History

A 6-month-old 9.5-kg (20.9-lb) sexually intact male Vietnamese potbellied pig was presented for castration. Findings on preoperative physical examination were unremarkable, and food was withheld from the pig for 8 hours in preparation for the procedure scheduled for the following day.

The morning of surgery, the pig received maropitant (0.84 mg/kg [0.38 mg/lb], PO) and gabapentin (10.5 mg/kg [4.7 mg/lb], PO). Approximately 4 hours later, dexmedetomidine (2.6 μg/kg [1.2 μg/lb]), ketamine (2.6 mg/kg), midazolam (0.30 mg/kg [0.14 mg/lb]), and butorphanol (0.30 mg/kg) were administered IM. A marked level of sedation with no resistance to restraint was achieved. An intravenous catheter was aseptically placed in a lateral ear vein, and anesthesia was induced with ketamine (5.3 mg/kg [2.4 mg/lb]) and diazepam (0.26 mg/kg [0.12 mg/lb]) administered IV to effect. The pig was positioned in sternal recumbency, and intubation was performed with use of a laryngoscope for direct visualization and a stylet (constructed of a wire completely within a polypropylene urinary catheter). Initial attempts at intubation with a 5.0-mm-internal-diameter cuffed endotracheal (ET) tube were unsuccessful because of resistance met at the level of the larynx. Successful intubation was then performed with a 4.5-mm-internal-diameter cuffed ET tube, proper placement of which was confirmed with findings of end-tidal CO₂ concentrations and capnography waveforms consistent with tracheal intubation. The ET tube was advanced to a premeasured distance not to extend beyond the thoracic inlet and was secured in place with a gauze tie around the jaw. The ET tube cuff was inflated until bypass leakage was no longer audible when a positive pressure ventilation of 20 cm H₂O was administered.

General anesthesia was maintained with 2% sevoflurane (vaporizer setting) in O₂ (0.5 L/min) delivered through a pediatric, semiclosed rebreathing circuit. The pig was allowed to breathe spontaneously (respiratory rate, approx 25 to 45 breaths/min; reference range, 20 to 40 breaths/min). The pig was instrumented for pulse oximetry, capnography, ECG, oscillometric blood pressure monitoring (cuff placed on a forelimb), and esophageal temperature monitoring with a multiparameter monitor. After the pig was positioned in dorsal recumbency, the surgical site was aseptically prepared, and 2% lidocaine hydrochloride (2.1 mg/kg [0.95 mg/lb], total) was injected into each spermatic cord and testicle bilaterally, dividing the total volume evenly among all 4 locations. In addition, lactated Ringer solution (5.0 mL/kg/h [2.3 mL/lb/h], IV) and ketamine (0.6 mg/kg/h [0.3 mg/lb/h], IV) were initiated, and supplemental heat was provided by a forced-air warming blanket.

Prior to surgery, gentamicin sulfate (5.2 mg/kg [2.4 mg/lb]) and flunixin meglumine (1.1 mg/kg [0.5 mg/lb]) were administered IV. In accordance with federal regulations, extralabel drug use was discussed with the owner, who conveyed that the pig was a pet and agreed that the animal would not enter the human food supply.

During surgery, the pig became mildly hypo-tensive (approx systolic arterial pressure, 80 to 95 mm Hg [reference range, 110 to 160 mm Hg]; mean arterial pressure, 50 to 65 mm Hg [reference range, 80 to 110 mm Hg]; and diastolic arterial pressure, 30 to 45 mm Hg [reference range, 70 to 90 mm Hg]); therefore, the delivered concentration of sevoflurane was decreased. The pig's heart rate and O₂ saturation of hemoglobin (Spo₂) remained stable, ranging from 65 to 75 beats/min (reference range, 50 to 150 beats/min) and 95% to 99% (reference range, 95% to 100%). However, hypothermia (35.8°C [96.4°F]; reference range, 37.8°C to 38.9°C [100°F to 102°F]) was noticed and continued into the postoperative period despite initiation of heat support by the use of a forced-air warming blanket. The surgical procedure took 39 minutes, and the total duration of general anesthesia was 55 minutes.

