Table 1 Fluid therapy terminology

Osmotic and oncotic pressure

— osmotic pressure is the hydrostatic pressure that would be required to resist the diffusion of water across a semipermeable membrane from a higher solute concentration to a lower solute concentration. Oncotic pressure is the portion of osmotic pressure which is due to large molecular weight particles, particularly proteins.

Fluid tonicity

— the effective osmolality of a solution in relation to a specific semipermeable membrane and therefore a useful way of describing a given fluid's in vivo behaviour. For example, although 5% dextrose has a similar ex vivo osmolality to 0.9% sodium chloride, after infusion the dextrose is taken up into cells, rendering the solution effectively hypo-osmolar with respect to the cell membrane, i.e. hypotonic; 0.9% sodium chloride remains isotonic due to the retention of sodium and chloride ions in the extracellular space.

Crystalloid

— solutions of glucose and/or electrolytes in water.

Colloid

— a dispersion of large molecules or ultramicroscopic non-crystalline particles in a carrier crystalloid. It includes gelatins, starches and dextrans.

Balanced solutions

— those with a composition more similar to plasma than to 0.9% sodium chloride. It is achieved by replacing a proportion of the chloride with stable organic anionic buffers such as lactate, gluconate or acetate.

Goal-directed haemodynamic therapy

— the use of cardiac output monitoring to guide fluid and inotrope therapy. Key physiological goals are targeted in specific treatment algorithms. This may be a predefined increase in global oxygen delivery or stepwise increases to wards a maximal cardiac stroke volume. This treatment—as compared with fluid dosing based on clinical assessment or ‘per weight’ basis—has been used in various forms for over 40 years. Early variants were guided by the pulmonary artery catheter, but several minimally invasive devices are now in use.