Signalment

The age of the foal or horse should be considered in the anesthetic planning process. Specific recommendations for anesthesia of the neonate can be found in Chapter 13 and those for the geriatric horse in Chapter 11. Breed considerations relative to temperament and body mass are important considerations with any anesthetic event, but even more so with horses with ischemic GI disease due to the prolonged nature of both the surgical procedure and recovery, as well as the accompanying systemic compromise. For example, utilizing alpha‐2 agonists or other sedatives in the recovery period may be more likely in light breed horses prone to excitation such as some Arabian horses. Conversely, large draft breeds are more predisposed to difficulty standing during recovery from anesthesia due to development of myopathies or neuropathies (Rothenbuhler et al. 2006) and tools to assist them to stand such as a sling or the Large Animal Lift (Large Animal Lift Enterprises LLC, Moses Lake, WA) may be necessary. Efforts toward reducing anesthesia duration in draft horses such as pre‐surgical clipping of the abdomen are ideal to minimize time under general anesthesia. Gender considerations do not differ from elective anesthetic procedures, but in an emergency situation, these considerations may be overlooked such as potentially avoiding the use of acepromazine in breeding stallions (Wagner 2009); special considerations for anesthetic management of the pregnant mare may be found in Chapter 12.

Assessment of the Acute GI patient

A variety of procedures are commonly performed during assessment of the acute GI patient that can directly impact the anesthetic period. Physical examination parameters will include assessment of presence of dehydration through mucous membrane texture and skin turgor along with presence of hypovolemia by assessment of heart rate, mucous membrane color, capillary refill time, pulse quality from the facial artery, jugular refill time, temperature of extremities, mentation, and urine output. Stabilization of the pre‐anesthetic patient with hypovolemia will be critical to success as most horses undergoing surgery for GI disease are positioned in dorsal recumbency, placing them at increased risk for hypotension and hypoxemia, even in the absence of pre‐existing hypovolemia.

Other parameters including packed cell volume (PCV), total protein (TP), and lactate in peripheral blood, provide more critical quantitative estimates of anemia, dehydration, hypovolemia, and poor circulation. Elevations in PCV and lactate in the preoperative period are associated with reduced probability of survival, particularly in the presence of relatively low peripheral TP in horses with surgical disease of the small intestine (Proudman et al. 2005). Decreases in TP and colloid oncotic pressure associated with anesthesia (Boscan et al. 2007), and continued intravascular losses associated with increases in endothelial permeability secondary to the systemic inflammatory response syndrome (SIRS) can further compromise the patient. Further, the use of synthetic colloids to promote intravascular volume support remains controversial due to increased risks relative to renal dysfunction and mortality in septic and critically ill human patients (Adamik et al. 2015). Convincing experimental studies and clinical investigations documenting this in equine and more broadly veterinary patients are however lacking.

Further initial assessment and therapeutic actions common in equine acute GI patients include abdominocentesis, passage of a nasogastric tube, and transcutaneous decompression of distended large intestine that may facilitate ventilation and improve oxygenation during the induction and early anesthetic period.

Gastric or Abdominal Distension

Gastric distention with fluid, gas, or feed material is a common finding in the adult acute surgical GI patient. Passage of a nasogastric tube (Figure 10.1) prior to induction of anesthesia is paramount to reducing the potential for aspiration pneumonia or the risk of gastric rupture upon induction. Presence of gastric distention may also exacerbate existing increases in intra‐abdominal pressure (Barrett et al. 2013).

Generalized distention of the abdomen may result from a variety of sources, most commonly from gas accumulation within the ascending colon and cecum. However, fluid distention of small intestine, colon, or the peritoneal cavity may also contribute to generalized abdominal distention and increases in intra‐abdominal pressure. Intra‐abdominal hypertension and its sequelae have been documented in the horse (Brosnahan et al. 2009) and horses with GI obstruction are at particularly high risk. Abdominal distension presents not only challenges with ventilation and oxygenation, but the presence of intra‐abdominal hypertension has the potential to contribute to hemodynamic derangements and multiple organ dysfunction (Roberts et al. 2016). Both transabdominal and transrectal trocarization in the preoperative period have been advocated as a method to decompress the abdomen in horses with severe large intestinal distension, as clinically relevant peritonitis is rare in those cases undergoing surgical exploration following trocarization (Schoster et al. 2020).

Pain

Many horses with surgical disease of the abdomen present in extreme pain making them fractious and sometimes difficult to stabilize in the preoperative period. Extreme pain may also make them refractory to sedation resulting in receipt of multiple doses of sedatives and analgesics such as xylazine, detomidine, and butorphanol. Compounding the situation is the need for rapid correction of the problem, as rapid correction of an ischemic process such as a colonic volvulus or a strangulating small intestinal lesion can markedly improve prognosis. General anesthesia must take place quickly to accomplish treatment of the underlying disease that may increase risk for anesthetic errors or result in insufficient volume resuscitation prior to induction. Interestingly, a large retrospective study correlated increased risk of intraoperative mortality with horses demonstrating less severe signs on pain on admission. This was attributed to either advanced devitalization of bowel since pain can subside as ischemia advances, or delays in surgical intervention due to less obvious signs of pain (Proudman et al. 2006).

Exertion, Exhaustion, Trauma

Depending on the duration and severity of disease, horses with surgical abdominal lesions may present in a state of exhaustion with self‐inflicted trauma from rolling and other colic‐associated behaviors. This may manifest in the recovery period as a prolonged anesthetic recovery and reluctance or inability to stand in the post‐operative period. This may be further influenced by high doses of drug received peri‐operatively.

