Perianesthetic Considerations

Anemic patients with packed cell volume (PCV) lower than 20% should receive a packed red blood cell (pRBC) transfusion before surgery because the oxygen‐carrying capacity of blood and the oxygen delivery to tissues are significantly impaired below this point. pRBC transfusion must also occur in patients with severe anemia undergoing mandibulectomy and maxillectomy owing to the increased risk of hemorrhage during these procedures [2]. The inferior alveolar artery, a branch of the maxillary artery, runs through the horizontal ramus of the mandible and is almost always ligated and transected during mandibulectomy (Figure 5.1). Blood supply to the maxillary region is provided by two major arteries, the major palatine, and the infraorbital arteries, both branches from the maxillary artery. Depending on the location of the tumor, one or both arteries may be ligated before maxillectomy. Rostral maxillectomy can be accompanied by diffuse bleeding from the highly vascularized nasal turbinates [6]. In this regard, the placement of two intravenous (IV) catheters may be prudent, as this allows simultaneous administration of blood, fluids, and inotropic support, if necessary.

Preoperative thoracic radiographs or computed tomography (CT) for the detection of pulmonary metastases is indicated for dogs and cats with oral neoplasia; the presence of visible pulmonary metastases may indicate an extremely poor prognosis and surgery may not be indicated [6]. Careful lung auscultation and thoracic radiographs are advised for evaluation of aspiration pneumonia in patients with clinical signs of dysphagia and diagnosed with large oropharyngeal masses or swelling [3].

Laryngeal visualization is often challenging in dogs and cats with large oral cavity masses (Figure 5.2) or masses located in the temporomandibular joint, which can complicate ET intubation and maintenance of a patent airway. General anesthesia without a secure airway may result in aspiration of gastric contents and blood, which can lead to pneumonia and small airway obstruction, respectively. In cases where ET intubation is expected to be difficult (i.e., patients with large pharyngeal masses, difficulty of opening the mouth, or for those already presenting dyspnea), preoxygenation with 100% oxygen via a tight‐fitting mask should be considered before anesthetic induction. In addition, diverse diameters of ET tubes, stylets, a laryngoscope with adequate lighting, and several laryngoscope blade sizes should be readily available. A flexible endoscope can also be valuable in visualizing the airway for ET intubation, if available. Alternatively, retrograde ET intubation can be performed in patients with restricted mouth opening. In this case, the anesthetist punctures the cricothyroid membrane with a needle or an IV catheter, a guide wire is inserted through the needle or catheter and advanced retrograde to the mouth opening. The guide wire is grasped with forceps once it is observed in the oral cavity and a cuffed ET tube is passed caudally over the wire into the larynx. The guide wire is removed, and the ET tube is advanced further caudally into the trachea [7]. Regardless of the selected ET intubation technique, a surgical tracheostomy pack should always be available, as tracheostomy may be necessary if several attempts at ET intubation fail.

An orally placed ET tube can sometimes hinder oral cavity and oropharyngeal surgery. In these cases, the portion of the ET tube that remains in the oral cavity can be deviated through a pharyngotomy or tracheotomy incision (see Chapter 13; Figure 5.3). For all methods of intubation, it is important that the ET tube and its cuff are properly inflated and lubricated to prevent blood and fluid from entering the lower airways. Gauze sponges are frequently placed in the oropharynx around the ET tube to help absorb fluids. However, these gauze sponges must be removed before the end of anesthesia and removal of the ET tube to prevent airway obstruction [2, 3]. Tying umbilical tape around the gauze to stack them together in the oropharynx and leaving the other end of the umbilical tape long enough to exit the mouth is a common way to prevent leaving packing material in the pharynx (Figure 5.4) [3].

Proper patient and anesthetic equipment positioning maximize the surgeon’s ability to visualize the surgical field. In some situations, the anesthetist and anesthetic equipment will be away from the patient head, which can make it difficult for the anesthetist to monitor the signs associated with anesthetic depth, and therefore changes associated with heart rate, arterial blood pressure, and respiratory rate and pattern should be used to help predict anesthetic depth.

A mouth gag is often needed to facilitate exposure of the oral cavity. However, in cats, spring‐loaded mouth gags have been associated with neurological deficits, including postoperative blindness. Blood flow to the feline brain is primarily supplied via the maxillary artery. The high and constant forces between the maxilla and the mandible applied by spring‐loaded mouth gags can compromise cerebral blood flow by stretching the maxillary artery. If a mouth gag is necessary, the use of a smaller and plastic mouth gag (e.g., a plastic needle cap or a 20 mm, cut 1 ml syringe barrel, depending on the size of the cat) between the upper and lower arcades is advised [8, 9]. Decreased duration of open‐mouth procedures should also be exercised during feline oral cavity surgery to minimize reduced maxillary artery blood flow.

