Emergency Fluid Therapy
Injuries with blood loss, exhaustion, acute rhabdomyolysis, and overheating are conditions that require emergency fluid replacement. Fluids can be administered for maintenance purposes, when fluid intake is physically not possible, or for replacement purposes when excessive losses have been incurred or ongoing losses are anticipated.
In athletic horses, replacement therapy is the mainstay of fluid therapy. Designing a replacement fluid therapy regimen requires consideration of the volume and type of fluids required as well as the route and rate of administration. The volume of fluid to give on a daily basis can be calculated using the following: volume to administer (L) = maintenance (60 mL/kg/day) + immediate losses (body wt [kg] × estimate of dehydration) + ongoing losses. Ongoing losses can be difficult to determine. Maintenance volumes are ~1L/hr for adult horses. Dehydration can be estimated by using clinical and laboratory parameters (see Equine Emergency Medicine: Physical and Laboratory Parameters for Estimation of Dehydration in Horses). These numbers should be considered in relation to the horse's clinical condition. For example, a nervous horse may have a transiently high heart rate in response to excitement and a high PCV because of splenic contraction.
Ongoing losses can be difficult to estimate, as losses through the GI tract are hard to measure. The equine GI tract secretes and reabsorbs the equivalent of the extracellular volume (~30% of body wt) on a daily basis. If ileus is present, the amount of reflux can be quantitated. If the large colon is not reabsorbing water (eg, diarrhea), losses can be significant. With severe diarrhea, ~50% of extracellular fluids can be lost on a daily basis.
This calculation provides only a crude estimate; volumes should be adjusted based on objective responses to fluid administration such as heart rate, pulse quality, capillary refill time, urine production, PCV, total protein, and creatinine. These parameters should be monitored as often as dictated by the horse's clinical condition. In a horse in severe shock, cardiovascular parameters may need to be monitored continuously, or at least every 15 min until an improvement is noted. In a horse with severe ongoing fluid losses, cardiovascular parameters should be monitored at least every 4 hr, and laboratory parameters as frequently as 4 times a day until stabilized. Following these evaluations, the estimate of fluid requirements can be adjusted.
Fluids available for administration in horses include crystalloids (fluids containing substances that freely cross the capillary membrane, including balanced electrolyte, saline, and dextrose solutions) and colloids (fluids that are retained in the vascular space for a certain number of hours because of their larger molecular size). Colloids include plasma, albumin solutions, dextrans, and hydroxyethylstarch. Crystalloids are most commonly used for replacement fluid therapy in athletic horses, whereas colloids are mostly reserved for resuscitation purposes. (Also see Fluid Therapy.)
Two basic types of crystalloids are available for horses: balanced electrolyte solutions (BES), which are solutions that contain electrolyte in concentrations similar to those in plasma, and saline solutions, which contain only sodium chloride. Although considered a crystalloid, dextrose solutions are rarely used alone. The decision to choose BES or saline is based, if available, on a serum chemistry profile. Saline is chosen if the sodium concentration is <125 mEq/L and there is no edema, there is a metabolic alkalosis, or the potassium is >5.9 mEq/L. Otherwise, a BES is used. If serum chemistries are unavailable, a BES is safe, unless hyperkalemic periodic paralysis is suspected, in which case saline, dextrose, and/or sodium bicarbonate should be used.
The addition of colloids to a fluid therapy regimen serves 2 purposes—preventing edema formation in hypoproteinemic states and sustaining the intravascular fluid volume. Products containing antibodies are also available for the treatment or prevention of endotoxemia, Rhodococcus equi pneumonia, West Nile virus infection, and clostridial diseases. Colloidal solutions are available in natural or synthetic forms. Natural colloids are plasma, serum products, or albumin. In general, plasma is selected when an increase in oncotic pressure is needed and coagulation factors or specific anticoagulants such as antithrombin III are required. Albumin solutions are not commonly used, as the intravascular half-life of albumin in states of compromised vascular permeability is short, and they do not have the added benefits of whole plasma. The synthetic colloid most commonly used in horses is hydroxyethylstarch. It is used to increase plasma oncotic pressure, and its effect is best evaluated either by clinical response (decreased edema) or increased oncotic pressure (measured by colloid osmometry). A refractometer cannot be used to monitor the effect of synthetic colloid administration.