Anticipated Electrolyte and Acid/Base Imbalances

In addition to fluid losses from dehydration and hypovolemia for the more severe forms of surgical colic, there are anticipated changes in electrolyte concentrations in horses and foals with various forms of GI disease. The most common electrolyte disturbances identified in adult horses with surgical GI obstruction are hypomagnesemia, hypocalcemia, and hypokalemia.

The prevalence of total or ionized hypomagnesemia is reported to be 17–54% in horses with GI disease (Garcia‐Lopez et al. 2001; Johansson et al. 2003). Magnesium is the second most abundant intracellular cation behind potassium and plays a critical role in over 300 enzymatic reactions involving ATP (Elin 1988). Hypomagnesemia is associated with increased cytokine production and systemic inflammation (Weglicki et al. 1992), and experimental endotoxin administration in horses results in acute decreases in both ionized and total magnesium concentrations (Toribio et al. 2005). Hypomagnesemia is concurrently observed with hypocalcemia in several GI diseases in horses (Garcia‐Lopez et al. 2001; Toribio et al. 2001, 2005), and it is also frequently observed with hypokalemia in other species (Martin et al. 1994). The presence of hypomagnesemia may make patients refractory to treatment of both hypocalcemia and hypokalemia until serum magnesium concentrations are corrected (al‐Ghamdi et al. 1994). This may place the patient at higher risk of post‐operative ileus (Garcia‐Lopez et al. 2001). Clinical signs of hypomagnesemia in horses that could have direct impact on anesthesia may include weakness, ataxia, ventricular arrhythmias, supraventricular tachycardia, atrial fibrillation, and seizures (Green et al. 1935; Marr 2004).

Hypokalemia in horses with GI disease is prevalent and believed to be associated with reduced intake, altered absorption, or excessive losses from secretory processes such as enteritis or colitis (Nappert and Johnson 2001). A whole‐body deficit of potassium is expected in anorectic horses with surgical GI disease and consequences may include skeletal muscle weakness, paralysis, irregularities of cardiac rhythm, perturbed intestinal motility, and abnormal acid–base status (Johnson 1995). Restoration of potassium levels intraoperatively at a rate not to exceed 0.5 mEq/kg/h is appropriate. Rarely, hyperkalemia may be observed in the peri‐operative equine GI patient and could be associated with intravascular hemolysis, severe metabolic acidosis, hyperkalemic periodic paralysis, severe myopathies, or malignant hyperthermia. Presence of serum hyperkalemia should be rapidly addressed with intravenous calcium, bicarbonate, or glucose and insulin to avoid fatal cardiac arrhythmias.

Hypocalcemia is associated with conditions commonly present in horses with surgical GI disease including colic, sepsis, endotoxemia, and acute renal failure. Signs of acute hypocalcemia may include muscle fasciculations, tremors, tetany, seizures, tachycardia, and cardiac arrhythmias. Hypocalcemia may also compound the development of ileus and can be addressed via supplementation intraoperatively at a dose of 50–100 ml of 23% calcium gluconate per 5 l volume of isotonic crystalloid (Johnson 1995). Fluid rates must be adjusted for the patient’s body weight to deliver 10–30 mg/kg over 30–60 minutes.

The presence of both metabolic and respiratory acidosis during anesthesia is higher in colicky horses compared with controls (Edner et al. 2007), although significant reductions in serum bicarbonate at the time of presentation were not evident in horses that recovered from their colic episode (Nappert and Johnson 2001). Volume repletion should precede any attempts to correct a metabolic acidosis with bicarbonate therapy as the majority of surgical GI cases with metabolic acidosis suffer from impaired perfusion and accumulation of lactate. Intra‐abdominal hypertension secondary to presence of gas or fluid accumulation in the GI tract, thoracic cavity (from diaphragmatic hernia), or peritoneal cavity can result in impaired ventilation and subsequent respiratory acidosis, resulting in increased requirements for ventilatory support.

Peri‐procedural Handling

Horses with acute GI disease can present unique challenges in the peri‐operative period. Preoperatively, they may present recumbent in a trailer and unable to stand or in a violently painful state where they continually collapse due to unrelenting pain. Some may require induction at a site distant from the surgical suite followed by transport during which there is a limited ability to monitor and support the patient. The Large Animal Rescue Glide (L.A.R.G.E., Inman, SC) can facilitate rapid transport of a recumbent horse from a trailer to the surgical suite when adequate numbers of people are available to facilitate movement of the horse. Special considerations for these critical patients may entail safety concerns for clinicians during IV catheter placement, anesthetic death from inability to properly stabilize the patient prior to induction, and impaired ventilation and oxygenation during the transport period. Endotracheal intubation and support of oxygenation and ventilation should be considered in these circumstances, with recognition that in the absence of rinsing out the oral cavity, contaminants may enter the airway. Solutions (e.g. hypertonic saline) with potential for improving circulation after small volume administration should be considered once the catheter is in place.

For mares presenting with colonic volvulus or other surgical GI conditions that are accompanied by their foal, attention must be paid to the foal to prevent injury from the mare and to provide a safe environment while the mare is in surgery. Alternatively, mares must be handled appropriately when the foal needs to undergo emergency surgery. To optimize the effectiveness of induction drugs along with sedatives for the mare, the mare and foal ideally should not be separated until the time of induction. Heavy sedation may still be required. Intravenous or intramuscular alpha‐2 agonists alone or in combination with acepromazine or an opioid may be used.

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