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Section IVOther important drugsintravascular space. Albumin follows resulting in a reduction of colloid oncotic pres-
sure and the formation of oedema.Crystalloids
Maintenance fluids only need to replace what is normally lost (i.e.∼70 mmol of
sodium and∼ 2 litres of water). Smaller amounts of potassium and other electrolytes
are also required.
Where crystalloids are used for volume expansion, 0.9% saline remains a com-
mon choice. However its chloride content is significantly higher than that found in
plasma and this will promote a hyperchloraemic acidosis, which may also impair
haemostasis and urine ouput.
Ringer’s solutions were designed around the solutions used to bathe cultured
cells. Hartmann added lactate, which allowed a reduced chloride concentration,
and therefore prevented hyperchloraemic acidosis formation when used in vivo.
However the addition of acetate instead of lactate may be advantageous in that it is
metabolized not only in the liver and kidney but by all tissues so that its buffering
capacity is retained during times of hypovolaemic shock. Both lactate and acetate
are metabolized to bicarbonate.Colloids
Gelatins
Gelatins are large molecular weight proteins that are commonly suspended in saline-
like solutions. They are used for plasma replacement and initially increase colloid
osmotic pressure. However the effects are not long lasting and therefore gelatins are
not considered as serious volume expanders.Formulation
The gelatins that were used initially had a very high molecular weight (100 000 Dal-
tons), which provided a large colloid osmotic effect and subsequent plasma vol-
ume expansion, however at low temperatures the solution became gel-like. As a
result lower molecular weight formulations were developed. These formulations
only increase the plasma volume by the amount infused; they do not draw water in
from the extracellular space.
Gelatins in use today are manufactured from bovine gelatin, which is heated,
allowing the protein to denature, and cooled, allowing new inter-chain bonds to form.
These bonds are either succinylated or urea cross-linked depending on the chemical
conditons during cooling. The resultant product has a range of molecular weights,
which is quoted in one of two ways: the weight average or the number average. The
number average best reflects the mean osmotically active particle weight of a colloid
and is the total weight of all the molecules divided by the total number of all the