Water plays a vital role in many physiological and homeostatic processes within the body and it constitutes approximately 60% of total body weight in dogs and cats. In health, it is obtained enterally from the diet (food and water), although water produced during normal metabolic processes also contributes to the total.

Most of the body's water (total body water) is contained within cells (intracellular fluid (ICF), equivalent to 40% of body weight), with the remainder distributed throughout the extracellular spaces (extracellular fluid (ECF), 20% of body weight). The ECF is found mainly within the interstitial spaces (interstitial fluid (ISF), 15% of body weight) while the rest is located intravascularly (plasma volume: 5% of body weight). The distribution of water between fluid compartments is achieved and maintained by a balance between hydrostatic, osmotic and oncotic pressures. However, the permeability of capillaries (Starling's law) also influences its distribution.

Animals are not always able to maintain normal body fluid balance, and fluids must be administered parenterally in an attempt to maintain or restore normality. There are many different fluids available for parenteral administration. However, they all fall into one of three broad categories: crystalloids, colloids or blood.

Crystalloids

Crystalloids are solutions of electrolytes and/or glucose in water. They are economical and readily available. The principal solute in most crystalloid fluids is the inorganic salt sodium chloride (NaCl). They may also contain buffers such as acetate, gluconate or lactate. They are generally isotonic with plasma and pass freely through the endothelial barrier. Following intravenous infusion, the distribution of these fluids is determined by their sodium concentration:

 Solutions containing isotonic concentrations of sodium will distribute throughout the entire ECF compartment as sodium is normally distributed uniformly throughout this compartment; i.e., for each 1000 ml of such fluid administered 250 ml will remain intravascularly and 750 ml will redistribute to the ISF space. This generally occurs within 1 hour of administration.

 Solutions containing less sodium, where the osmotic activity is maintained with the inclusion of glucose, will distribute fluid throughout all body water compartments rather than just the ECF compartment; e.g., 1000ml of a 5% glucose solution will contribute 1000 ml of 'free water' with less than 100 ml retained intravascularly following infusion.

There are a number of crystalloid fluids available with a veterinary licence in the UK and they include:

 0.9% NaCl (isotonic saline; 'normal' saline), e.g., Vetivex 1®; Aqupharm No1®. Normal saline is used widely by veterinary surgeons. Although it is isotonic with plasma it contains a higher concentration of sodium and chloride than normal plasma. Consequently, infusion of large volumes of 'normal' saline can cause hyperchloraemic metabolic acidosis and it should be used with caution if administered as a maintenance fluid. Normal saline has specific indications for the treatment of conditions such as hypochloraemia or hyponatraemia and it is a common choice for patients with hyperkalaemia and hypercalcaemia.

 7.2% NaCl (hypertonic saline), e.g., Vetivex 20®. Hypertonic saline is generally used for fluid resuscitation in patients where administration of large volumes of fluid may be difficult, and for its cardiovascular benefits. It will draw water from the ISF compartment intravascularly, and so the fluid 'lost' from the ISF space must also be considered in the fluid therapy plan.

 Hartmann's solution (lactated Ringer's solution; compound sodium lactate), e.g., Vetivex 11 Isolec®; Aqupharm No11®. Hartmann's solution is often referred to as a balanced electrolyte solution and contains sodium, chloride, potassium and calcium in addition to lactate. It is isotonic with plasma. The lactate is metabolised to generate bicarbonate and is an aid to the treatment of metabolic acidosis. Hartmann's solution is used widely by veterinary surgeons and is an appropriate fluid for resuscitation.

 Ringer's solution, e.g., Vetivex 9®; Aqupharm No9®. Ringer's solution has the same composition as Hartmann's without the addition of lactate.

 Glucose is a common additive to intravenous solutions. Several solutions are available including 5% glucose (Vetivex 6®), 5% glucose and 0.9% NaCl (Vetivex 3®; Aqupharm No3®) and 4% glucose with 0.18% NaCl (Vetivex 18®; Aqupharm No18®). The addition of glucose to water renders the water isotonic and following infusion the glucose will contribute briefly to the tonicity of the plasma. However, this effect is short lived as the cells, in the presence of insulin, take up and metabolise the glucose administered leaving 'free water'. Solutions containing a concentration of glucose of 5% or less have a very low calorific value and should not be used as a source of parenteral nutrition. Infusion of large volumes of 5% glucose will lead to hyponatraemia. It is unsuitable for fluid resuscitation in shock because the fluid will distribute across all body fluid compartments as discussed above. 5% glucose with 0.9% NaCl is a hypertonic solution and does not contain 'free water'. Following administration of 1000 ml of 4% glucose with 0.18% NaCl, 200 ml of the fluid may be considered to act in a similar way to 0.9% NaCl and distribute throughout the body accordingly, while the remaining 800 ml will effectively be 'free water'.

Colloids

Colloids are macromolecules that are retained intravascularly and that exert colloid osmotic pressure (COP). The COP of colloids helps to retain the water in which they were administered intravascularly and so they will produce greater volume expansion of this compartment than crystalloids. Furthermore some colloids will also attract additional fluid from the ISF compartment. However, it is important to remember that the positive effect on vascular volume is still temporary. Colloids available for parenteral administration may be either natural (derived from whole blood, i.e., plasma or albumin) or synthetic (gelatins, dextrans and starches). There are few colloids with a veterinary licence in the UK.

Natural Colloids

 Plasma. Frozen and fresh frozen canine plasma is now available in the UK for purchase from the Pet Blood BankUK. However, plasma can be obtained locally if facilities are available to allow its separation from whole blood. Plasma is a useful replacement fluid in hypovolaemic animals and is suitable for patients that are hypoproteinaemic or hypoalbuminaemic. Plasma also has the added benefit of containing some or all of the natural clotting factors and can be used in the treatment of clotting factor deficiencies and coagulopathies.

 Albumin (4% and 20% human albumin solution). In normal dogs and cats, the protein albumin is the most important colloid contributing up to 80% of COP. Albumin for parenteral administration is a naturally occurring plasma protein derived from pooled human plasma that is heated and sterilised by ultrafiltration. The administration of albumin is indicated in hypovolaemia or hypovolaemic shock, hypoproteinaemia or hypoalbuminaemia. Its use has been advocated to reduce peripheral and pulmonary oedema, but care must be taken in patients with compromised vascular endothelial integrity.

Synthetic Colloids

 Gelatins (e.g., Haemaccel®; Gelofusine®). These are modified beef collagens. A molecular weight below the renal threshold can mean short vascular retention times. They are indicated for short-term volume expansion, e.g., vasodilation following administration of anaesthetic agents.

 Starches (e.g., Hespan®; Hemohes®; Pentaspan®). A range of synthetic colloids based on hydroxyethyl starch (HES) is available. They are made from amylopectin derived from plant starch. They are delivered in fluids containing sodium and chloride. Starches can interfere with coagulation in a similar manner to dextrans.

 Dextrans (Dexran 70, 6% solution; Dextran 40, 10% solution). The dextrans are polysaccharides produced by bacterial fermentation of sucrose. Dextrans with a molecular weight below the renal threshold are excreted by this route, while the remaining molecules are broken down by dextranases prior to excretion. Dextrans can interfere with coagulation.

 Oxyglobin®. This is derived from purified bovine haemoglobin. It does not contain red blood cells and it is delivered in lactated Ringer's solution. This synthetic colloid is novel, as it not only provides oncotic support but also oxygen-carrying capacity. This fluid is hyperoncotic.