Veterinary medicine is filled with debate on which is the most appropriate therapy for a patient. Fluid therapy is no different within this context. The debate between colloids and crystalloids is one that still has not yet been resolved. In spite of numerous studies undertaken, colloids and/or crystalloid therapy are yet to be shown to be superior from one another. It is important to realise that fluid therapy is vital in saving patients' lives, but seldom if ever cures the underlying disease present in the patient. Fluids do cure peritonitis, babesiosis, severe infections, etc., but many of these conditions cause hypovolaemia and shock, and that does kill patients rapidly. Fluid therapy is therefore largely a supportive measure that is undertaken, prevents deaths from certain causes but does not cure the cause. With this in mind, we need to think of how fluids are distributed in the body.

Sixty percent (40 L) of the body is made up of water. This water is divided into two main compartments, namely the intracellular and extracellular compartments. The intracellular compartment is the largest and holds 60% (25 L) of total body water. Intracellular fluid has potassium as the dominant cation, while magnesium and sodium make up the remainder. The dominant anion is phosphate followed by smaller quantities of sulphate. The protein content of cells is 4 times higher than extracellular fluid. The extracellular fluid makes up 40% (15 L) of total body fluid and is divided into two components - extravascular and intravascular components. The extravascular component consists of 66% (10 L) of fluid while the intravascular component is 33% (5 L). The intravascular portion is then divided again into two - red cells and plasma. The dominant cation of extravascular fluid is sodium with smaller quantities of potassium and calcium. The dominant anion is chloride followed by bicarbonate. These differences are shown in Table 1. Fluid is distributed within this system based on osmotic pressure modified by selectively permeable membranes, maintenance of electroneutrality, maintenance of electrolyte constitutes of the various compartments and the availability of free water.

Table 1. Composition of body fluids

All fluid therapy is generally administered into the intravascular compartment, and from here it is distributed and managed. A complete understanding of the makeup of the fluid we administer is important to understand how these fluids will distribute within the body. Some important concepts are that water usually follows a concentration gradient and is not actively moved between compartments. Water moves to maintain osmotic neutrality between compartments. This means that if sodium is pumped from one compartment to another, an equal volume of water will follow.

Table 2. Composition of fluids

Ions are in mmol/L
*Type: M = Maintenance, R = Resuscitation

Water is known to distribute freely between all body water compartments. Pure water itself is not generally administered, but 5% dextrose water (similar osmolarity to plasma) can be given. Once the glucose has been removed, free water is then available for redistribution. This fluid readily redistributes between all compartments. Most crystalloid solutions redistribute between the compartments with the net effect that between 3 to 5 L of crystalloids are required to increase plasma volume by 1 L.

At this point in time, it is important to revisit an important pathophysiological concept of extravascular water. The accumulation of extravascular water is also known as oedema. When crystalloids are given, only 20–30% remains in the vascular space after an hour. This means that the remaining 70–80% has accumulated in the extravascular space. If the extravascular fluid accumulated in the lungs, it would be known as lung oedema. The concept that the rapid administration of crystalloid fluids can result in lung oedema has been known since the Vietnam War, where this was called Da Nang lung. As lung oedema impairs oxygenation of blood, oedema in other organ systems impairs oxygen delivery to tissues. The amount of crystalloids infused in trauma patients is directly correlated to the development of abdominal compartment syndrome. And so, we can now enter the debate - are crystalloids and/or colloids better for our patients?

It is important to be reminded that no study is yet to show that more patients will survive if colloids are used compared to crystalloids and vice versa. What has been shown is that hypovolaemia is detrimental to our patients and specifically if hypovolaemia is coupled to anaemia. In the setting of acute blood loss, the restoration of circulating volume is more important than necessarily which fluids are used to do it.

Saline has been recommend as a resuscitation fluid. It has been recommended as the fluid of choice in patients with hyperkalaemia, as it contains no potassium. This has been debated, as the acidosis (hyperchloraemic acidosis) it produces may further exacerbate the hyperkalaemia. In the treatment of hyperkalaemia, bicarbonate is given to cause alkalosis. This results in hydrogen ions being transported out of cells in exchange for potassium being transported into cells to correct the alkalosis. As saline causes an acidosis, hydrogen is now taken up by cells and potassium released. The acidosis may further exacerbate the acidosis of trauma when used during the resuscitation of patients. For these reasons, balanced resuscitation fluids are preferred - Ringer's lactate, Balsol, etc. Saline has an important role to play in patients with hyponatraemia.

To correct dehydration - fluid is required to move from the vascular compartment through all the compartments and into cells. In order to achieve this effectively, free water along with the required missing electrolytes are required. The cause of dehydration will determine what electrolyte imbalances are present. Fluid can be moved into cells more effectively with glucose and potassium than with higher sodium containing fluid. Maintenance fluids are high in potassium and usually contain glucose, which makes them suitable except that potassium cannot be infused rapidly into the vascular compartment. A half-strength Darrow's solution may be a suitable alternative with moderate potassium and glucose concentrations.