The goal of fluid therapy in shock states is to rapidly expand circulating blood volume to improve perfusion and oxygen delivery. Isotonic crystalloids must be administered at a rapid rate of up to 60–80 mL/kg in the first hour (~1 blood volume) for maximal beneficial effects. Hypertonic saline can rapidly expand circulating volume by redistributing extravascular fluids into the vascular space. Because of redistribution, hypertonic solutions in horses have a short duration of effect (~45 min). Colloid solutions can be used to sustain the effect of hypertonic crystalloid solutions to several hours. For resuscitation, a combination of hypertonic saline (4 mL/kg) and hetastarch (4 mL/kg) has the most beneficial and sustained effects.
The flow rate of fluids through an administration system is directly proportional to the diameter of the line and inversely proportional to the viscosity of the fluid and the length of the line. Teflon® or polyurethane 14-gauge catheters are used routinely in adult horses. When gravity flow is used, a rate of 2–3 L/hr can be achieved when fluids are ~10 ft higher than the jugular vein. For more rapid flow, 10- or 12-gauge catheters with large-bore connecting sets can be used, but 10-gauge catheters are more thrombogenic. Finally, both jugular veins can be cannulated for increased fluid administration, and a pressure bag system or a pump used to increase the flow rate. Peristaltic pumps can cause endothelial damage and increase the risk of thrombosis.
Nasogastric intubation is an essential and possibly life-saving procedure performed in cases of equine colic. The tube is passed in the ventral meatus, using the thumb to keep it directed correctly. If a hard structure (the ethmoidal area) is encountered, the tube should be redirected more ventrally. Once the pharynx is reached, a soft resistance is felt. The tube can be turned 180° to direct the curvature dorsally toward the esophagus. The horse is stimulated to swallow, and the tube is then pushed in the esophagus. Blowing into the tube to dilate the esophagus helps facilitate insertion. If the horse coughs, the tube should be withdrawn and the procedure repeated until it is correctly positioned. The tube is advanced into the stomach (14th rib). If difficulty is encountered in passing the cardia, 60 mL of mepivacaine can be injected into the tube. Once the tube is in place, if there is no spontaneous reflux, the stomach should be lavaged (ie, it should not be assumed that any reflux will come out spontaneously). Medication should never be administered by nasogastric tube to a horse with colic without checking first for reflux. To do so, the tube is filled with water using a pump, and the end of the tube is directed downward to verify the presence of gastric contents. Subtracting the amount pumped in from the amount obtained determines “net” reflux.
Nasogastric reflux is not normal. Occasionally a small amount of reflux (≤1 L) is obtained if a horse has had a tube in place for a long time. When reflux is obtained, the amount, character, and timing in relationship to the onset of colic is noted. In addition, the response to gastric decompression should be noted.
Typically, reflux accompanies small-intestinal ileus, either functional or mechanical. Lesions of the proximal small intestine produce large amounts of reflux early in relationship to the onset of the colic. With lesions of the distal small intestine (ileum), there is initially no reflux, but it usually is found several hours after the onset of colic. Occasionally, large colon disease can be associated with reflux, if the colonic distention exerts pressure on the duodenum as it curves over the base of the cecum.
Foul-smelling, fermented, or copious bloody reflux is associated with anterior enteritis. With intestinal obstruction, the reflux is usually composed of fresh feed material and intestinal secretions. Reflux originating from the small intestine is alkaline, whereas reflux composed of gastric secretions is acidic. Because gastric outflow obstruction is rare in horses, pH is usually not measured. Response to gastric decompression should be noted. Horses with functional ileus show relief of pain, and the heart rate decreases in response to decompression. Horses with a mechanical obstruction usually remain painful, although some horses respond. The rest of the examination should focus on determining whether functional or mechanical ileus is present. The amount of reflux obtained should be noted, and the volume of fluids given IV should be adjusted accordingly. Horses with functional ileus generally need gastric decompression every 4 hr, although if severe, every 2 hr may be required. The nasogastric tube should be left in place only as long as required, as it will cause pharyngeal and laryngeal irritation in some horses.
Abdominocentesis is important in the evaluation of abdominal disease (eg, colic, weight loss, or postoperative problems). Ultrasonography can be used to determine the best location for obtaining a fluid sample, which can be collected using an 18-gauge needle, a catheter, or a cannula. The fluid is collected using sterile technique and placed in an anticoagulant or culture tube.
Normal values for abdominocentesis include a total protein <2.5 g/dL and WBC <5,000 cells/μL. On cytology, neutrophils comprise ~40% of cells; the remainder are lymphocytes, macrophages, and peritoneal cells. With intestinal strangulation, protein increases in the first 1–2 hr. After 3–4 hr of strangulation, RBC are present, and after 6 hr or more, WBC increase gradually, as intestinal necrosis progresses.
