Conditions associated with the oral cavity and pharynx

Patients with trauma, swelling and/or edema, or space-occupying masses of the pharynx, oral cavity, and sometimes the head and neck frequently require general anesthesia. These patients are often difficult to intubate to secure and maintain a patent airway. (Figure 16.1) Fractures of the mandible, maxilla, or temporomandibular joint may not permit examination to determine the range motion of the jaw. Masticatory myositis may prevent opening of the mouth even when the patient is anesthetized. Intubation via a tracheostomy may be necessary in these patients. Although lesions of the tongue are usually readily seen, problems at the base of the tongue or in the back of the pharynx may not be realized until the patient is sedated or ready to be intubated. Some patients may not allow any examination of the oral cavity without sedation, so the nature of the problem may be completely unknown. General anesthesia without a secure airway may result in aspiration, which can be fatal. Evaluation of available diagnostic imaging prior to the procedure is valuable to determine the type and severity of the lesions present. These can help determine the extent of difficulty that may be expected with intubation and the potential for airway compromise following extubation. Preparation for placement of a tracheostomy tube should be performed prior to induction in cases where there is high suspicion that oral intubation may not be possible. Preoxygenation using a face mask with the flow meter set at 5 L/min will prolong the time until hypoxemia develops, and should be utilized in all patients in this category.

Intramuscular (IM) or subcutaneous (SC) administration of alpha₂ agonists or μ agonist opioids in dogs or cats may cause emesis. Dogs and cats with oral or pharyngeal masses are at high risk for aspiration pneumonia, so the use of medetomidine, dexmedetomidine, morphine, and other drugs that commonly induce emesis should be avoided. In dogs, a combination of IM acepromazine (0.02–0.05mg/kg) and IM or intravenous (IV) hydromorphone (0.05–0.2 mg/kg) will induce a neuroleptanalgesic state with sufficient muscle relaxation to enable examination of the mouth and jaw. If the animal’s laryngeal function or its ability to open its mouth is questionable, IV fentanyl (0.005 mg/kg) should be substituted for hydromorphone because there is a risk of vomiting associated with hydromorphone administration. Older or debilitated patients may respond well to midazolam (0.2 mg/kg) in combination with butorphanol (0.2–0.3 mg/kg), both given IM or IV. If the patient is amenable, placement of an IV catheter before premedication is useful to allow titration of drugs IV. Premedication with an anticholinergic agent is recommended to avoid opioid-induced bradycardia. An anticholinergic will also decrease salivation, which may be helpful in patients with oral or pharyngeal lesions.

Figure 16.1. Example of a difficult intubation from a large mass that completely obstructed the nasal passages and partially obstructed the pharynx in a cat. Reprinted from Byron J.K., Shadwick S.R., Bennett A.R. 2010. Megaesophagus in a 6-month-old cat secondary to a nasopharyngeal polyp. J Feline Med Surg, 12: 322–324, with permission from Elsevier and Dr. Julie Byron.

In cats, a low dose of ketamine (4 mg/kg IM) combined with acepromazine (0.05 mg/kg IM) usually enables examination of the oral cavity. An anticholinergic should also be given when ketamine is used to prevent salivation. If an IV catheter is in place, very low doses of propofol (1 mg/kg) or alfaxalone (0.5–1 mg/kg in dogs; 1–2 mg/kg in cats) can be used following premedication in dogs and cats to deepen sedation if needed. When induction agents are used even at low doses, attention to anesthetic depth is paramount, as intubation may be needed in some patients.

