Fluid therapy is indicated for horses with many clinical conditions. It can be used to maintain and/or restore plasma volume and electrolyte and acid–base equilibrium in conditions ranging from adipsia to severe dehy-
dration. In some cases, the goal is to create a state of overhydration in order to promote diuresis (e.g., exercise myopathy) or hydrate the content of the gas- trointestinal (GI) tract (e.g., GI impactions).
Fluid therapy is currently administered to horses almost exclusively via the IV route,1 which is not the most physiologic or economic option in many clinical situations. Because of a horse’s enormous body size, large volumes of fluids are required (sometimes more than 100 L/day), and fluids for IV therapy are expen- sive. The natural route for hydration is the GI tract, which is highly effective at absorbing large amounts of fluids, electrolytes, and other nutrients. Because the GI mucosa is a naturally selective barrier for absorption, nonsterile solutions can be administered through a nasogastric (NG) tube, and iatrogenic imbalances are less likely than with IV fluid therapy.
Besides the physiologic benefits, enteral fluid therapy may result in reduced costs because inexpensive nonsterile fluids can be used. However, NG adminis- tration of fluids may be contraindicated in many cases, such as in horses with complete GI obstruction or those with severely compromised GI motility, and complications can occur. Current textbooks barely mention the enteral route as an option for fluid therapy in horses, and many veterinarians are unaware of the potential benefits of this route of administration. This article presents some practical guidelines on how to administer enteral fluid therapy to horses.

PATIENT SELECTION
The patient must have a relatively normal GI tract that is able to tolerate enteral administration of fluids. A patent nasal passage, pharynx, and esophagus are necessary to permit the passage of the NG tube. If the NG tube cannot be passed (e.g., pharyngeal obstruc- tion), enteral fluid therapy can still be used if an esophagostomy is performed. However, esophagostomy is an invasive procedure and is contraindicated in most cases. In horses with conditions such as adynamic ileus and complete GI obstruction, NG administration of fluids is contraindicated because fluids would not move aborally but contribute to gastric and intestinal disten- tion. Enteral fluid therapy should be used only in recumbent horses if the animals can be maintained in sternal recumbence to avoid reflux and aspiration. Enteral fluid therapy should not be used alone in horses with severe hypovolemia and shock because immediate plasma expansion is indicated and can be achieved only with IV fluid therapy.
TUBE SELECTION
A tube with an outer diameter and length compatible with the size of the horse is recommended. The com- mercial NG tubes (large-bore tubes; Jorgensen Labora- tories, Loveland, CO) traditionally used for horses with colic can be used for enteral fluid therapy (Figure 1). For average-size horses, a medium-size NG tube (outer diameter, 15.9 mm) designed for colicky horses is appropriate. For ponies, yearlings, or foals, a tube with a smaller caliber, such as a small size (outer diameter, 12.7 mm) or foal size (outer diameter, 9.5 mm), is rec- ommended. If fluid therapy is going to be administered for a short period (e.g., no more than 2 days), large- bore tubes are appropriate; however, for prolonged treatments, this type of tube may cause significant damage to the mucosa of the upper respiratory tract and discomfort for the horse.
Another option is a commercial tube designed for enteral nutrition. Enteral feeding tubes (Mila Interna- tional, Florence, KY) have the advantage of having a very small outer diameter (4.7 or 6 mm), producing less discomfort for the horse, and producing less pressure on the mucosa. When an enteral feeding tube is used, it is safe to allow a full-size horse to eat (Figure 2).2 How- ever, when a large-bore NG tube is used, it is better to keep the horse muzzled due to the risk of interference with swallowing and feed aspiration. The small-bore enteral feeding tube has one disadvantage: This type of tube may not be useful to check for gastric reflux or to drain fluids from the stomach in case the horse develops intolerance to enteral fluid therapy because it is likely that food particles will occlude the tube.

TUBE INSERTION
To avoid damage to the nasal passage, pharynx, lar- ynx, and esophagus when inserting a feeding tube in horses, tube manipulation must be gentle. It is better to use a lubricant jelly (e.g., KY Jelly; Johnson & Johnson) to minimize friction and prevent mucosal damage. Appropriate restraint is also indicated, although most horses may not require any restraint other than a mildly tight twitch or even just a halter. Repeated tube inser- tion and removal may increase the risk of damage by tube passage. When fluids are being administered for several days, it is better to avoid removing the tube more than once daily, and the other nostril should be used for tube reinsertion.
Small-bore enteral feeding tubes come with a wire stylet, which is placed within the tube to reduce pliabil- ity and facilitate tube insertion. Before tube insertion, remove the wire stylet, lubricate it with a lubricant jelly, and then replace the wire; this will facilitate wire removal when the tube is in place. The tube tip should be placed in the caudal esophagus or stomach. To rule out the presence of gastric reflux before enteral fluid therapy is started, the tube tip must be placed in the stomach and suction must be applied on the external tip of the tube. To avoid inadvertent administration of fluids into the respiratory tract, the position of the NG tube must be checked before initiating fluid therapy.
When a small-bore tube is used, it is not easy to feel the resistance of the esophagus while passing the tube, nor is it easy to feel the tube in the esophagus by palpa- tion of the neck. Passing the tube in the trachea is likely to produce coughing, unless the horse is severely depressed or sedated. If the horse does not cough while the tube is being inserted, a small volume (100 to 500 ml) of water or isotonic electrolyte solution can be administered, which will likely produce coughing if the tube has been inserted in the trachea. For definitive confirmation of appropriate tube position, either radi- ography (Figure 3) or ultrasonography of the neck (Figure 4) or endoscopy of the pharynx (Figure 5) can be performed.
When a large-bore tube is used, the resistance pro- duced by the collapsed esophagus can be easily per- ceived while the tube is being passed. If the horse is not very fat, it is usually possible to feel the tube in the esophagus by palpation of the neck or even see the dis- tention in the jugular groove produced by the advance- ment of the tube tip. In the middle of the neck, the esophagus is located dorsolaterally to the trachea, usu- ally in the left side.