Preanesthetic Medication

Sedative drug selection will depend on multiple factors, including patients’ signalment, temperament, physical status, and disease localization (Table 5.2). In patients with partial airway obstruction and those in whom preoxygenation is included before induction of anesthesia, sedative drugs that decrease anxiety may be desired.

The potential for severe pain exists in patients undergoing mandibulectomy and maxillectomy, as bone and nerves will be resected during these procedures. Therefore, these cases warrant a multimodal analgesic approach [10]. Full mu‐opioid agonist drugs should be included in the premedication with the goal of providing preemptive analgesia (Table 5.2).

Vomiting after premedication should be avoided in patients with an inability to open the oral cavity and with partial airway obstruction. Therefore, drugs such as alpha‐2‐adrenergic agonists [11] and full mu‐agonist opioids [12, 13] should be used with caution or via appropriate routes (e.g., IV versus IM). Meperidine and methadone are notable exceptions, as they are less likely to cause vomiting in small‐animal patients [14, 15]. The advantage of using full mu‐opioid agonists (i.e., intense analgesia) must be weighed against several factors, including options of opioids available, procedural pain level, and the risks associated with emesis. Therefore, in patients in whom vomiting is undesirable, but analgesia is required, methadone may be a better premedication drug to administer if the chosen route is IM [14].

Anesthetic Induction

In patients with airway obstruction or an inability to open the oral cavity, anesthetic induction should be rapidly achieved without excitement (Table 5.2). For this reason, IV anesthetic induction techniques with propofol (2.0–8.0 mg kg⁻¹ IV), alfaxalone (0.5–4.0 mg kg⁻¹ IV), or ketamine–diazepam or ketamine–midazolam (5.0 mg kg⁻¹, 0.25 mg kg⁻¹ IV) are overwhelmingly preferred to mask induction with inhalation anesthetics.

Anesthetic Maintenance

Anesthetic maintenance in patients undergoing oral cavity surgery should be achieved using inhalation anesthetics (e.g., isoflurane, sevoflurane, or desflurane) or injectable techniques (total or partial IV anesthesia; TIVA or PIVA). Intraoperative analgesia can be accomplished by combining nitrous oxide with the inhalation anesthetic or by using a multimodal approach. Local or regional blocks can be very effective for providing analgesia for mandibulectomy and maxillectomy. Depending on the resected area, regional blocks of the infraorbital, maxillary, mental, or mandibular nerves can be performed with the administration of 0.5–1.0 ml kg⁻¹ of lidocaine, bupivacaine, or ropivacaine (Table 5.3) [16]. Ultrasound‐guided techniques can be used to allow local anesthetic visualization and distribution in the region of the maxillary nerve [17], and a nerve stimulator can aid in the localization of the proximal mandibular nerve [18]. Specific descriptions of these techniques can be found elsewhere [17, 18]. Intraoperatively, analgesia can also be achieved by a continuous rate infusion (CRI) of an opioid (e.g., fentanyl, morphine, hydromorphone, or methadone) or combined with multimodal CRIs such as IV lidocaine and/or ketamine (Table 5.2). This approach provides the additional benefit of reducing the amount of inhalation anesthetic required, which will improve hemodynamic stability for the patient and may also prevent central sensitization of pain [10, 19].

Anesthetic Recovery

On termination of inhalation anesthetics, gauze sponges should be removed from the caudal pharynx and the oral cavity [2]. Fluid evacuation with a suction tube before termination of anesthesia may also prevent fluid aspiration. Postoperative swelling of the oral mucous membranes may cause airway obstruction. This can be minimized by the administration of corticosteroids (e.g., dexamethasone 0.1–0.2 mg kg⁻¹ IV).

Tracheal extubation should be delayed until a well‐developed swallowing reflex is present. When the patient is ready for extubation, the ET tube should be removed with the cuff at least partially inflated to help ensure that any blood clots and fluid left in the pharynx are expelled through the mouth rather than being aspirated or swallowed [36]. During recovery, these patients should be monitored for signs of airway obstruction or pain.

Analgesics such as opioids (e.g., fentanyl, hydromorphone, methadone) and nonsteroidal anti‐inflammatory drugs (NSAIDs) (e.g., carprofen [2.0–4.0 mg kg⁻¹ in dogs; 2.0–4.0 mg kg⁻¹ single dose in cats], meloxicam [0.2 mg kg⁻¹ in dogs; 0.1–0.2 mg kg⁻¹ single dose in cats], robenacoxib [2.0 mg kg⁻¹ for up to 3 days in dogs and cats]) should be provided as needed. However, if corticosteroids have been previously administered to the patient, caution should be taken when deciding if NSAIDs are an appropriate analgesic to administer.