Patients with gastrointestinal (GI) disease are at risk for airway compromise (see Table 16.1) and should be observed at all times following premedication and in recovery. Supplies for reintubation should be available in case airway obstruction occurs following extubation. Sedation alone may result in airway compromise, especially when lesions extend into the nasal passage(s). Swelling or edema associated with surgical manipulation or positioning may result in airway compromise that is not apparent until the patient is extubated. Keeping the patient in sternal recumbency and the mouth open in such patients will usually improve the volume of air flow. These patients should remain intubated until conscious, swallowing well, and able to remain in sternal recumbency without assistance. Pulse oximetry is very helpful to evaluate oxygenation in recovery. For comparison, oxygen saturation should be measured in recovery after anesthesia has been discontinued but while the patient is still on oxygen, after the patient has been breathing room air for 5 minutes, and after extubation. Extubation should not be performed unless an SpO₂ ≥95% (or a PaO₂ > 80mmHg) can be maintained. If the patient is breathing well, but remains hypoxemic, oxygen supplementation is necessary. If needed, opioids and benzodiazepenes can be reversed with naloxone or flumazenil, respectively.

Table 16.1. Anesthetic concerns/complications during GI procedures

Conditions associated with the gastrointestinal tract

Many procedures for diagnosis and treatment of GI disease are now often performed using flexible or rigid endoscopy. Endoscopy is performed under general anesthesia for a variety of procedures, including examination and/or biopsy of the intestinal lining, biopsy of masses, removal of foreign bodies or polyps, treatment of esophageal strictures with bougienage or balloon dilation, and placement of feeding tubes. Patients are usually in lateral or sternal recumbency so ventilation and access to the patient is better than when in dorsal recumbency (Figures 16.2 and 16.3). Often, the procedure is performed in partial darkness; some sort of light source should be available for the anesthetist. Endoscopic examination involves distention of the gastrointestinal tract (GIT) via the instillation of air so that lesions of the mucosal layer are easily viewed. Distension of the stomach occurs, sometimes significant enough to affect ventilation and venous return to the heart. The distention can also be painful, especially when esophageal strictures are present. Patients anesthetized for bougienage or balloon dilation of the esophagus may require additional analgesic therapy during this part of the procedure.

Figure 16.2. Adult dog anesthetized for upper GI endoscopy. Courtesy of Dr. Jeannette Cremer.

Figure 16.3. Cat anesthetized for endoscopic placement of a feeding tube. Courtesy of Dr. Christina Braun.

Gastric distention must be monitored closely; excessive air should be removed as needed. The stomach should be emptied of air at the end of the procedure. Fluid and gastric contents often leak into the oral cavity during endoscopy; the pharynx should be thoroughly examined and cleaned. The security of the endotracheal (ET) tube should be checked regularly, as dislodgement can occur during passage of the endoscope or feeding tube. This can be avoided by marking the tube in some fashion when it is properly positioned so that any slippage can be quickly identified. Using pieces of plastic IV fluid tubing to secure the ET tube during endoscopy may also help avoid dislodgement, since ET tubes slip easily when gauze strips get wet. Supplies for reintubation should be readily available, including extra IV anesthetic, gauze sponges, and long forceps to clean the pharynx. Pneumothorax and/or pneumoperitoneum can result when the esophagus or stomach is distended from tearing or rupture of preexisting pathology or during removal of foreign bodies, and may not be realized until difficulty with ventilation or a fall in cardiac output develops. Pulse oximetry is useful to identify decreasing oxygen saturation that is seen with this complication. The swallowing reflex is difficult to abolish under general anesthesia and can affect retroflex endoscopic examination of lesions in the caudal choanal area of the nasopharynx. Bilateral maxillary nerve block with a short acting local anesthetic such as lidocaine may facilitate retroflex examination with the endoscope.³

