Depending on the duration of illness and the underlying cause (e.g., fluid loss in a horse with pneumonia differs dramatically from that in a horse with severe colitis and large-volume diarrhea), fluid deficits in horses with SIRS can range from mild dehydration to severe hypovolemic shock. Fluid deficits result from increased losses such as can occur with ileus and high-volume reflux, diarrhea secondary to enterocolitis, or third-space sequestration such as can develop with 360-degree volvulus of the large colon. Sick, febrile horses frequently have low water consumption, which exacerbates the water deficits. In addition to losses or decreased intake, SIRS affects both vascular integrity (increased permeability) and tone (decreased vasoconstrictive responses), as well as cardiac function. Consequently, whereas total body water volume may remain unchanged, there can be a decrease in effective circulating volume. Clinical findings associated with the alteration in cardiovascular integrity and function include hemoconcentration, hypoalbuminemia, and hypotension, as well as ineffective cardiovascular compensatory responses.

In a horse with normal colloid oncotic pressure and normal vascular integrity, isotonic crystalloids are a useful and effective fluid for rapid restoration of circulating volume and tissue perfusion. If needed, 20 L of crystalloids can be given rapidly through a 12-gauge intravenous catheter and large-bore tubing. However, because electrolytes are rapidly diffusible across the endothelium, 75% to 80% of infused crystalloids will redistribute to the interstitium and intracellular spaces within an hour of infusion. In horses with increased vascular permeability, the percentage of redistribution can be even higher and can result in interstitial edema and further exacerbation of oxygen delivery deficits. Hypertonic saline is an effective fluid for rapid, transient restoration of effective circulating volume. The high tonicity of hypertonic saline (7.2% NaCl solution) results in approximately 4 L of expansion in intravascular volume for every 1 L infused. The extravascular fluid volume is pulled into the vascular space from cells, the interstitium (in response to the gradient in tonicity between interstitium and plasma), and third spaces (such as intraluminal ingesta in the gastrointestinal tract). Similar to isotonic crystalloids, the volume expansion associated with administration of hypertonic saline is short lived, and additional fluid administration is required to maintain the volume expansion. In addition to its volume-expanding effects, hypertonic saline administration decreases bacterial translocation, leukocyte activation, endothelial cell adhesion molecule expression, and intestinal damage and increased short-term survival in a rat model of strangulated small bowel obstruction.

Colloids are solutions containing molecules of large molecular weight (such as albumin) that do not freely pass through healthy capillary membranes. Infusion of colloids results in increased plasma colloid osmotic pressure and favors movement of fluid into the vascular space, in accordance with the Starling principle. Natural colloids include plasma, concentrated albumin, and whole blood. Synthetic colloids available in the United States include hetastarch, dextran, and oxyglobin. Synthetic colloids have the advantage of ease of use and storage. Hetastarch is the most commonly used synthetic colloid in equine medicine and consists of a heterogeneous population of amylopectin molecules ranging in size from 30 to 2300 kDa. Attachment of hydroxyethyl groups to the glucose chains retards metabolism by amylase, permitting longer circulation time. The larger molecules of hetastarch may act as “plugs” in leaky vessel walls, effectively decreasing permeability defects and further fluid losses associated with SIRS.

In 2011, a large volume of research on hetastarch was retracted because of author misconduct. Subsequent to this retraction, a large-scale review of randomized controlled trials comparing hetastarch with other resuscitative fluids (excluding the retracted work) found an increased risk for renal injury and mortality in patients receiving hetastarch. Subsequent to this review, the U.S. Food and Drug Administration has issued a warning on the use of this synthetic colloid in adults (humans) with sepsis, preexisting renal disease, and other critical illness. Administration of heta­starch has also been associated with prolongation of prothrombin time and partial thromboplastin time in horses, and horses at risk for bleeding should undergo monitoring of coagulation parameters. Natural colloids have the advantage of providing albumin, antibodies, complement, and clotting factors, but disadvantages are that initiation of treatment will be delayed while products thaw, and infusions must be administered slowly because of the risk for allergic or anaphylactic reactions. The colloid oncotic pressure of commercial plasma is less than that of presently available synthetic products, so on a per-liter basis, synthetic colloids are more effective in restoring and maintaining plasma volume.

Blood is also a useful colloid; however, time is required for collection of fresh whole blood, and unless the recipient is anemic or has acute hemorrhage (requiring replacement of red blood cell mass and oxygen-carrying capacity), the benefits of blood administration do not outweigh those of plasma.

The choice of replacement fluid and volume administered depends on the severity of the deficits and other clinicopathologic findings. In a horse with severe hypovolemia and normal to high values for colloid oncotic pressure, initial treatment with hypertonic saline (2 to 4 mL/kg), followed by administration of isotonic crystalloids, can rapidly restore circulating volume while emergency treatment is instituted. If colloid oncotic pressure falls or signs of improved cardiovascular function are not seen (e.g., a decrease in the heart rate and capillary refill time and an increase in extremity temperatures, central venous pressure, jugular refill, and urine output), infusion of colloids should be considered to try to improve circulating volume and perfusion. Typical therapy would begin with a bolus dose of about 10 mL/kg, followed if necessary by continuous rate infusion at 1 mg/kg per hour, with total dose not to exceed 20 to 25 mL/kg per day. Frequent clinical monitoring, with particular attention paid to jugular fill, central venous pressure, urine production, and blood pressure, is critical for assessing response to fluid therapy.