Perianesthetic Considerations

The purpose of preoperative fasting is to withhold liquids and solids for a specified period before surgery to reduce gastric volume and acidity. These goals are particularly important for patients with gastroesophageal diseases that are prone to regurgitation. When considering preoperative fasting, one needs to balance the risk of dehydration, administration of preoperative oral medications, and hypoglycemia versus the risk of GER, regurgitation, esophageal injury, and aspiration pneumonia. Traditionally, in small‐animal medicine, preoperative fasting times have been 8–12 h for scheduled procedures with the rationale that the stomach would be empty prior to anesthetic induction, since complete gastric emptying in dogs and cats ranges from 7–15 h [76]. More recently, the American Society of Anesthesiologists (ASA) has modified their guidelines for preoperative fasting in healthy humans undergoing elective procedures; these guidelines may additionally need to be adjusted for people with coexisting diseases or conditions that can affect gastric emptying or fluid volume. With that in mind, the ASA Practice Guidelines for Preoperative Fasting recommend: (i) clear liquids may be ingested for up to 2 h before anesthetic procedures; (ii) a light meal may be ingested for up to 6 h before elective procedures; and (iii) fasting time of 8 h or more may be needed in cases of patient intake of fatty foods and/or meat. These conclusions were based on randomized clinical trials that showed no differences in findings regarding gastric volume and pH when people could have a light meal 4–6 h before a procedure compared to overnight fasting [77].

In veterinary medicine, the incidence of GER in healthy dogs after shorter fasting time has been reported (Box 5.1). When fed a light meal (one‐half of their daily ration in canned food) approximately 3 h before elective procedures, 3/60 dogs experienced GER during anesthesia compared to 12/60 dogs that were fed the same amount and type of food but 10 h prior to anesthetic induction [56]. These results were confirmed as dogs did not have significantly increased gastric volume when fed canned food at one‐half of their daily ration 3 h before anesthesia; in addition, the gastric acidity was reduced [54]. However, in a more recent report, dogs that were fed a light meal 3 h prior to anesthesia were associated with significantly greater odds of both GER and regurgitation compared to dogs fasted for 18 h [49]. The differences can be due to diet composition which may have different gastric emptying time or the preanesthetic administration of opioids (or other drugs) that could have also influenced gastric motor function and affected residual gastric volume content. While shorter preoperative fasting may be desirable in veterinary medicine and has been the new standard of practice in people, there have been no randomized controlled trials assessing the impact of preoperative fasting for dogs and cats with GI disorders, and veterinarians have followed recommendations based on small clinical studies and experience. Based on that, the American Animal Hospital Association (AAHA) has recommended shorter preoperative fasting times for water and food for healthy dogs and cats [78]. For patients with a history of, or at risk for, regurgitation, fasting times for food and water should be kept between 6 and 12 h. In addition, the clinician should consider feeding 10–25% of the normal amount of food in the form of a wet ration 4–6 h prior to induction. Regardless of fasting duration, the anesthetist should recognize that esophageal diseases can increase the likelihood of GER, regurgitation, and related morbidities, and that additional strategies may be appropriate, such as rapid induction and ET intubation, as well as readiness to suction the pharynx and airway. GI foreign bodies are considered medical emergencies, and fasting is usually not possible. If patients are not fasted, preoperative and intraoperative regurgitation may occur with potential of aspiration.

Preoperative fasting for young animals (i.e., 8–16 weeks of age) undergoing surgery, such as ligation and transection of the ligamentum arteriosum in puppies and kittens with persistent right aortic arch (Figure 5.5), should be kept to a minimum and limited to no greater than 4–6 h. In cases of malnourished animals, preoperative fasting should be decreased to 1–2 h [2, 39]. In these young animals, other anesthetic considerations regarding size and nourishment must also be considered. For example, blood glucose should be monitored during the procedure, maintenance fluids may have to be supplemented with dextrose, and hypothermia should be aggressively prevented [2, 42].

