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.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
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.