Pharmacology for Anaesthesia and Intensive Care

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9780521704632c20 CUFX213A/Peck 9780521618168 December 28, 2007 14:10

20 Intravenous fluids

Processes of distribution

There are essentially three components to intravenous fluids: water, electrolytes

(principally sodium) and large molecules (gelatins, starches, albumin). It is no sur-

prise that each behaves differently because from a molecular point of view they are

very dissimilar.

Water has no charge but can be encouraged to be polar and will distribute rapidly

across all compartments in the body, resulting in a minimal increase in plasma

volume. Sodium carries a charge and is distributed rapidly into the extracellular

space, whereas potassium is transported into cells. Therefore, solutions with a high

sodium content are distributed across the extracellular space resulting in a greater

effect on plasma volume compared with 5% dextrose but still not a profound volume

increase. By way of contrast, fluids with a significant component of large molecules

remain largely contained in the plasma and as a result the contribution to the plasma

volume is more significant.

However there are additional factors that govern fluid movement between the

compartments, and these are linked in the Starling equation. They are:


The shape and size of the molecules


Hydrostatic pressure gradients (i.e. the actual pressure in certain anatomical

spaces)


Oncotic pressure gradients (i.e. the pressure generated by the components within

these anatomical spaces)


The time over which the fluid is given


The endothelial barrier

The Starling equation

Fluid movement=k[(Pc+πp)−(Pi+πp)]

where k=filtration constant for the capillary membrane

Pc=capillary hydrostatic pressure

Pi=interstitial hydrostatic pressure

πp=plasma oncotic pressure

πi=interstitial fluid oncotic pressure

Sothe administration of intravenous fluid depends on the specific aims in any given

situation. For example when a patient is starved pre-operatively for a long period, it

may become appropriate to administer maintenance fluid which consists of water

and electrolytes. However if the patient is given bowel prep then the amount of water

and electrolytes will need to be increased in order to maintain normality. Pyloric

stenosis is a specific example in which careful fluid and electrolyte replacement are

vital prior to surgery.

The picture may be complicated further as vascular permeability increases in

burns, trauma, sepsis and surgery allowing immunological mediators to leave the