GER may still happen during anesthesia regardless the duration of food withholding. The incidence at which patients with esophageal diseases have GER during anesthesia is unknown, but one can assume that this occurs at higher rate than in patients without GI disease, especially in patients with clinical signs of esophagitis. In dogs and cats without history of esophageal or GI disorders, GER during anesthesia occurs frequently with an incidence rate varying from 12% to 67% [15, 48, 49, 52–56, 58–60, 66, 67, 69, 72, 79–81]. However, clinically, GER is usually not diagnosed during anesthesia because it requires the use of an esophageal pH meter or a pH impedance catheter to be detected. Only a minority of patients that experience GER regurgitate (0–15%) during anesthesia [15, 48, 50, 53,58–60, 66, 67, 69, 80, 81], making it an insensitive clinical sign for recognizing GER. For this reason, medications to prevent GER and reflux events, to increase the pH of gastric secretions, or to improve GI motility preoperatively have been advocated by some clinicians. It should be noted that the efficacy of these drugs, when administered preoperatively, in reducing the frequency of reflux events in healthy dogs and cats that experienced GER during anesthesia is equivocal. In part, the lack of drug efficacy in reducing reflux may be due to differences in methodology of measuring pH. In some cases, administration of these drugs preoperatively, particularly proton‐pump inhibitors (PPIs), may still be beneficial in reducing gastric acidity, helping to reduce esophageal mucosal and lung damage after aspiration if reflux or regurgitation does occur [2, 81, 82]. In humans, the ASA guideline for preoperative fasting and the use of pharmacologic agents found inconclusive evidence to routinely recommend administration of metoclopramide, histamine‐2 antagonists (H2As), PPIs, or antiemetics prior to anesthesia to reduce the incidence of GER, emesis, or pulmonary aspiration. As such, these drugs are only recommended in people at increased risk of aspiration pneumonia [77]. Similarly, the AAHA Anesthesia and Monitoring Guidelines for Dogs and Cats recommend antiemetic, antiacid, and promotility drugs prior to anesthesia for patients with history of, or at risk for, regurgitation [78]. Clinically, H2As and PPIs are commonly administered prior to anesthesia with the goal to decrease gastric acidity. Ranitidine (2 mg kg⁻¹ IV) administered 6 h prior to anesthesia did not prevent or decrease the frequency of GER in dogs [83]. Although famotidine administered every 12 h is superior to famotidine every 24 h or ranitidine to decrease gastric acid secretion and increase gastric pH, H2As are overall less efficacious than PPIs in dogs and cats to treat gastroduodenal ulceration or reflux esophagitis [84]. In addition, H2A tachyphylaxis may occur within 3–13 days [85], which could give a false sense of security if patients with esophageal diseases receiving this therapeutic regimen for that period would reflux or regurgitate during anesthesia. Currently, there are no studies in dogs and cats that determine if the preanesthetic administration of H2As and, potentially, a short period of increasing esophageal pH present any benefit in the anesthetized patient [86]. Therefore, additional studies of H2As prior to anesthesia to reduce esophageal pH are warranted before they are routinely recommended. In clinically healthy dogs and cats, PPIs such as omeprazole and pantoprazole are the most effective in increasing gastric pH compared to H2As [86]. In addition, omeprazole administered orally to dogs as a single dose 4 h before anesthesia and to cats twice, 18–24 h and 4 h before anesthesia, decreased the incidence of GER with pH <4. However, reflux event frequency was not decreased in both dogs and cats that had GER [81, 82]. In addition, esomeprazole administration IV 12–18 h and 1.0–1.5 h before anesthetic induction in dogs undergoing orthopedic surgery significantly increased the gastric and esophageal pH. Like the studies with omeprazole, the preanesthetic administration of esomeprazole did not decrease reflux event frequency. By contrast, esomeprazole administration IV combined with cisapride, a prokinetic drug, before anesthesia significantly decreased GER events [87], likely due to increase in LES pressure [88]. Therapy with PPIs (omeprazole 0.5–1.0 mg kg⁻¹ PO or pantoprazole 1.0 mg kg⁻¹ IV) should be started at least 2 days before anesthesia in order to be most effective [82, 84]. Recently the American College of Veterinary Internal Medicine published a Consensus Statement on the administration of gastroprotectants to dogs and cats and has made several recommendations to the practice of acid suppressants for the management of GI diseases, including the prevention of reflux esophagitis. The consensus opinions on the use of acid suppressants for management and prevention of reflux esophagitis, which pertains to the scope of this chapter, including: (i) there is lack of benefit for administration of H2As on a once‐daily basis in dogs and cats to treat reflux esophagitis; (ii) monotherapy with an H2A given twice daily is inferior to PPIs given twice daily to dogs and cats; (iii) acid‐suppressing agents may be beneficial for prevention of esophagitis secondary to GER, particularly in animals when these are associated with an anesthetic procedure; and (iv) administration of PPIs does not decrease GER, but may prevent injury by increasing the pH of the refluxate [86].