The kidneys continuously filter plasma and produce urine. They excrete electrolytes and waste products. The kidneys require a continuous supply of water. Colloids generally do supply much free water, and so all patients would require free water. Resuscitation fluids are generally high in sodium and these fluids are often used to maintain patients for days in the clinical setting. The high sodium load results in diuresis as the sodium is dumped through the kidneys with water. Secondly, resuscitation fluid either contains no potassium or very little potassium. Hypokalaemia is commonly seen in critically ill patients who require supplementation. Maintenance fluids are high in potassium, low in sodium and contain free water once the glucose is utilised metabolically. In critically ill patients, it does make sense to use a combination of resuscitation and maintenance fluids as part of standard therapy. The maintenance fluids are run at maintenance rates while the resuscitation fluids are used to restore intravascular volume and maintain circulation. They may be continuously or intermittently administered. Maintenance solutions contain magnesium and calcium. Magnesium is an important ion and should be monitored and supplemented in critically ill patients. Maintenance solution due to their high osmolarity should not be administered through a peripheral line, as tissue necrosis may follow.

Hypertonic saline has been advocated as a resuscitation fluid in trauma patients. Recent evidence supports its use as a resuscitation fluid in cranial trauma patients and may be associated with better outcomes than conventional fluid therapy. The hypertonic saline causes dehydration of the brain or rather reduces or prevents brain oedema due to the hydroscopic effect. Small volumes are required of hypertonic saline for resuscitation. The consequence is a massive sodium load that needs to be eliminated afterwards and the diuresis that follows. A hyperchloraemic acidosis is also seen.

In cranial trauma patients and patients with pulmonary contusions, high volume administration of resuscitation fluids can result in brain oedema and pulmonary oedema. Fluids should be used cautiously in these patients, and a low volume resuscitation should be considered. Starches are also tremendously useful in these patients, as they should remain in the vascular compartment and not leak out. Another scenario is the patient with a haemorrhagic gastroenteritis. They usually present a haemoconcentrated with a normal albumin and total protein. The natural inclination is to give them lots of crystalloids for resuscitation with an invariable result of pulmonary oedema. A resuscitation plan based more on colloids and not ignoring the requirement for crystalloids can be used successfully. A similar plan for haemoconcentrated Babesia patients may be relevant.

Albumin is vital for life. In critically ill patients, albumin usually decreases rapidly as the endothelium becomes dysfunctional. Albumin usually sits in balance between central circulation and extravascular compartments. Normally, it takes 24 hours to complete the cycle. With the rapid exodus in sepsis, albumin decreases. Administering frozen plasma to increase albumin is never very successful, as large volumes are required and the exodus continues. Human concentrated albumin solutions have been used to increase albumin levels, but again these do not result in long-term increases. When the patient gets better and endothelial function returns, albumin levels usually rise rapidly. Starches are useful to use in these patients, as they are larger than albumin and are less likely to leak out. This results in a better maintenance of circulating volume. If plasma osmolarity is maintained at normal values, the liver will not synthesise albumin. Colloid administration should be decreased or "weaned off" over a day or two to allow the liver to start production of albumin.

Liver failure is often associated with coagulation disorders. Many rodenticides contain warfarin-like products that result in coagulation disorders. The administration of fresh frozen plasma in these patients can be lifesaving.

A number of artificial colloids are available on the market. They are sold as hetastarch, pentastarch, tetrastarches and dextrans. The dextrans are older colloids and are considerably cheaper than the modern generation of starches. Allergic reactions are known to occur with the gelatins. The hydroxyethyl starches are a polymer of amylopectins that differ by molecular weight and branching of the chains. This determines their clinical properties in the patients. Amylopectin consists of branching chains of α-1, 4-linked glucose and α-1, 6-linked glucose. This is easily hydrolysed by plasma amylases. The type of starches are determined by its degree of substitution. The hydroxyl group of the glucose subunits are substituted for hydroxyethyl groups. The substitution improves the stability of the compounds. The substitution is expressed as a ratio. Hetastarch has a high degree of substitution 0.6–0.7, pentastarch moderate substitution 0.5, and tetrastarch low substitution 0.4. The molecular weight for the starches is also described. The starches can have a broad range of weight from a few thousand to several million daltons in weight. The starches are then described as hetastarch 450/0.7, pentastarch 200/0.5 (HAES-steril) and tetrastarch 130/0.4 (Voluven). The lower substitution results in less accumulation in tissues and more rapid elimination from the body. This also reduces its effect on coagulation. Manufacturing processes have improved, and the variation in molecular size of modern generation of starches is small. This makes the lower molecular weight and low substituted molecules more suitable. Renal failure and an increase in mortality has been associated with the use of hydroxyethyl starches.

Lastly, it important that fluids are given on a goal directed approach. Fluids should be given to achieve a resuscitation end point not just a particular volume. This approach along with cardiovascular support is vital for successful management of patients.