IV fluids & fluid balance: practice problems with step-by-step solutions

1Why fluids are counted

The body needs a steady supply of water to replace what it loses through urine, sweat and breathing. A patient who cannot drink enough may receive maintenance fluids through an IV line.

Nurses also keep an intake and output record. Every drink, IV bag and IV medication goes in the intake column. Urine, vomit and drain fluid go in the output column. The difference shows whether the patient is gaining or losing fluid.

This topic has three parts: how much maintenance fluid a child needs, how to add up an intake and output record, and when an infusion will finish.

2Maintenance fluids: Holliday–Segar

The Holliday–Segar method estimates daily maintenance fluid from body weight. The weight is split into bands, and each band has its own amount per kg:

• the first 10 kg: 100 mL/kg/day, or 4 mL/kg/h;
• the next 10 kg (from 10 to 20 kg): 50 mL/kg/day, or 2 mL/kg/h;
• every kg above 20: 20 mL/kg/day, or 1 mL/kg/h.

The hourly version is called the 4-2-1 rule and the daily version the 100-50-20 rule. They give slightly different numbers, because 100 mL/day is about 4.2 mL/h. Use the version the question names.

Each band only counts the kilograms inside it. A 15 kg child gets 4 × 10 for the first 10 kg and 2 × 5 for the other 5 kg.

3Intake, output and balance

Add everything that went in, add everything that came out, and subtract:

• Intake: oral fluids, tube feeds, IV infusions (rate × hours) and IV medications or flushes. Ice chips count as half their volume, because ice melts to about half its volume in water.
• Output: urine, vomit, drains and other measured losses.

A negative balance means the patient lost more than they took in. A positive balance means a net gain.

Urine output is often checked per kg of body weight per hour:

4The method step by step

For maintenance fluids:

1. Convert the weight to kg if needed (lb ÷ 2.2).
2. Split the weight into the bands: up to 10 kg, 10 to 20 kg, above 20 kg.
3. Multiply each part by its rate and add the parts.

For an intake and output record:

1. List every intake item. Halve the ice chips. Multiply an IV rate by the hours it ran.
2. Add the intake, then add the output.
3. Subtract: intake − output. For urine output per kg per hour, divide by weight and hours.

For infusion times: time = volume left / rate, and volume infused = rate × time. Turn decimal hours into minutes (× 60) before you add them to a clock time.

5A worked example

Follow every step. Try to predict the next one before you open it. All values are made up for practice.

6Common mistakes

• Multiplying the whole weight by one rate instead of using the bands.
• Counting ice chips at their full volume.
• Forgetting the IV infusion or IV medications in the intake.
• Subtracting the wrong way: balance is intake minus output.
• Reading decimal hours as minutes: 4.17 h is 4 h 10 min, not 4 h 17 min.

7How exam questions look

• Easy: maintenance fluids with the Holliday–Segar 4-2-1 rule (mL/h) or the 100-50-20 rule (mL/day), sometimes with the weight in pounds.
• Medium: an intake and output record for one shift: the total intake, the fluid balance, or the urine output in mL/kg/h.
• Hard: infusion completion time: the hours until a bag is empty, the volume left at a given clock time, or a bag whose rate is changed partway.

All values in these questions are made up for practice and are not clinical guidance. Real fluid prescriptions depend on the patient and on local protocols.