Metoclopramide is a dopaminergic receptor antagonist with antiemetic properties. In addition, it is thought to increase the LES resting tone and promote gastroduodenal motility. However, metoclopramide at 0.5 mg kg⁻¹ PO did not increase LES pressure measured by high‐resolution manometry in unpremedicated awake healthy Beagles [88]. Clinically, metoclopramide is recommended as a prokinetic in the treatment of esophagitis because esophageal inflammation may cause esophageal hypomotility. Also, it is routinely administered to patients who are at risk of GER and regurgitation during anesthesia; however, clinical studies are conflicting in this regard and there is no consensus if metoclopramide administration perioperatively decreases the risk of GER, esophagitis, and aspiration pneumonia. For example, high‐dosage IV administration (1.0 mg kg⁻¹) followed by a high‐dosage CRI (1.0 mg kg⁻¹ h⁻¹) in healthy dogs undergoing orthopedic surgery decreased the risk of GER by 54% [79]. By contrast, the same high‐dosage CRI (1.0 mg kg⁻¹ h⁻¹) did not influence the incidence of GER in dogs undergoing ovariohysterectomy, which could have been a result of a small number of dogs experiencing GER in the studied groups [83]. Lower dosages similar to that required to attain an antiemetic and gastric prokinetic effect (0.4 mg kg⁻¹ IV followed by a 0.3 mg kg⁻¹ h⁻¹ CRI) also did not decrease the incidence of GER compared to a negative control group [79]. Interestingly, preoperative metoclopramide at a lower dosage CRI (2.0 mg kg⁻¹ d⁻¹) combined with maropitant (1 mg kg⁻¹ IV) did not decrease the incidence of postoperative clinical GER, defined in the study as passive gastric reflux observed in the oral cavity (i.e., regurgitation) combined with signs of nausea. The authors speculated that metoclopramide could also have increased gastro‐oral reflux due to anxiety from metoclopramide‐induced extrapyramidal effects [89]. When metoclopramide (0.5 mg kg⁻¹) was administrated in combination with famotidine or omeprazole in brachycephalic dogs undergoing airway surgery, there was a lower incidence of postoperative regurgitation, and preoperative administration of these drugs is recommended for brachycephalic breeds. However, brachycephalic dogs with history of regurgitation are still more likely to regurgitate intraoperatively regardless of treatment with GI‐protectant drugs [71]. Therefore, it can be concluded that the risk of GER and regurgitation during anesthesia still exists when metoclopramide is administered regardless of the CRI dose in healthy patients, and observation of clinical signs for esophagitis is warranted postoperatively. Care must be taken when extrapolating these data from healthy patients to patients with esophageal disorders who already have a predisposition to GER and regurgitation (e.g., brachycephalic breeds), and, therefore, these patients, when receiving metoclopramide perioperatively, should be closely monitored for signs of regurgitation and aspiration pneumonia.

Maropitant (1.0 mg kg⁻¹), an antiemetic drug that acts as a neurokinin‐1 receptor (NK1) antagonist, administered IV 45–60 min prior to anesthetic premedication with acepromazine and hydromorphone did not significantly decrease the incidence of GER in dogs undergoing elective surgical procedures when compared to dogs treated with saline [90]. Similarly, preoperative administration of maropitant (1.0 mg kg⁻¹ IV) did not prevent postoperative GER/regurgitation in dogs [89]. Currently, there is not sufficient evidence that maropitant decreases the incidence of GER or regurgitation, and therefore maropitant should not be recommended with the intent to prevent anesthesia‐induced GER.

However, both metoclopramide (0.2 mg kg⁻¹ IM) [91, 92] and maropitant (1.0 mg kg⁻¹ SC or IV) [90, 91,93–99] are effective antiemetics administered prior to opioids combined with dexmedetomidine or acepromazine in healthy dogs and cats. Their administration in dogs and cats with esophageal disease preoperatively may be desirable with the goal of preventing vomiting, thus decreasing further damage to the esophagus and the risk of aspiration pneumonia. Metoclopramide should be administered 30–45 min in dogs and maropitant 30–60 min in dogs and 20 h in cats before the administration of an opioid or an alpha‐2‐adrenergic agonist premedication to be most effective.

In cases where the patient is emaciated, anesthesia should be postponed until the animal has an improvement in body condition score. However, in certain situations, treatment for nutritional debilitation requires the placement of a gastrostomy or PEG tube, both of which require general anesthesia. In these situations, the anesthesia period should be kept as short as possible with perianesthetic considerations aimed at the individual patients’ physical status and comorbidities.