The pig was positioned in sternal recumbency for recovery, and 22 minutes after the end of anesthesia, atipamezole (0.05 mg/kg [0.02 mg/lb], IM) was administered. Extubation occurred within 10 minutes following the injection. Initially, the pig was eupneic and appeared calm; however, approximately 15 minutes after extubation, the pig showed signs of excitement, including vocalization and increased intentional movement, that progressed to signs of respiratory distress, including tachypnea and intermittent stertor. Therefore, phenylephrine (0.1 mg/kg [0.05 mg/lb]) and lidocaine (1.0 mg/kg [0.45 mg/lb]), diluted with sterile saline (0.9% NaCl) solution to a final volume of 0.4 mL, were administered intranasally for treatment of a potential airway obstruction secondary to nasal congestion; diazepam (0.2 mg/kg [0.09 mg/lb], IV) was administered for sedation; and supplemental O₂ was provided by face mask. Despite these efforts, the pig developed marked dyspnea and cyanosis. Reintubation with use of a laryngoscope and a 4.5-mm-internal-diameter ET tube was unsuccessful because of resistance at the level of the larynx; however, intubation with a 4.0-mm-internal-diameter ET tube was successful. Afterward and because of resistance during intubation, ketamine (1.6 mg/kg [0.7 mg/lb], IV) and diazepam (0.08 mg/kg [0.04 mg/lb], IV) were administered to effect for adequate sedation. Intermittent positive pressure ventilation was initiated; however, the pig remained tachypneic, and its Spo₂ consistently ranged between 80% and 90%.

Question

What were possible causes for respiratory distress in this pig?

Answer

Pigs are obligate nasal-breathing species, and respiratory distress in the pig of the present report could have had various underlying causes, including upper airway obstruction caused by laryngeal edema or spasm (alone or in combination) from intubation trauma,^1,2 obstruction of the nares (eg, nasal congestion) or displacement of the soft palate during intubation, or regurgitation and aspiration³; spontaneous pneumothorax from positive-pressure ventilation⁴; or perforation of the larynx or trachea and secondary pneumothorax or pneumomediastinum,⁵ alone or in combination. Causes of lower airway disease and subsequent respiratory distress could have included pulmonary edema, atelectasis, or exacerbation of chronic lung disease.

Following reintubation of the pig, its lung sounds were considered decreased. Point-of-care ultrasonography was performed, and pneumothorax was identified. Bilateral thoracocentesis was performed and removed approximately 400 mL of air from the left hemithorax. Afterward, the pig's respiratory rate decreased and Spo₂ normalized. General anesthesia was again maintained with 1% to 2.5% sevoflurane (vaporizer setting) in O₂ (0.5 L/min) delivered through a pediatric, semiclosed rebreathing circuit, and the pig was allowed to breathe spontaneously. Thoracic radiography was performed and revealed a large volume of emphysema ventrally around the cervical portion of the trachea, mild to moderate pneumomediastinum, and mild bilateral pneumothorax (Figure 1). Our primary suspicion was a tear of the larynx or the cervical portion of the trachea.

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Left lateral (A) and ventrodorsal (B) thoracic radiographic images of an anesthetized 6-month-old 9.5-kg (20.9-lb) castrated male Vietnamese potbellied pig that developed respiratory distress after extubation following completion of surgical castration. There are gas lucencies consistent with emphysema ventral to the cervical portion of the trachea (black arrows; A), pneumomediastinum (yellow arrows; A), and bilateral pneumothorax (white arrowheads; B). The inflated cuff of the ET tube (black arrowheads; A) can be seen.

Citation: Journal of the American Veterinary Medical Association 257, 8; 10.2460/javma.257.8.809

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Surgical exploration of the ventral aspects of the larynx and cervical region of the trachea revealed a 3-mm tear in the lateral wall of the right ventricle of the larynx. The defect was oversewn with 3-0 poliglecaprone 25^a suture placed in a simple interrupted Lembert pattern, and a tracheotomy, with placement of an ET tube through the temporary tracheostomy, was performed. Although placement of a tracheostomy tube is not routine after tracheal or laryngeal tear repair, it was performed in this pig because of increased concern for upper airway obstruction owing to multiple difficult intubations. In addition, because of the pig's disposition, a tracheostomy tube would have allowed for easier emergency intervention if needed. Partial closure of the ventral midline cervical incision was performed with 3-0 poliglecaprone 25^a suture placed in a cruciate pattern. Thoracic radiography was repeated and confirmed that the ET tube placed through the tracheostomy terminated at the bifurcation of the trachea (not shown).

Postoperatively, the pig received buprenorphine (0.010 mg/kg [0.005 mg/lb], IV), and supplemental oxygen was discontinued 5 minutes after the end of anesthesia. The pig remained eupneic and normoxemic and recovered well. Several hours after surgery, the ET tube was removed from the tracheostomy, which was left open to heal by second intention. The pig was discharged from the hospital 6 days later with no further complications.

Discussion

A laryngeal or tracheal tear is a risk of intubation in pigs and can be attributed in large part to their unique anatomy.^1,5 Pigs have thick tongues, long and narrow oropharyngeal spaces, excessive tissue in the oropharyngeal region, and an elongated soft palate that can obscure visualization of the epiglottis. In addition, pigs have long pharyngeal diverticula and wide angles of the lateral ventricles to the trachea that can make placement and appropriate advancement of an ET tube difficult.^1,2 Even in the hands of trained personnel, the act of intubation itself often leads to some degree of inflammation and potential physical injury in pigs.^5,6 Although it could not be determined definitively when during the anesthetic procedure that the laryngeal tear occurred in the pig of the present report, we suspected that the tear most likely happened during intubation attempts with the initial, larger ET tube. Alternatively, the tear could have occurred with preplacement of the stylet into the larynx prior to intubation.