Enterocentesis sometimes is seen and should be differentiated from intestinal rupture. With enterocentesis, cytology reveals plant material, bacteria, and debris, but no cells. The horse's clinical condition is not consistent with rupture, although in early rupture (2–4 hr), clinical signs may not be seen. Cytology of abdominal fluid with intestinal rupture shows neutrophils, bacteria, and bacteria that have been phagocytized by neutrophils.
Blood contamination that occurs during the procedure should be differentiated from internal hemorrhage or severely devitalized bowel. Blood from skin vessels usually swirls in the sample and spins down when centrifuged, leaving the sample clear. If an abdominal vessel is punctured, blood will also spin down. All fresh blood contamination shows platelets, which are not present with blood >12 hr old. If the spleen is accidentally punctured, centrifugation reveals a PCV higher than the peripheral PCV. In internal hemorrhage, blood is hemolyzed (leading to a reddish supernatant after centrifugation), there are no platelets, and erythrophagocytosis may be seen. Ultrasonography also reveals fluid swirling in the abdomen. Because calcium edetate in the sample will falsely elevate the total protein, it is useful to shake it out of the tube.
Abdominal surgery increases the total protein (TP) for 3–4 wk and WBC for up to 2 wk. Neutrophils appear nondegenerate. After an enterotomy or an anastomosis, degenerate neutrophils and occasional bacteria may be seen in the first 12–24 hr. The WBC count remains elevated for 2 wk, but the neutrophils appear nondegenerate on cytology, and there are no bacteria. The TP remains elevated for 1 mo after surgery. If septic peritonitis is present, signs will be consistent with bacterial infection (eg, fever, depression, anorexia, ileus, pain, endotoxemia). The WBC and TP are markedly elevated. On cytology, >90% of cells are neutrophils, and they appear degenerate. Free and phagocytized bacteria are seen.
Trocarization is useful to decompress the abdomen when abdominal compartment syndrome is present (severe distention associated with pain and dyspnea). Trocarization should be performed only for large colon distention, never to decompress the small intestine. Thus, it is important to identify the segment of intestine that is involved prior to the procedure. In adult horses, this can be done by rectal palpation. In foals or small horses, radiographs and/or ultrasonography can be used. The distended segment of large colon must also be close to the body wall so it can be safely reached. The most common site for trocarization is the right upper flank area, just cranial to the greater trochanter at the location of the cecal base. After decompression, the trocar is removed, and an antibiotic (usually gentamicin) is infused as the catheter is withdrawn.
Peritonitis and local abscessation are the 2 most common problems encountered after trocarization. The horse is observed for 24 hr for signs of peritonitis. Peritonitis is confirmed with abdominocentesis, and systemic broad-spectrum antibiotics are administered until it is resolved. A local abscess can be drained externally.
If possible, the planned incision site for a tracheostomy should be clipped, prepped, and infiltrated with a local anesthetic. In acute respiratory distress, this may not be possible, and the procedure is done without preparation.
An 8- to 10-cm longitudinal incision is made at the junction of the proximal and middle third of the neck, just above the “V” made by the junction of the sternothyrohyoideus muscles. Care must be taken to stay on midline, to favor drainage. The sternothyrohyoid muscles are separated on the midline, and the trachea is exposed. A transverse incision is made between 2 tracheal rings, taking care to avoid damaging the tracheal cartilages. If the horse's head is elevated during the procedure, the tracheal incision should be made distal in relationship to the skin incision, to avoid covering the incision when the head is lowered. In emergency situations, a J-type tracheostomy tube is used because of ease of insertion. When the horse is calm or if the situation is not critical, a self-retaining tube is preferred for maintenance, as J-tubes tend to fall out. If the animal is to be ventilated, a silicone-cuffed tube allows for closed-system ventilation.
The tracheostomy tube should be cleaned daily and changed as needed. Petroleum gel applied around the incision helps to avoid skin scalding. In general, but particularly in foals, tracheostomy tubes should be removed as early as possible to avoid permanent tracheal deformity. One method used to determine when the tube can be removed is to temporarily occlude the tube and determine whether the horse can breathe without it. Once the tube has been removed, the site is cleaned of exudate twice daily and allowed to heal by second intention. It will generally close in 10–14 days and heal in 3 wk.
Last full review/revision March 2012 by Pamela Anne Wilkins, DVM, MS, PhD, DACVIM-LA, DACVECC