60% of body mass is fluid. (a 70kg man is made up of 42kg (42L) of water).
11 litres (approximately one third) are extracellular (3L plasma, 8L interstitial fluid) and 28 litres (approximately two thirds) are intracellular (RBCs accounting for 2 litres of this).

Source of loss (average daily loss)

Increased by…

Urine (1.5L)

Diabetes, diuretics

Faecal (100ml)

Diarrhoea, laxatives

Insensible – skin (500ml)

Exercise, high temperatures, fever

Insensible – lungs (500ml)

Hyperventilation, dry inhaled air

Total average loss – 2.5L

Ions needing replacement:
•  Sodium – 1-1.4 mmol/kg/day
•  Potassium – 0.7-0.9 mmol/kg/day
•  Calcium and Magnesium if nil by mouth for a few days

Calculating required amounts

100 ml/kg/day for first 10kg
50ml/kg/day for second 10kg
20ml/kg/day for the remainder

e.g. for a 15 kg toddler: 1.25 L/day
for a 70 kg man: 2.5 L/day

Fluid Types

Crystalloid: a true solution, passing freely through a semi-permeable membrane – i.e it can spread from the blood into other compartments, expanding the total fluid volume.
e.g. Saline, Hartman's, 5% dextrose

Colloid: a suspension of particles, too large to pass through a semi-permeable membrane – i.e. it expands intravascular volume only.
e.g. Gelofusin, albumin

Rehydration fluids are given to patients who have become dehydrated (due to decreased intake or increased loss). Check the U+Es and choose a solution that will replace any ion deficits.

Replacement fluids are given to counteract any unusual losses (e.g. vomiting or blood loss).

Complications of Fluid Therapy

•  Infection
•  Allergy to gelofusion
•  Overtransfusion – easily done in patients with heart or kidney failure but hard in the young or fit.