Patients in good condition can be anesthetized for endoscopy as for most elective procedures. If vomiting is a significant part of the primary disease, drugs that cause emesis such as alpha₂ agonists (medetomidine and dexmedetomidine) and the μ agonist opioids hydromorphone or morphine can be avoided in favor of butorphanol. IM administration of acepromazine 15 minutes prior to administration of a μ agonist opioid decreases the incidence of vomiting; however, this antiemetic effect is not 100% reliable.⁴ Although most of the commonly used opioids have been used for endoscopy in small animals, a side effect of opioids is increased tension of sphincters in the GIT. Passage of the endoscope through the pyloric sphincter could therefore be affected by the use of opioids. Two studies have looked at this effect of opioids in small animals. Morphine in combination with atropine was associated with some difficulty passing the endoscope into the duodenum in dogs. Glycopyrrolate combined with morphine and either atropine or glycopyrrolate combined with meperidine were not different from saline.⁵ Premedication of cats using hydromorphone, hydromorphone and glycopyrrolate, medetomidine, or butorphanol administered IM did not interfere with passage of the endoscope.⁶ It is not known if hydromorphone in dogs could interfere with endoscopy; however, reversal can be accomplished with butorphanol or naloxone if needed. Buprenorphine has a long duration and is difficult to reverse and has not been studied; its effects on endoscopy are unknown. Induction drugs should be chosen based on patient status; halothane, isoflurane, or sevoflurane are suitable for endoscopy. Nitrous oxide should not be used for endoscopy since it diffuses readily into the intestinal lumen; it would cause further GI distention and pollution of the environment.

Patients that require endoscopy for intestinal biopsy, esophageal stricture, or feeding tube placement may be quite debilitated and/or hypoproteinemic (Figure 16.4). They are often more sensitive to effects of anesthetic drugs because of decreased protein binding and lack of body mass for redistribution of drugs. Combinations of shorter acting, reversible sedatives such as midazolam and butorphanol are useful for debilitated patients requiring endoscopy. Thiopental should be avoided for induction. Propofol or midazolam/ketamine can be used for induction, at lower doses, to effect, for intubation. Isoflurane or sevoflurane are preferred for maintenance. Low infusion rates of a colloid (1–2 mL/kg/h) with no or a very low (< 5 mL/kg/h) infusion rate of crystalloid solution to replace standard IV infusion therapy can be helpful to avoid hypotension and hemodilution.

Figure 16.4. Debilitated dog with an esophagitis and stricture following a previous exploratory laparotomy to be anesthetized for bougienage of the esophagus. Courtesy of Jennifer Adams, DVM.

During general anesthesia for esophageal foreign bodies (+/− endoscopy), relaxation of the striated muscular layer of the esophagus may aid in removal of the obstruction with less damage to the mucosal layer. Skeletal muscle relaxation is increased at deeper planes of anesthesia and can be further enhanced by administration of a neuromuscular blocker (NMB). In cats, the proximal two-thirds of the esophagus have a striated muscle layer, whereas in dogs, the entire esophagus contains striated muscle.⁷ A depolarizing muscle relaxant such as succinylcholine should not be used since it initially causes contraction of skeletal muscle and results in damage to the esophagus. Administration of a short-acting nondepolarizing NMB agent—for example, atracurium, 0.1–0.2mg/kg IV—may facilitate endoscopic or surgical access to the foreign body. Administration of NMBs must always be accompanied by tracheal intubation and support of ventilation. Although rare, histamine release may occur following administration of atracurium. Bronchoconstriction and vasodilation may follow, causing hypoxemia and hypotension, respectively. Antihistamine and/or bronchodilator therapy may counteract these effects; however, slow IV administration should prevent or reduce histamine release. Atracurium breaks down spontaneously in the plasma and is metabolized by plasma esterases. The duration of neuromuscular block usually lasts 20–35 minutes but can be extended by alkalosis and hypothermia, which slow the elimination of the drug.

Anesthetic considerations for colonoscopy depend primarily on the patient’s condition since interference is not expected with the airway, and sphincters are not present in the colon. GI hemorrhage is often a preexisting problem in these patients and some will be anemic. Distention is not generally as severe as that seen in upper GI endoscopy, but can sometimes cause discomfort. Excessive hemorrhage and colonic perforation have been reported, but are rare complications of colonoscopy.⁸ Some patients will be subjected to both upper and lower GI endoscopy; considerations as described above will apply to these patients as well.