Since regurgitation is commonly associated with esophageal disease, thoracic radiographs should be taken preoperatively to evaluate for the presence of aspiration pneumonia. Patients with aspiration pneumonia should be treated aggressively before being anesthetized for esophagoscopy, surgery, or feeding tube placement. If anesthesia cannot be delayed until aspiration pneumonia resolves, preoxygenation with 100% oxygen before anesthetic induction is recommended [2]. In addition, the trachea should be immediately intubated after induction, and positive pressure ventilation should be instituted to maintain oxygen saturation above 90%.

Dogs presenting with esophageal diseases that appear to be dehydrated or hypovolemic based on physical examination, CBC, and serum biochemical variables should receive fluid resuscitation before anesthesia. Tachycardia, pale mucous membranes, prolonged capillary refill time, and cold extremities are signs of hypovolemic shock and should be treated with isotonic crystalloids (60–90 ml kg⁻¹ in one‐quarter dose increments) and/or HES solutions (5–20 ml kg⁻¹, when indicated) and reassessed before anesthetic induction (Table 5.4). It should be noted that cats in shock might present with bradycardia and hypothermia [100]. Treatment of acid–base and electrolyte disturbances should be initiated before anesthesia if possible (see the subsection titled “Laboratory Data ” under the section titled “Gastric and Small Intestinal Diseases ”).

In general, the anesthetic protocol is not aimed at avoiding drugs that decrease LES pressure because many of the drugs administered with the purpose of producing anesthesia or analgesia will interfere with protective mechanisms against GER (Box 5.1). Most drug selection is based on the physical status of the patient, mentation, pain level, and surgical procedure (Table 5.2). Whenever possible, drugs that produce emesis should be avoided to prevent further damage to esophagus and aspiration pneumonia.

Preanesthetic Medication

The GI effects of anticholinergic drugs have been well described. Atropine and glycopyrrolate decrease LES tone at doses commonly used for prevention or treatment of bradycardia [5361–63]. At high doses, they inhibit hydrogen‐ion secretion by gastric parietal cells, and at the standard recommended doses for preanesthetic medication, they have minimal effect on the pH of gastric secretions in dogs [101]. Therefore, the argument for including anticholinergic drugs in the preanesthetic medication is not justifiable due to their effect on the LES, likelihood of GER, and lack of effect to reduce gastric acidity at clinical doses.

The administration of drugs that may induce vomiting or GER (e.g., alpha‐2‐adrenergic agonists and full mu‐opioids) in the preanesthetic period of patients with an esophageal foreign body is questionable. Sharp foreign body objects may abrade or lacerate the esophageal mucosa, causing esophagitis. Sharp objects may also perforate the esophagus and, occasionally, the great vessels. If emesis is induced by preanesthetic drugs in these patients, further damage may occur [2, 102]. Acidic gastric‐fluid reflux can induce further damage to the esophagus. Even so, opioids are usually included in the preanesthetic medication of these patients to provide analgesia even if foreign body removal is performed with noninvasive techniques (e.g., esophagoscopy) wherein pain is less intense. However, opioids that do not cause vomiting such as butorphanol (0.1–0.4 mg kg⁻¹), buprenorphine (0.01–0.03 mg kg⁻¹), methadone (0.1–1.0 mg kg⁻¹), and fentanyl (0.002–0.005 mg kg⁻¹) have been recommended for this type of procedure [42]. The administration of morphine, hydromorphone, and oxymorphone has been shown to increase the risk of regurgitation, GER, or vomiting significantly [13–15, 50, 60]. Therefore, methadone or fentanyl is preferred in the case of esophageal surgery where pain is more likely to be more intense. In addition, IV administration may result in less vomiting altogether, as drugs may bypass the chemoreceptor trigger zone (which induces vomiting) and act to suppress vomiting at the vomiting center [103]. Maropitant may be administered 30–60 min prior to opioid administration to prevent emesis [90, 91,93–99].

Acepromazine (0.02–0.05 mg kg⁻¹) can be administered to provide sedation in patients with esophageal foreign bodies that are hemodynamically stable. When administered before opioids, acepromazine decreases the incidence of vomiting [13], although it increases the chances of GER [53]. Diazepam or midazolam (0.2 mg kg⁻¹) may be preferred in depressed and hypovolemic patients. Furthermore, diazepam has been associated with a significant reduction in GER episodes [53].

Intense sedation or muscle relaxation should be avoided in patients with megaesophagus. Muscle relaxation of the striated muscle layer of the esophagus can increase the likelihood of regurgitation before anesthetic induction. Intense sedation may impair the patients’ ability to protect their airway, increasing the chances of aspiration pneumonia. Acepromazine, alpha‐2‐adrenergic agonists, and benzodiazepines should be used with caution in these patients.

