Emergency Fluid Therapy
Conditions that may require emergency fluid replacement include injuries with concurrent blood loss, exhaustion, acute rhabdomyolysis, hyperthermia, and circulatory shock secondary to systemic illness. Fluids may be needed for maintenance purposes when oral intake is physically not possible. Replacement fluids are required when excessive losses have been incurred or ongoing fluid losses anticipated.
Design of a fluid therapy regimen requires consideration of the volume required, the type of fluids needed, and the route and rate of administration. The volume to be administered over 24 hr can be calculated using the following formula: volume to administer (L) = maintenance rate (60 mL/kg/day) + estimated fluid deficit (body wt [kg] × estimate of dehydration) + estimate of ongoing losses. Maintenance fluid requirements are ~1 L/hr for adult horses (~450 kg) but are much lower if the animal is not eating. Dehydration can be estimated by using clinical and laboratory parameters (see Table 1: Physical and Laboratory Parameters for Estimation of Dehydration in the Horse). These numbers should be considered in relation to the horse's age and clinical condition and are only an estimate of the deficit. For example, a nervous horse may have a transiently high heart rate in response to excitement and an increased PCV because of splenic contraction.
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Ongoing losses are difficult to estimate, because losses from the GI tract are hard to measure. The equine GI tract secretes and resorbs the equivalent of the extracellular fluid volume (~30% of body wt) on a daily basis. If small-intestinal ileus or obstruction is present, the amount of gastric reflux obtained can be easily quantitated to gauge losses. If the large intestine is not resorbing water (eg, diarrhea), fluid losses can be significant but difficult to measure. With severe diarrhea, ~50% of the extracellular fluids can be lost daily.
The formula to calculate volume of fluid to administer provides only a crude estimate of needs; volumes administered should be adjusted based on responses to treatment, such as heart rate, pulse quality, capillary refill time, urine production, PCV, serum total protein, creatinine, and lactate. Reassessment is required to adjust the daily fluid requirements for any horse receiving supplemental fluids, and the timing should be dictated by the horse's clinical condition. In horses in severe shock, cardiovascular parameters may need to be monitored continually (eg, every 15 min) until improvement is noted. In horses with severe ongoing fluid losses (eg, diarrhea, anterior enteritis), cardiovascular parameters should be reassessed every 4 hr and laboratory parameters measured as frequently as 4 times a day until the horse stabilizes.
After determination of volume required, the type of fluid to be administered should be selected. Fluid choices include crystalloids (fluids containing substances that freely cross the capillary membrane) and colloids (fluids retained in the vascular space for a certain number of hours because of their larger molecular size). Crystalloids are most commonly used for replacement fluid therapy, whereas colloids are more often reserved for resuscitation purposes. (Also see Fluid Therapy.)
Two general types of crystalloids are available: balanced electrolyte solutions (BES), which are solutions of electrolytes 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, and are usually added as a supplement to a BES when indicated by the needs of the individual horse. The decision to choose BES or saline can be based on a serum chemistry profile. Saline is chosen if the sodium concentration is <125 mEq/L, if there is a metabolic alkalosis, or if the potassium is >5.9 mEq/L (eg, acute hyperkalemic periodic paralysis, uroabdomen). In most emergent cases, however, a BES is used.
The addition of colloids to a fluid therapy regimen serves two purposes: it prevents edema formation caused by hypoproteinemia, and it maintains the intravascular fluid volume. Plasma products that contain antibodies to treat or prevent a variety of conditions (including endotoxemia, Rhodococcus equi pneumonia, West Nile virus infection, clostridial diseases, and snake envenomation) are also available. Colloid solutions are available in natural or synthetic forms. Natural colloids are plasma, serum products, or albumin. In general, fresh or fresh frozen plasma is selected when an increase in colloid oncotic pressure is needed, and coagulation factors or specific anticoagulants such as antithrombin III are required. Albumin solutions are not commonly administered to horses, because the intravascular half-life of albumin in diseases with compromised vascular permeability is short. They also do not have the added benefits of whole plasma. Synthetic colloids include dextran and hydroxyethyl starch. The synthetic colloid most commonly administered to horses is hydroxyethyl starch (hetastarch). It is used to increase plasma oncotic pressure, and its effect is best evaluated by clinical response (decreased edema) or increased oncotic pressure (measured by colloid osmometry). A refractometer that measures total protein cannot be used to monitor the effect of synthetic colloid administration.