Because a laryngeal or tracheal tear can lead to severe clinical consequences, understanding recommendations for diagnosis and treatment of this condition is essential. First, the occurrence of a tear must be recognized. Tracheal or laryngeal tears are not only associated with intubation but can also be caused by trauma or other iatrogenic means (eg, esophageal surgery).^7,8 Patients with such tears may exhibit clinical signs such as cough, hemoptysis, dyspnea, or cyanosis, alone or in combination.⁷ A laryngeal or tracheal tear may be diagnosed with radiography, CT, or tracheoscopy, and findings may include subcutaneous and mediastinal emphysema, pneumothorax, or pneumomediastinum, alone or in combination.^7,8 Emergency treatment is based on the patient's degree of respiratory distress and may include thoracocentesis, supplemental oxygen, and intubation beyond the level of the tear. Longer-term treatment options include medical and surgical treatments, such as administration of antimicrobials to prevent infection, the combined use of closed thoracic drainage with negative-pressure suction, administration of supplemental oxygen, and cage rest to allow the tear to heal on its own.^7,8 This is acceptable for small, uncomplicated tears in animals with mild, nonprogressive clinical signs. However, for animals with severe or progressive respiratory distress or complex tears, surgical treatment should be pursued and may include repair of the tear or resection and anastomosis.⁷ Either a thoracotomy or transcervical approach may be needed on the basis of the location of the tear. Better outcomes have been reported with earlier repair, and although intubation may be maintained in patients that require continued ventilation, other patients can often be extubated once the repair has been completed.⁸

Despite the risks, intubation is indicated in pigs when maintenance of general anesthesia is required for longer than a few minutes or when the pig would need to be positioned in dorsal recumbency.^2,3 Pigs that are sedated or anesthetized but not intubated are at an increased risk of aspiration.^2,3 Further, pigs that are sedated or anesthetized may be at risk of respiratory depression, with development of hypoxemia and hypercapnia.^2,3 These risks are of particular concern in pigs because, owing to their disposition, pigs often need to be heavily sedated to allow for the intended procedure.

Because tracheal or laryngeal tear is a known risk associated with intubation in pigs, it is important to use intubation techniques that optimize safety and efficacy. In general, it is recommended that when a pig is intubated, it should be positioned in dorsal or ventral recumbency, gauze strips should be placed behind the upper and lower canine teeth to open the pig's mouth, and a laryngoscope should be used to visualize the larynx.² The arytenoid cartilages may be sprayed with lidocaine to decrease laryngospasm, and the ET tube is then inserted into the trachea and secured with a gauze tie. If resistance is met, rotation of the ET tube 180° to allow continued passage of the ET tube can be attempted. Correct ET tube placement can be confirmed by laryngoscopy to visualize tube placement, bilateral thoracic auscultation, or capnography.²

Studies^9–12 on how variations in orotracheal intubation techniques can improve the safety and efficacy in pigs have been conducted. For example, a study⁹ on the effect of patient positioning on intubation times shows that intubation was markedly faster when patients were positioned in ventral (sternal) recumbency, compared with dorsal recumbency. It was proposed that ventral recumbency not only facilitated better visualization of anatomic landmarks during intubation but also allowed for optimal extension of the animal's head and better handling of equipment by the person performing intubation owing to the better ergonomic working position.⁹ However, others advocate for positioning pigs in dorsal recumbency for intubation, especially for pigs < 50 kg (110 lb) and when personnel are accustomed to intubating humans.¹⁰Additionally, when pigs positioned in dorsal recumbency are intubated, no assistance is needed to hold the jaw open, and in some instances, neither a stylet nor laryngoscope may be necessary.¹⁰ In the pig of the present report, we used a stylet to facilitate intubation. Relatedly, a study¹¹ shows that intubation is markedly faster when a stylet guide is used (vs not used) to aid intubation in pigs when performed by anesthesiologists and individuals without experience intubating pigs.¹¹ Furthermore, because of known difficulties with intubation in pigs, use of laryngeal mask airways has been described in pigs,¹² and although not specifically evaluated for use in pigs, other tools, such as an ET tube whistle or an airway exchange device, may prove useful.

Although tracheal intubation is often in the best interest of a pig undergoing general anesthesia, it is not without risk. Veterinarians should be aware of possible complications with intubation in pigs and be comfortable with techniques required to diagnose and treat them, should any occur. Findings in the pig of the present report underscored that there remains room for innovation in optimizing intubation in pigs because, although recommended techniques for intubation were followed, a laryngeal tear still occurred in the pig of the present report.