Equine fluid therapy

a) Crystalloids

Crystalloids contain water, sodium or glucose, other electrolytes plus a buffer. The fluid may be hypotonic, isotonic or hypertonic relative to plasma. They distribute between the intravascular (25%) and interstitial compartments (75%) within one hour; thus, 1L of crystalloids results in a 250ml increase in plasma volume. Distribution into the interstitial compartment is beneficial if the animal is dehydrated as well as hypovolemic, but it will also promote tissue oedema formation, which may compromise perfusion.

Distribution in the intravascular compartment will result in a decrease in the total protein concentration (through dilution), which, in turn, will decrease the colloid osmotic pressure (COP), thus favoring further fluid loss into the interstitium, which again may be beneficial or detrimental depending on the individual case. These factors should all be borne in mind when considering the fluid therapy plan.

Isotonic solutions

Isotonic polyionic solutions are the most commonly used crystalloid solution in equine veterinary practice. Their electrolyte composition is similar to plasma; however, the potassium concentration is too low for maintenance requirements so 10 - 20 milliequivalent (mEq) potassium chloride (KCl)/L   should be added . When choosing a fluid, it should be remembered Hartmann’s solution contains calcium, which will bind anticoagulants in blood products and that these fluids contain a buffer (lactate, acetate or gluconate) metabolized (by the liver, muscle or most cells, respectively) to bicarbonate, which will affect the acid-base balance.

Isotonic saline (0.9%  NaCl) is hypernatremia and hyperchloremic relative to plasma and lacks other electrolytes. it’s used in cases of hyponatremia and hypochloraemia, rather than for resuscitation or maintenance.

Hypertonic solutions

Hypertonic saline (7% to 7.5% NaCl) is hypertonic relative to plasma and so initiates movement of water into the intravascular space from the interstitium, resulting in rapid expansion of the circulating volume. The plasma volume expansion achieved is two to four times that of the infused volume (2-4 ml/kg). However, this will result in intracellular dehydration, so it should be followed up with 10L of isotonic fluids for every 1L of hypertonic saline administered in two-and-a half hours.

Hypotonic solutions

A5% dextrose is a hypotonic solution that will replace water without electrolytes. Thus, it is only used in animals that are hypernatraemic and hyperchloraemic. It should be used with caution as it can cause hyperglycaemia with rapid administration, which will subsequently result in osmotic diuresis, and so further fluid loss.

 b) Colloids

-  Contain high molecular weight substances which don't readily migrate across    capillary walls such as large sugar or protein molecules and are a mix of large and small molecules.

- The advantages of colloids over crystalloids are the large molecules improve oncotic pressure and provide rapid intravascular volume replacement by expanding the plasma volume by 100% of the volume infused, thus improving microvascular perfusion with less tissue oedema formation.

- The disadvantages are they are affected by alterations in capillary permeability, do not correct dehydration (interstitial losses).

Colloids available   include plasma, whole blood and the synthetic colloid (gelofusine,

 pentastarch and hetastarch).

plasma

Albumin is the primary component of plasma contributing 65% to 75% of plasma collected from a suitable donor or is commercially available in the form of hyperimmune plasma. immunoglobulins (in failure of passive transfer due to  failure of  colostrum intake ), clotting factors, anticoagulants and platelets.

Horses receiving plasma should be closely monitored for signs of hypersensitivity including tachycardia, pyrexia and urticaria. If signs occur, the infusion should be stopped and corticosteroid therapy IV may be necessary.

Whole blood

Whole blood is only indicated in cases of acute severe haemorrhage or haemolysis. No universal donor exists, but, ideally, the donor should be Aa and Qa antibody and antigen negative, although this requires a donor to have been blood typed in advance. Suitable alternatives include a young, healthy gelding that has never received a blood product, or a mare that has never had a foal.

Usually, cross matching is not performed the first time a blood or plasma transfusion is given, but should be performed for all subsequent transfusions, regardless of the donor.

Gelofusine

The synthetic colloid gelofusine contains succinylated gelatin and sodium chloride. It has been shown to expand the blood volume by 68% one hour after infusion and by 30% after six hours.  

Side effects reported include:

- proteinuria (not clinically significant),

- hyperchloraemia (which may cause metabolic acidosis) and anaphylaxis.  

- 10ml/kg and 20ml/kg gelofusine resulted in haemodilution and an increase in COP in healthy.

- Ponies were not associated with clinically significant adverse effects on haemostasis or renal parameters.