Lessons · Medicine foundations · maintenance fluid by the 4-2-1 rule
The 4-2-1 rule: maintenance fluid in three slices
Per hour, the rule gives 4 mL per kilogram for the first 10 kg of body weight, 2 mL per kilogram for the next 10 kg, and 1 mL per kilogram for every kilogram beyond 20, added together.
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Not yet checked by a physician. This lesson was written for Hone. Practice material, not professional advice. What that means.
What it is for
It is the arithmetic everybody is asked to do out loud in a teaching session, and the mistake everybody makes the first time is applying 4 mL per kilogram to the whole person. For a 70 kg adult that is 280 mL an hour instead of 110, and the shape of the error is the same every time: the slices were ignored.
How to think about it
Three lines, one per slice, then add. First slice: always 10 kg at 4, so always 40 mL/h once the person is over 10 kg. Second slice: always 10 kg at 2, so always 20 mL/h once they are over 20 kg. Third slice: whatever is left over 20 kg, at 1. For anyone above 20 kg the sum is just 60 plus the kilograms above 20.
Worked example
A 25 kg child, with the rule given by the problemTwenty-five kilograms crosses into all three slices.
First 10 kg: 10 × 4 = 40 mL/hThe first slice is the same for everybody over 10 kg.
Next 10 kg: 10 × 2 = 20 mL/hThe second slice, also fixed once the weight is over 20 kg.
The remaining 5 kg: 5 × 1 = 5 mL/hTwenty-five minus twenty. Only this slice changes with weight.
40 + 20 + 5 = 65 mL/hSixty-five millilitres an hour.
Your turn
A 70 kg adult, with the same 4-2-1 rule. The first two slices give 40 and 20 mL/h. Write the line that gives the total.
40 + 20 + (70 − 20) × 1 = mL/h
Solve one, graded on the server
The trap
Using 4 mL per kilogram for the whole weight. The rule is three slices and the rate per kilogram falls as you move up through them; a single multiplication cannot produce it.