The goal of fluid therapy for treatment of shock is to rapidly expand circulating blood volume to improve tissue perfusion and oxygen delivery. Isotonic crystalloids should be administered at a dosage of up to 60–80 mL/kg in the first hour (equal to the circulating blood volume) for maximal benefit. This fluid rate, the “shock dose,” should be given in boluses of 20 mL/kg (approximately 10 L at a time), and the horse should be reassessed between each bolus to determine whether additional boluses are needed. Because of the large volumes of BES required, hypertonic fluids or colloids may be given first to immediately support the circulation until the shock dose of crystalloids can be administered. At a dosage of 2–4 mL/kg, hypertonic saline (7.5%) can rapidly expand the circulating volume by redistributing extravascular fluids into the vascular space. Because of redistribution, hypertonic solutions have a short duration of effect (~45 min) in horses. Colloid solutions can be used for a more sustained effect. Hydroxyethyl starch has been reported to increased oncotic pressure for up to 24–36 hr. However, dosages of colloids >10 mL/kg/day have caused coagulopathies. For resuscitation, a combination of hypertonic saline (4 mL/kg) and hetastarch (4 mL/kg) may have the most beneficial and sustained effects.
The flow rate of fluids through an administration set is directly proportional to the diameter of the line and inversely proportional to the viscosity of the fluid and the length of the infusion set. Polytetrafluoroethylene or polyurethane 14–gauge catheters are routinely used in horses. A rate of 7 L/hr can be achieved when fluids are >2 feet above the jugular vein, and faster rates can be achieved through gravity flow if the fluid bags are raised even higher. For more rapid flow, 10- or 12-gauge catheters with large-bore connection sets can be used, but 10-gauge catheters are more thrombogenic. Other ways to increase fluid administration rates include cannulating both jugular veins or using a pressure bag system or peristaltic pump. Complications of peristaltic pumps are endothelial damage and an increased risk of venous thrombosis.
An alternative route of fluid administration is use of an indwelling nasogastric tube. Case selection is key, and horses that are in shock, are >8% dehydrated, or have positive net gastric reflux are not candidates for oral fluids. Equine electrolyte solutions are preferred, or a homemade mixture can be formulated using 5.27 g sodium chloride, 0.37 g potassium chloride, and 3.78 g bicarbonate per liter of water. The daily fluid rate is divided into boluses and administered using a bilge pump. The horse should be checked for reflux before administration. Volumes <8 L every 2–4 hr are typically well accepted. If a smaller gauge feeding tube is in place, a continuous delivery system from a nonsterile IV set and fluid bags or a carboy can be used, and the rate should be determined in a manner similar to that of an IV infusion rate.
Nasogastric intubation is an essential and possibly life-saving procedure performed routinely in cases of equine colic to decompress the stomach and to provide therapy. After the horse is adequately restrained, the nasogastric tube is passed into the ventral meatus, using the thumb to keep the tube directed correctly. If a hard structure is encountered (eg, the ethmoid or nasal turbinates) and the tube is difficult to pass, the tube should be redirected more ventrally. Once the pharynx is reached, a soft resistance is felt when the tube contacts the larynx and/or esophageal opening. The horse's head is flexed at the poll, and the tube can be turned 180° so that the curvature of the tube is directed dorsally toward the esophagus. Swallowing is stimulated with gentle pressure against the esophageal opening or by blowing small puffs of air into the tube, and the tube is then passed into the esophagus by coordinating with the swallow reflex.
Once the tube is in the esophagus, mouth suction on the tube will be met with negative resistance. The tube may also be palpated in the proper position dorsal to the larynx or on the left side of the neck above the jugular vein. If the tube is in the trachea, shaking the trachea will result in a “rattle.” If the horse coughs, or if air is noted when suction is applied by mouth, the tube should be withdrawn into the pharynx and the procedure repeated until the tube is correctly positioned. Once the tube is confirmed in the esophagus, intermittently blowing into the tube will help to dilate the esophagus and facilitate insertion into the stomach. The tube is advanced into the stomach until level with the twelfth rib. If difficulty is encountered in passing the tube through the cardia, 60 mL of mepivicaine may be injected into the tube and followed with air or water.
Once the tube is in place, if there is no spontaneous reflux, the stomach should be lavaged. It should not be assumed that any excess fluid in the stomach will drain spontaneously. Medications should never be administered by nasogastric tube without checking first for net reflux. To do so, the tube is filled with ~2 L of water using a pump or funnel with gravity flow to establish a siphon effect, the pump removed, and the end of the tube directed downward to verify the presence of gastric contents. Subtracting the amount of water pumped in from the amount of fluid obtained determines the “net” reflux. Horses should be lavaged with at least 8 L of water to confirm lack of net reflux before any therapy is administered.
