Oral fluid treatment for horses with colic (Proceedings)

Fluid Therapy

There are two major reasons horses with colic may require fluid therapy: hypovolemia and endotoxemia (or sepsis). Hypovolemia results from decreased intake, loss of fluid (typically sweat or reflux), and sequestration of fluid (typically in horses that have intestinal obstruction). Endotoxemia exacerbates hypovolemia by triggering sequestration and leakage of fluid in peripheral capillary beds. The most obvious example of this is a horse with congested gums, in which endotoxin-induced elaboration of prostanoids (particularly PGI2) results in inappropriate vasodilation of capillary beds. The blood that is pooled in these capillary beds becomes de-oxygenated, resulting in progressive deterioration of gum color (from red to purple). Some horses with severe colic will present with predominantly hypovolemia, particularly horses with volvulus of the large colon, in which fluid is sequestered in the large colon, but there has been insufficient duration of disease to trigger endotoxemia. Many horses will present with both hypovolemia and endotoxemia, such as a horse with a small intestinal strangulating obstruction, in which fluid may be sequestered in the lumen of the gut (hypovolemia) and endotoxin has started to leak from degenerating bowel.

Estimation of Dehydration

Treatment of hypovolemia should begin with an estimate of the degree of dehydration, which can be based on the following:

The degree of dehydration can be confirmed by running a packed cell volume and total protein. Once the degree of dehydration has been estimated, the percentage is multiplied by the horses body weight to give the fluid deficit (e.g., 8% dehydration x 500kg = 40L). Approximately half of the fluid deficit can be administered by stomach tube. The horse may be able to begin to drink after initial treatment, or may require additional oral fluid therapy.

Oral versus Intravenous Fluid Therapy

Fluid therapy should be considered in horses with impactions that are refractory to treatments such as flunixin and liquid paraffin. Fluids can be administered either orally, or intravenously. The latter has been extensively used by referral hospitals taking care of refractory impactions, and in many instances veterinarians will give excessive IV fluids in the hope of over-hydrating colonic ingesta. Recent studies suggest that this is unlikely to occur, so the main reason for giving IV fluids is to make sure any dehydration is corrected, and to make sure the horse continues to receive a maintenance level of fluids. This is because, although it is apparently difficult to overhydrate colonic contents, dehydration will result in dehydrated colonic contents because colonic fluid will be used to make up reduced circulating volume.

Practical Oral Fluid Therapy

An alternative to intravenous fluids is continuous infusion fluids via a small bore nasal esophageal feeding tube. The methodology for this has recently been described by Lopes et al. Old fluid bags can be used, using water containing electrolytes, and an infusion line attached to the nasal-esophageal tube. A commercial oral rehydration solution formulation can be used, such as those used for calves (without the dextrose component). Alternately a solution consisting of 5.37 grams NaCl, 0.37 grams lite salt, and 3.78 grams NaHCO3 per liter of water can be used. Straight water or saline both cause electrolyte disturbances when given as a continuous infusion. In the studies by Lopes et al., rates of 3-5L/hr were tolerated. However, some horses have developed colic with this rate of infusion and needed to have fluid removed from the stomach. The author therefore prefers a rate of 1-2L/hr. The major advantage of this method is that hydration of colonic contents is greatly increased compared to IV fluids, and impactions that may be refractory to IV fluid therapy may be resolved. A final alternative would be repeated water/ electrolyte treatments by regular stomach tube (for example, 2-3 gallons 2-3 times/ day).