FLUID ADMINISTRATION
To minimize the risk of excessive gastric distention, fluids should be administered by gravity but not with a pump. When a small-bore tube is used, it is better to put the solution in a container, such as a plastic jug or an empty IV fluid bag (Figure 6). With a large- bore tube, the same
containers can be used or the fluids can be administered through a large funnel (Figure 1) because this type of tube permits rapid administration of large boluses. Simi- lar to the process of administering IV fluid therapy, en- teral fluid therapy can be administered to start fluid therapy slowly with small doses and grad- ually increase the rate of administration over a few hours. In most cases, fluid temperature does not seem to be critical since gastric emptying is not affected by the administration of cold fluids.6 However, when the environment is very cold or if the horse has severe mal- nutrition or hypothermia, it is better to use warm fluids (38°C). When the environment is very hot or for con- ditions such as postexercise dehydration, cold fluids may help to combat hyperthermia.

FLUID COMPOSITION
Easy-to-make nonsterile electrolyte solutions or even tap water can be used for enteral fluid therapy. Fluid composition should be chosen based on the horse’s clinical condition, the volume of fluids needed, and duration of the treatment. For a horse with functioning kidneys that is being fed and is going to receive enteral fluid therapy for a short period of time (e.g., 2 days), fluid composition is less critical.
On the other hand, in horses with anorexia or kidney failure that need enteral fluids for several days, there is a higher risk of severe electrolyte imbalance. Tap water, which is almost electrolyte free, can be used if given in small volumes or for a very short period (1 day) but can produce life-threatening hyponatremia and hypo- chloremia if large doses are used, especially in horses with kidney disease or diarrhea. A 0.9% NaCl solution (made with 9 g of table salt/L of water) would be another option, but can produce hypernatremia, hyper- chloremia, and acidosis due to the high concentration of sodium and chloride (each 154 mEq/L).7 A solution containing 4.9 g of both table salt (NaCl) and Lite Salt (Morton Salt) has been described.6 In this solution, the concentration of sodium, potassium, and chloride is about 126, 33, and 158 mEq/L, respectively. In equine patients with normal kidneys, the high potassium con- centration of this solution may not be a problem because a horse’s diet usually has a large amount of potassium; however, the high concentration of chloride may lead to hyperchloremia and acidosis if large vol- umes are used. Alternatively, a solution with sodium, potassium, and chloride concentrations, closer to what is found in plasma (about 141, 4, and 105 mEq/L, respectively), can be produced with 5.6 g of table salt, 0.6 g of Lite Salt (50% NaCl, 50% KCl), and 3.4 g of baking soda (NaHCO3) per liter of water. Rapid admin- istration of large volumes (10 L/hr for 6 hours) of this solution to normal horses did not produce significant changes in plasma electrolytes.8
Contrary to what has been shown in humans and other mammals, the importance of adding glucose to the elec- trolyte solution to promote sodium and water absorption has not been proven in horses.6 Thus the authors have not used electrolyte solutions containing glucose for routine enteral fluid therapy in horses.

TREATMENT MONITORING
Continuous monitoring for signs of intolerance to enteral fluid therapy is mandatory. Abdominal pain may be produced by excessive gastric distention and may be managed by interrupting fluid administration, although in some cases drainage of gastric content and treatment with analgesics may be required. Signs of intolerance to enteral fluid therapy may not mean that NG adminis- tration of fluids is absolutely contraindicated. In many cases, the horse may be just intolerant to the rate of fluid administration, and a reduction in rate may be the only change necessary. As with IV fluid therapy, hydra- tion status (skin tent, mucous membrane aspect, capil- lary refill time, urine aspect, urine specific gravity, fecal aspect, packed cell volume, and plasma protein) and plasma electrolytes must be monitored frequently (at least once every 12 hours) and used to guide changes in rate of infusion and fluid composition. If enteral fluid therapy alone is unable to maintain hydration status and electrolyte balance (e.g., in a horse with severe colitis or a horse that cannot tolerate the volume of fluids required by the enteral route), complementary adminis- tration of IV fluid therapy is indicated.

TREATMENT TERMINATION
Clinical evaluation will indicate the appropriate moment to terminate enteral fluid therapy. The same approach used for IV fluid therapy of gradual withdrawal (to slowly reduce the rate of infusion) to allow the horse to readapt to voluntary intake of water is indicated.