Anesthetic Induction

Dogs and cats with esophageal disease are at high risk of regurgitation and aspiration pneumonia. For this reason, it is advised to have a suctioning system set up during the induction period. Anesthetic induction should be rapid with immediate ET intubation. Therefore, mask induction with inhalation anesthetic should be avoided, and IV anesthetic techniques are preferred. Propofol (2.0–8.0 mg kg⁻¹ IV) or alfaxalone (0.5–4.0 mg kg⁻¹ IV) are rapid‐acting, and the airway can be immediately secured, particularly in patients with megaesophagus. Alternatively, other drugs with fast onset, such as ketamine–diazepam/midazolam, can be used in patients with esophageal disorders [42].

Endotracheal intubation should be performed with the patient in sternal recumbency with the head elevated. Application of pressure to the cricoid region has been documented in humans to reduce regurgitation [104]; however, this has not been advocated for dogs and cats. If regurgitation is observed during induction, the airway of the patient should be secured with a properly sized and lubricated cuffed ET tube and its cuff immediately inflated. Afterward, the patient’s head should be dropped below the thoracic inlet, and the airway and oropharynx should be suctioned. The ET tube should be connected to a breathing system and 100% oxygen supplemented. Oxygenation and ventilation with the aid of a pulse oximeter and capnograph should be monitored at this time. If the procedure can be delayed, it is advised to allow the patient to recover from anesthesia and be observed for signs of aspiration pneumonia.

Anesthetic Maintenance

Anesthetic maintenance in patients with esophageal obstruction or megaesophagus is achieved with inhalation anesthetics (e.g., isoflurane, sevoflurane, or desflurane) diluted in 100% oxygen. Nitrous oxide should be avoided in patients with esophageal obstruction, as it can accumulate in viscous organs and dilate the esophageal portion distal to the obstruction [2].

Owing to the striated muscle layer in the esophagus, especially in dogs, central muscle relaxant administration (e.g., diazepam and midazolam) or short‐acting neuromuscular blocking agents (e.g., atracurium 0.1–0.4 mg kg⁻¹ IV) may be indicated for patients with esophageal foreign bodies. These drugs may help to relax the striated muscular layer of the esophagus, reduce esophageal tone, and facilitate endoscopic manipulations and foreign body removal [2, 42]. Positive pressure ventilation is required in patients that receive neuromuscular blocking agents, and, ideally, neuromuscular function should be monitored by accelerometry. Deep anesthetic planes required with inhalation anesthetics to achieve muscle relaxation are not recommended in patients who are hemodynamically unstable.

Intraoperatively, a balanced isotonic crystalloid solution (e.g., LRS) should be administered up to a rate of 5–10 ml kg⁻¹ h⁻¹. Supplemental fluid boluses of isotonic crystalloids may be indicated depending on the patient’s hydration and hemodynamic status. In most cases, blood pressure can be monitored noninvasively, and pulse oximetry and capnography can provide valuable information in the event of esophageal perforation or aspiration [42].

Most general anesthetics can increase the chances of GER or regurgitation because of their effects on the LES tone in dogs and cats [65–69]. Although the incidence of regurgitation is low in healthy anesthetized patients [15, 48, 50, 53,58–60, 66, 67, 69, 80, 81], it is more likely to occur in patients with esophageal disorders. Continuous monitoring for regurgitation during anesthetic maintenance, and immediate treatment if regurgitation occurs, can help prevent complications from occurring.

If regurgitation is observed during anesthesia, suctioning the refluxate from the oropharyngeal cavity and esophagus can minimize the chances of the patient aspirating and avoids damage to the esophageal mucosa, respectively. However, only suctioning of the refluxate from the esophagus has minimal impact in increasing the pH within the lumen of the esophagus. If regurgitation is observed, suctioning should be followed by esophageal lavage and sodium bicarbonate instillation [105]. Esophageal lavage with tap water can increase esophageal pH above 4.0, and infusion of a small volume of sodium bicarbonate can increase the pH above 6.0–7.0 for up to 3 h [105, 106]. However, esophageal lavage with tap water after esophageal suctioning may not be necessary if sodium bicarbonate is instilled, especially when all or most of the regurgitant volume is suctioned from the esophagus. Although esophageal lavage with tap water increases the esophageal pH, instillation of sodium bicarbonate after suctioning the esophagus readily buffers the pH of the esophageal lining, rendering any dilutional effect of the lavage inconsequential. The esophagus can be suctioned using a smooth tipped 8–12 Fr catheter. To ensure correct placement of the catheter and avoid suctioning from the stomach, the distance from the mandibular incisors to the 10th rib should be measured. Caution should be taken when inserting the catheter into the oropharynx to not enter the trachea. Therefore, it is critical that the airway is protected, and the ET tube cuff appropriately inflated. Esophageal suctioning should only be applied, as the catheter is withdrawn to avoid trauma to the esophagus. If the esophagus is lavaged with tap water, the lavage should be continued until clear fluid is retrieved. Sodium bicarbonate solution can be prepared using a 1:1 ratio of 8.4% sodium bicarbonate and water for injection; a volume of 0.6 ml kg⁻¹ of this solution has been recommended for instillation into the esophagus [105, 106]. Pantoprazole (1.0 mg kg⁻¹ IV) may be administered to dogs and cats following regurgitation.