Exploratory laparotomy

Exploratory laparotomy (EL) always requires general anesthesia with variable levels of anesthetic management depending on the specific problem and the patient status. It is both a diagnostic and therapeutic procedure for many diseases of the GIT, and is one of the most challenging types of cases for both surgeon and anesthetist. The preanesthetic status of these patients varies from healthy with no apparent systemic abnormalities to those with life-threatening conditions. Some problems may not be apparent until a diagnosis is made during surgery. A wide spectrum of potential anesthetic and surgical problems should be anticipated and prepared for prior to anesthesia. General considerations for EL will be discussed first followed by concerns specific to some of the more common disease conditions seen.

Many anesthetic considerations are similar because of the positioning, the surgical techniques utilized, and patient status. Dorsal recumbency is required for an EL, and is associated with several potential anesthetic problems or complications. Ventilation is compromised in dorsal recumbency compared to lateral or sternal since the abdominal organs lie against the diaphragm. This can be further exacerbated when abdominal distention is present from excessive abdominal fluid or hemorrhage, distention of the GIT, or large masses. Hypoventilation can result in hypoxemia, hypercarbia, and inadequate anesthetic depth in some patients. Manual or mechanical ventilation is necessary in these patients. Postoperative pulmonary complications are common in patients who have undergone a laparotomy, especially when vomiting, regurgitation, or peritonitis is present. Mortality is greater in patients that develop pulmonary complications.⁹ Hypotension is more common in dorsal recumbency than sternal or lateral, and can be exacerbated by the patient’ s status, positive pressure ventilation, and surgical manipulation of abdominal organs or masses. Rolling into dorsal recumbency can result in a precipitous drop in blood pressure, especially in large dogs and when abdominal distention is present. Clinical signs of anesthetic depth and blood pressure should be checked immediately upon repositioning and again within a few minutes to detect hypotension. Intravascular access and anesthetic monitoring is more difficult once patients are positioned in dorsal recumbency. Placement of an extra peripheral venous catheter or a central venous catheter in some cases prior to or following induction should be considered in those patients where numerous IV therapies may be required. (Figure 16.5) Monitoring equipment should be secured carefully to the patient and all wires, probes, and tubing should be safely positioned before the surgical drapes are applied. Access to the mucous membranes and the eyes for monitoring of anesthetic depth should be ensured. Exposure of abdominal organs, in addition to thermoregulatory dysfunction, is caused by anesthetic drugs. Prolonged recovery, cardiac arrhythmia, and increased incidence of wound infection are associated with significant hypothermia. GER is seen more often in intra-abdominal procedures.¹⁰ Surprisingly, the incidence of GER has not been associated with positioning, in spite of the finding that the lower esophageal pressure was decreased in dorsal recumbency versus other positions. As previously mentioned, GER increases the risk of aspiration pneumonia, postanesthetic esophagitis, and esophageal stricture.^10,11 During anesthesia, the head should be positioned to direct drainage of reflux fluid away from the head while still allowing access to the eyes for monitoring. The pharynx should be examined and cleared of any reflux fluid or debris prior to recovery. Lavage of the distal esophagus with tap water and instillation of dilute bicarbonate following lavage has been suggested for patients with extensive regurgitation; however, this may not always prevent repeat episodes of GER or the development of esophagitis and/or esophageal stricture.¹² Prophylactic therapy with antacids and prokinetic agents is not always successful in prevention of GER and esophagitis. However, a recent study showed that the incidence of GER was decreased in patients given omeprazole 4 hours prior to anesthesia.¹³ In recovery, patients with GER should be positioned in sternal recumbency with the head elevated and supported and should not be extubated until a vigorous swallow reflex is present. The ET tube should be removed with the cuff still partially inflated to help remove any gastric contents that may be present in the proximal trachea. Signs of esophageal dysfunction postoperatively include coughing, salivation, dysphagia, difficulty swallowing, and retching, regurgitation, vomiting, and/or anorexia in spite of continued interest in food, and should be treated aggressively to prevent further damage.

Figure 16.5. Bulldog with abdominal distention caused by a large splenic mass following induction of anesthesia. He is receiving a blood transfusion during preparation for an EL to remove the mass. Courtesy of Jennifer Adams, DVM.

Stay updated, free articles. Join our Telegram channel

Join