Net nasogastric reflux is not normal. Occasionally, a small amount of reflux (<1 L) is obtained if a horse has had an indwelling nasogastric tube. In most horses with colic, >2 L net reflux is abnormal. When reflux is obtained, the amount, character, and timing in relation to the onset of colic should be noted, as well as any clinical response to gastric decompression.
Typically, reflux accompanies small-intestinal ileus, either functional or mechanical. Lesions of the proximal small intestine or pylorus produce large amounts of reflux early in relation to the onset of colic. With lesions of the distal jejunum and ileum, there is initially no reflux, but nasogastric reflux usually becomes productive several hours after the onset of colic. Occasionally, large-colon disorders can be associated with net reflux if the colonic distention exerts pressure on the duodenum as it courses over the base of the cecum. Foul-smelling, fermented, or copious bloody reflux is associated with anterior enteritis and should be treated as a biohazard because of the association of this disease with clostridial and Salmonella spp organisms. With mechanical obstruction, including strangulating lesions and ileal impactions, 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 signs of relief, and the heart rate may decrease in response to decompression. Horses with a mechanical obstruction usually remain in pain, although some do respond. The rest of the physical examination of horses with reflux should focus on determining whether functional or mechanical ileus is present. The amount of reflux obtained should be noted after each decompression, and the volume of fluids given IV adjusted accordingly. Horses with functional ileus generally need gastric decompression every 2–4 hr. The nasogastric tube should be left in place only as long as required, because it can cause pharyngeal and laryngeal irritation. Esophageal rupture has been described in severe cases.
Abdominocentesis is important in the evaluation of abdominal disease (eg, weight loss, colic, peritoneal effusions, or postoperative complications). Ultrasonography can be used to determine the best location to obtain a fluid sample, which can be collected using an 18-gauge needle, a catheter, or a teat cannula through a stab incision. Most commonly, the sample is obtained just to the right of midline, caudal to the descending pectoral muscle on the dependent abdomen. This will prevent the site of the abdominocentesis from interfering with a surgical incision if an exploratory laparotomy is needed, and it will also help to avoid the spleen. The body wall is much thicker off midline, and the length of the instrument used to sample the fluid should be considered. A needle tap should be avoided with severe small-intestinal distention to reduce the risk of bowel puncture and iatrogenic peritonitis. The fluid is collected using sterile technique and placed in a tube with anticoagulant for analysis and in a culture tube if peritonitis is suspected. It is useful to shake out the EDTA from the tube before sampling, because EDTA will falsely increase the total protein.
Normal values for abdominocentesis include a total protein <2.5 g/dL and WBC counts of <5,000 cells/μL. Repeated abdominocentesis will not significantly alter these values above reference ranges. On cytology, neutrophils comprise most of the 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, RBCs are present, and after ≥6 hr, WBCs increase gradually as intestinal necrosis progresses. Degenerate neutrophils will be noted on cytology, and the gross appearance is serosanguinous. Peritoneal lactate is also increased with intestinal ischemia (>4 mmol/L) or is found to increase over time if serial abdominocenteses are performed.
Enterocentesis sometimes occurs, especially in cases of sand colic, and should be differentiated from intestinal rupture. With enterocentesis, cytology reveals plant material, bacteria, and debris but few cells. The horse's clinical condition is not consistent with rupture, although in early rupture (~2–4 hr), clinical signs of endotoxemia (eg, depression, tachycardia, congested mucous membranes, shock) may not be seen. Cytology of abdominal fluid compatible with intestinal rupture shows a large number of toxic and degenerate neutrophils, in addition to bacterial organisms, plant material, 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 or an abdominal vessel usually swirls in the sample and spins down when centrifuged, leaving the sample clear. Fresh blood contamination shows platelets, which are not present in blood >12 hr old. If the spleen is accidentally punctured, centrifugation of the sample reveals a PCV the same or higher than the peripheral PCV. In internal hemorrhage, the blood in the sample is hemolyzed, there are no platelets, and erythrophagocytosis may be seen. When centrifuged, the supernatant will remain reddish. Abdominal ultrasound will reveal fluid swirling on the abdomen. If vascular compromise of the bowel occurs, hemolysis of RBCs that leak from damaged capillaries will result in a serosanguineous fluid, with a red supernatant after a spin.
