21st Century fluid therapy

Intravenous fluid therapy

In humans and animals, fluid therapy is a core element for the resuscitation of critically ill patients, since fluid therapy is essential to decrease mortality and accelerate recovery. In equids fluid therapy is a fundamental component of the treatment for diseases, such as SIRS, sepsis, colic, hypovolemia and shock. The purpose of fluid therapy is to restore effective blood volume, correct hypotension, improve cardiac output, normalize tissue oxygenation and correct electrolyte and acid base abnormalities. The ultimate aim is to prevent organ dysfunction⁸. Thus fluid therapy is the mainstay in the treatment of SIRS and sepsis induced hypovolaemia.

Sepsis is a complication of critical illness with high degree of mortality. In horses with gastrointestinal disease SIRS is a common complication, and as in man it carries a high mortality rate. In adult horses SIRS commonly originates secondary to gastrointestinal disease (i.e. colitis), due to bacterial and endotoxin translocation through the gut wall10. The most current Surviving Sepsis Campaign guidelines recommend crystalloids over hydroxyethyl starches (HES) as the preferred fluid for resuscitation. The most commonly used type of intravenous fluid therapy for resuscitation in humans and equids are the polyionic isotonic crystalloid fluids, Lactated Ringers and 0.9 % sodium chloride.

Intravenous fluid therapy effects and sife effects

Intravenous fluids expand the intravascular space. However depending on the fluid type, leakage into the extravascular space varies in speed and degree. Crystalloids fluids counteract the movement into the extravascular space due to the osmotic pressure exerted by its solutes. Whilst colloids create oncotic pressure gradients to keep fluids within the intravascular space⁷. Thus in theory the blood volume expansion may be proportional to the solute tonicity or oncotic power. Colloid fluids remain in the intravascular space longer than crystalloids, thus less fluids are needed to achieve similar hemodynamic effects, as shown by the recent CHEST trial. Colloids have been used for rapid and long lasting circulatory stabilization, although data supporting this practice is lacking.

The major complications of fluid resuscitation are pulmonary and interstitial oedema. There are concerns with the use of colloids with regards to immune effects in critical illness, acute kidney injury, coagulopathy, increased risk of death and higher costs. However, the administration of large volumes of 0.9% sodium chloride has been associated with hyperchloraemic metabolic acidosis due to increased plasma chloride and decreased strong ion difference. Furthermore hyperchloraemia may cause renal vasoconstriction and decrease glomerular filtration rate, leading to acute kidney injury and higher mortality.

Enteral fluid therapy

Where enteral fluids can be used they have many advantages not least from a financial point of view, particularly in the management of impactions, oral supplementation of electrolytes, provision of nutrition and treatment of dehydration when present without hypovolemia. Interestingly NICE guidelines recommend that justification is required as to why intravenous fluids are chosen over enteral fluids and that if the former are used should be stopped as soon as practical.

Water absorption and blood flow

Water absorption from the gastro-intestinal tract in the normal horse primarily occurs in the large intestine, and more specifically the large colon. In an adult horse a volume of up to 100L of fluid and associated secretions is absorbed during the course of the day^27-29. In the hypovolemic horse, in order to protect the vital organs, blood flow is diverted from the gastro-intestinal tract. Once blood flow is reduced, so too are gastrointestinal motility and absorption. In addition, obtunded, hypovolemic horses have a reduced thirst drive. It is for this reason that using oral fluid therapy in hypovolemic animals is unsuccessful at best and detrimental in certain scenarios.

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