops/h; ÷ 60 ≈ 41.7 ≈ 42 drops/min.
Remaining after 3 hours
125 × 3 = 375 mL given; 1,000 − 375 = 625 mL remaining.
Practical safety
Check against the prescription; monitor the patient; risks of fluid overload.
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First, 1 litre is 1,000 mL1. Over 8 hours, that's 1,000 divided by 8, which is 125 mL per hour. That's what I'd set the pump to.
For the gravity set: 125 mL per hour at 20 drops per mL is 125 times 20, which is 2,500 drops per hour. Divide by 60 to get per minute: 2,500 divided by 60. 60 times 40 is 2,400, leaving 100, and 100 divided by 60 is about 1.7. So about 41.7, which I'd round to 42 drops per minute2.
After 3 hours, 3 times 125 is 375 mL should have gone in, so 625 mL should be left in the bag.
That last calculation is actually a useful practical check3 — if you looked at the bag after 3 hours and there was much more or much less left, you'd know something was wrong.
This matters because giving fluid too quickly can cause fluid overload, especially in older patients or people with heart or kidney problems, where it can build up in the lungs. So I'd also want to check the patient themselves, not just the numbers.
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