Abdominal surgery increases the total protein and WBCs in the abdominocentesis. With a sterile peritonitis, the WBC count will remain increased for up to 2 wk, and cell counts of 40,000 cells/dL have been reported at 6 days after surgery. Neutrophils will appear nondegenerate on cytology, and there are no bacteria. The total protein peaks at 6 g/dL 6 days after surgery and may remain increased for 1 mo. After an enterotomy, or resection and anastomosis, degenerate neutrophils and occasional bacteria may be seen in the first 12–24 hr but should resolve in time. If septic peritonitis is present, clinical signs will be consistent with bacterial infection (eg, fever, depression, anorexia, ileus, colic, endotoxemia). The WBC and total protein in the abdominocentesis will be markedly increased. On cytology, >90% of cells are neutrophils and will appear degenerate. Free and phagocytized bacteria are seen. A serum–peritoneal glucose difference >50 mg/dL or a peritoneal fluid pH <7.2 with a peritoneal glucose <30 mg/dL is supportive of a diagnosis of peritonitis.
Trocarization is useful to decompress the abdomen when abdominal compartment syndrome is suspected (increased intra-abdominal pressure resulting in severe abdominal distention, dyspnea, and perfusion abnormalities). Trocarization is performed only for distention of the cecum and large intestine, and never to decompress the small intestine or stomach. Thus, it is necessary to identify the segment of intestine involved before the procedure. In adult horses, this is done by transrectal palpation combined with transabdominal ultrasound. Abdominal auscultation may note a “ping” after percussion indicative of a gas-filled organ. In foals or small horses, radiographs and/or ultrasound can be used. The distended segment of large intestine must also be against the body wall so it can be safely reached. The most common site for trocarization is the upper right flank, just cranial to the greater trochanter at the location of the cecal base. Ultrasound is used to confirm a gas-filled structure at the chosen site, to avoid other organs or large blood vessels.
In adult horses, a 14-gauge, 12.5-cm over-the-needle catheter is typically used, whereas in neonates, a 5-cm needle may be adequate. The area is clipped, sterilely prepared, and infiltrated with a local anesthetic. The catheter is inserted aseptically and attached to a long extension set that is placed in a nonsterile cup of water to monitor progress. Gas released will produce bubbles, and the catheter may be repositioned as the bowel decompresses. A successful trocarization may take >30 min to decompress the bowel. When removing the catheter, an antibiotic (usually ~5 mL of gentamicin) is infused as the catheter is withdrawn.
Complications after trocarization include peritonitis, hemorrhage, and a local subcutaneous abscess. The horse should be observed for 48 hr for signs of peritonitis, including abdominal pain and fever. Peritonitis is confirmed with abdominocentesis and treated with systemic broad-spectrum antibiotics until it is resolved. Hemorrhage is typically self-limiting, and local abscesses can be drained percutaneously.
Tracheostomy is used as an emergency procedure for conditions resulting in acute upper airway obstruction (eg, arytenoid chondritis, snakebite). The incision site is selected at the junction of the proximal and middle third of the neck, above the “V” formed by the paired sternomandibularis muscles. If possible, the skin should be clipped, aseptically prepared, and infiltrated with a local anesthetic. In acute respiratory distress, this may not be possible, and the procedure is performed without sterile preparation.
The incision (6–8 cm) is made on midline, through the skin and between the paired sternothyrohyoideous muscles. The trachea is exposed, and a transverse incision is made between two tracheal rings, taking care to avoid the tracheal cartilages. The incision will extend to ~30% of the circumference of the trachea. If the horse's head is elevated or extended, the tracheal incision should be made caudal relative to the skin incision, to avoid covering the tracheostomy site when the head is lowered. A J-type silicone tracheostomy tube may be used and secured in place to the mane with gauze. The cuff should not be inflated, unless the horse is to be ventilated, to prevent tracheal necrosis. Alternatively, a self-retaining tube may be preferred because of the tendency of J-tubes to fall out.
The tracheostomy tube should be removed, cleaned, and replaced twice a day or more often if secretions occlude the lumen. Petroleum jelly applied to the surrounding skin will help to reduce skin scalding. Complications are rare (eg, cartilage deformity, intraluminal granulation tissue, mucosal structure) but can be reduced by removing the tube as soon as possible. One method to determine when the tube can be removed is to temporarily occlude the lumen by hand and observe whether the horse can breathe without it. After tube removal, the site is cleaned with saline twice daily and allowed to heal by second intention. The trachea will generally close in 10–14 days and heal completely in 3 wk.
Last full review/revision September 2014 by Amelia S. Munsterman, DVM, MS, DACVS, DACVECC