If reflux aspiration is observed while the animal is anesthetized, the airway should be suctioned to remove irritants, and oxygen saturation and breathing patterns should be monitored. If oxygen saturation starts to drop, positive pressure ventilation should be instituted. Peak inspiratory pressure generated by a normal tidal volume (10–20 ml kg⁻¹) should be observed. Generation of high peak inspiratory pressure (>20 cm H₂O) with a normal tidal volume can indicate bronchospasm. Blood gas analysis can help assess the degree of oxygen exchange impairment.

Esophageal perforation with life‐threatening tension pneumothorax should be suspected with sudden changes in breathing patterns, subsequent drops in oxygen saturation, decreased breath sounds on thoracic auscultation, and increased resistance to manual ventilation [42]. If these occur, immediate thoracocentesis should be performed and positive pressure ventilation instituted. A decision of exploratory thoracotomy should then be made to remove the esophageal foreign body and repair the esophagus [2, 40]. In cases of thoracotomy for esophageal surgery, positive pressure ventilation must be continued, and appropriate pain management should be administered intraoperatively and postoperatively. Air and fluid must be evacuated via a thoracostomy tube or thoracocentesis after the procedure.

Anesthetic Recovery

Observing for regurgitation and airway protection during recovery is still crucial. Premature extubation can lead to aspiration if regurgitation occurs. Patients should be maintained in sternal recumbency, with the head elevated until the animal regains full consciousness and laryngeal reflexes. The ET tube cuff should be kept inflated (at least in part) during extubation to remove any fluid or content proximal to the ET tube cuff, which will minimize the chances of aspiration [36]. If the patient regurgitates on extubation, its head should be dropped below the thoracic inlet, and suctioning of the airway should be provided. Oxygenation with the aid of a pulse oximeter must be monitored during recovery from anesthesia.

Postoperative analgesia should be provided accordingly to the intensity of pain produced by the procedure and damage to the esophageal mucosa in the case of foreign bodies. NSAIDs have been identified as a factor associated with intraoperative regurgitation [50]; therefore, these drugs should be avoided in patients with esophageal disorders.

Perianesthetic Considerations

The degree of dehydration should be evaluated based on physical examination and laboratory findings. Ideally, fluid deficit and electrolyte and acid–base abnormality correction should take place before anesthesia to improve the patient’s hemodynamic stability throughout the anesthetic procedure [42]. Isotonic (0.9%) saline is the fluid of choice for metabolic alkalosis. Since hypokalemia can lead to skeletal muscle weakness, decreased GI motility/ileus, and cardiac arrhythmias, it should be corrected with potassium chloride supplementation before anesthesia. When administered IV, potassium chloride generally should not be infused at rates >0.5 mEq kg⁻¹ h⁻¹ to avoid potential adverse cardiac effects [112]. Hypokalemia can also predispose the myocardium to the refractory effects of class I antiarrhythmic drugs such as lidocaine and procainamide. These drugs are commonly used during anesthesia in patients with diseases that can cause ventricular arrhythmias such as GDV. Hypokalemic patients in shock should be resuscitated with an isotonic crystalloid solution (e.g., 0.9% saline or LRS) before adding potassium chloride to the fluids. If a patient is severely hypokalemic (serum potassium <2 mEq), it is prudent to start potassium as a separate infusion and not with the resuscitation fluid bag to not exceed the recommended potassium infusion rate (0.5 mEq kg⁻¹ h⁻¹) [73]. Dehydration from vomiting in patients with metabolic acidosis should be corrected with a balanced isotonic crystalloid solution containing lactate or acetate as a buffer. The administration of sodium bicarbonate is necessary only in the treatment of patients with severe metabolic acidosis who have not responded to fluid therapy (pH <7.15) [113]. In these patients, bicarbonate deficit should be corrected with sodium bicarbonate according to the following equation:

(5.1)

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