IV Fluids and Fluid Balance: cram pack

Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.

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1Formulas and methods

IV fluids & fluid balance

Holliday–Segar per hour
4 mL/kg (first 10 kg) + 2 mL/kg (next 10 kg) + 1 mL/kg (each kg above 20)
Holliday–Segar per day
100 / 50 / 20 mL/kg for the same bands
Fluid balance
intake − output
Ice chips
count as half their volume
Urine output
mL/kg/h = mL / (kg × h)
Infusion time
time (h) = volume left (mL) / rate (mL/h)
Volume infused
mL = rate × time

IV infusion rates

Pump rate
mL/h = volume (mL)/time (h)
Drip rate
drops/min = volume (mL) × drop factor / time (min)
Infusion time
time (h) = volume (mL)/rate (mL/h)

Method

  1. Pump rate in mL/h: volume divided by time in hours.
  2. Drip rate in drops/min: volume × drop factor divided by time in minutes.
  3. For time remaining: volume left divided by the rate.
  4. Round as the question or the equipment requires.

2Practice problems

1.IV fluids & fluid balanceMedium
IntakeOutput12-hour shiftsoup250 mLmilk200 mLtea200 mLIV infusion100 mL/h × 12 hIV antibiotic2 × 100 mLurine300 mLurine275 mLurine350 mLurine400 mLwound drain100 mL

Practice exercise (made-up values, not clinical guidance). The record shows a patient's intake and output for one 12-hour shift. The patient weighs 82 kg. Find the average urine output U in mL/kg/h. Round to two decimal places.

82 kg12 hurine: 300 mLurine: 275 mLurine: 350 mLurine: 400 mLwound drain: 100 mL
U = mL/kg/h
2.IV fluids & fluid balanceHard
500 mL→ 200 mL/hstart 11:30ttotal​ = ? h120 mL/h × 3 h

Practice exercise (made-up values, not clinical guidance). A 500 mL infusion starts at 11:30 at 120 mL/h. After 3 hours the rate is changed to 200 mL/h and is not changed again. How many hours t after the start will the bag be empty? Round to one decimal place.

500 mL120 mL/h · 3 h200 mL/h
t = h
3.IV infusion ratesMedium
V = ?15 gtt/mL50 gtt/min6 h

A drip runs at 50 gtt/min through a set of 15 gtt/mL. How many millilitres V does the patient receive in 6 hours?

50 gtt/min15 gtt/mL6 h
V = mL
4.IV infusion ratesHard
100 mg100 mL20 gtt/mL75 mg/h? gtt/min

100 mg of a drug is diluted in 100 mL and given by gravity through a set of 20 gtt/mL. The order is 75 mg per hour. Find the drip rate R in drops per minute.

100 mg/100 mL75 mg/h20 gtt/mL
R = gtt/min

3Solutions

1.IV fluids & fluid balance

Answer: U = 1.35 mL/kg/h

Intake and output: intake counts everything that goes in (oral fluids, IV infusions, IV medications and flushes, tube feeds); ice chips count as half their volume. Output counts urine, drains, vomit and other losses. Balance = total intake − total output; a negative balance is a net loss. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Total urine

300 + 275 + 350 + 400 = 1325 mL

Per kilogram per hour

U = 1325/(82·12) = 1.347 ≈ 1.35 mL/kg/h

Answer

U = 1.35 mL/kg/h
2.IV fluids & fluid balance

Answer: t = 3.7 h

Infusion time: time (h) = volume to infuse (mL)/rate (mL/h); use the volume still left in the bag, not the bag size. Volume infused = rate × time. Convert decimal hours to hours and minutes with the fraction × 60 (0.5 h = 30 min, not 50 min). The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

First part

120·3 = 360 mL
500 − 360 = 140 mL left

Second part

140/200 = 0.7 h

Total time

t = 3 + 0.7 = 3.7 h

Answer

t = 3.7 h

That is about 3 h 42 min, so the bag runs out at about 15:12.

3.IV infusion rates

Answer: V = 1200 mL

Infusion rates: a pump is set in mL/h = volume/time in hours. A gravity drip is counted in drops per minute: gtt/min = volume (mL) × drop factor (gtt/mL)/time (min). The drop factor is printed on the tubing package: macrodrip sets deliver 10, 15 or 20 gtt/mL (blood sets often 10 gtt/mL in the US, 15 in the UK) and microdrip sets 60 gtt/mL. Always use the drop factor the question or package gives.

Rate in mL/h

50·60/15 = 200 mL/h

Volume

V = 200·6 = 1200 mL

Answer

V = 1200 mL
4.IV infusion rates

Answer: R = 25 gtt/min

Dose-based infusions: concentration = amount of drug in the bag/bag volume. Pump rate (mL/h) = dose per hour/concentration, and dose per hour = mL/h × concentration. Convert per-minute doses to per-hour (× 60) and keep the mass units the same (1 g = 1000 mg, 1 mg = 1000 mcg).

Concentration

100/100 = 1 mg/mL

Volume per hour

75/1 = 75 mL/h

Drip rate

R = 75·20/60 = 25 gtt/min

Answer

R = 25 gtt/min

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