History

A 6-year-old 8.4-kg (18.5-lb) castrated male domestic shorthair cat was examined by the small animal emergency service at the University of Illinois Veterinary Medical Teaching Hospital because of progressive subcutaneous emphysema of the head, neck, and thorax of 1 week's duration. Signs developed shortly after a routine dental procedure had been performed by the referring veterinarian.

Physical examination revealed severe subcutaneous emphysema extending from the head to the base of the tail and distally to the level of the carpal and tarsal joints. The cat was bright, alert, and responsive. Rectal temperature was 40°C (104°F), pulse rate was 160 beats/min, and respiratory rate was 40 breaths/min. Results of electrocardiography were unremarkable. Systolic blood pressure, measured by means of a Doppler ultrasonic method, was 140 mm Hg. Pulse oximetry was performed with the probe placed on the ear, and oxygen saturation was 100%. Thoracic auscultation was attempted, but cardiac and pulmonary sounds could not be heard because of the severe subcutaneous emphysema. Fleas were evident during the physical examination.

A CBC and serum biochemical profile were performed. Abnormalities included neutrophilia (13.9 × 10³ neutrophils/μL; reference range, 2.5 to 12.5 × 10³ neutrophils/μL), lymphopenia (0.639 × 10³ lymphocytes/μL; reference range, 1.70 to 7.00 × 10³ lymphocytes/μL), hypophosphatemia (3.5 mg/dL; reference range, 4.0 to 7.0 mg/dL), hyperglycemia (173 mg/dL; reference range, 65 to 129 mg/dL), hypercholesterolemia (141 mg/dL; reference range, 63 to 130 mg/dL), mild metabolic acidosis (pH, 7.366 [reference range, 7.38 to 7.49]; bicarbonate concentration, 18.1 mmol/L [reference range, 21.2 to 27.8 mmol/L]), and high lactate concentration (3.2 mmol/L; reference range, < 2.5 mmol/L). Thoracic radiography revealed severe subcutaneous emphysema, pneumomediastinum, pneumoretroperitoneum, and mild pneumothorax (Figure 1).

Lateral radiographic projection of the thorax and abdomen in a cat with subcutaneous emphysema secondary to a tracheal tear. Notice the extensive hypolucent area (between arrows) dorsal to the spinal column.

Citation: Journal of the American Veterinary Medical Association 234, 12; 10.2460/javma.234.12.1539

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Lateral radiographic projection of the thorax and abdomen in a cat with subcutaneous emphysema secondary to a tracheal tear. Notice the extensive hypolucent area (between arrows) dorsal to the spinal column.

Citation: Journal of the American Veterinary Medical Association 234, 12; 10.2460/javma.234.12.1539

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Lateral radiographic projection of the thorax and abdomen in a cat with subcutaneous emphysema secondary to a tracheal tear. Notice the extensive hypolucent area (between arrows) dorsal to the spinal column.

Citation: Journal of the American Veterinary Medical Association 234, 12; 10.2460/javma.234.12.1539

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Tracheoscopy was planned to further investigate the cat's problems, and general anesthesia was considered necessary to facilitate the examination. An American Society of Anesthesiologists status of III was assigned on the basis of the cat's compromised physical condition. Additionally, concerns were raised regarding maintaining an acceptable anesthetic plane and a patent airway during tracheoscopy in a potentially compromised patient.

Question

What is the most likely cause of the clinical signs in this cat? What is the appropriate treatment plan for this patient? What considerations and possible complications must be taken into account when formulating an anesthetic plan?

Answer

The most likely cause of subcutaneous emphysema and dyspnea in a cat with a history of a recent anesthetic event is tracheal rupture. To achieve a definitive diagnosis, direct visualization of a tracheal injury via tracheoscopy, advanced imaging techniques such as computed tomography, or exploratory surgery is required. General anesthesia is often needed to perform these diagnostic tests. To devise an appropriate anesthetic protocol, several issues must be addressed. First, endotracheal intubation is usually not possible when performing tracheoscopy because the diameter of the endoscope is often larger than the inner diameter of the largest endotracheal tube that can be used to intubate a cat. Additionally, if the tracheal defect is in a more proximal location, an endotracheal tube may block visualization of the defect. Second, use of inhalant anesthetics in the presence of a tracheal defect may allow carrier gas and vaporized anesthetic to escape from the respiratory tract and into the surrounding environment. This can result in difficulties in maintaining an appropriate anesthetic plane and expose personnel to waste anesthetic gases.

Treatment and Outcome

A total IV anesthesia technique was selected for the cat. The cat was premedicated with hydromorphone (0.1 mg/kg [0.045 mg/lb], IV), and supplemental oxygen was administered with an anesthesia machine by adjusting the oxygen flow to a rate of 2 L/min and placing the breathing circuit Y piece near the cat's face to provide an oxygen-enriched environment. After 15 minutes of oxygen administration, anesthesia was induced with propofol (4 mg/kg [1.8 mg/lb], IV, to effect). Monitoring consisted of continuous electrocardiography, indirect measurement of blood pressure, and pulse oximetry.^a During tracheoscopy, anesthesia was maintained by administering additional boluses of propofol as needed. During the procedure, 100% oxygen was insufflated through the biopsy channel of the endoscope.

Tracheoscopy revealed a 28-mm-long tear in the dorsal membrane of the trachea at the approximate level of the sixth and seventh cervical vertebrae. Once the lesion and location were identified, the endoscope was removed and a noncuffed 4-mm (internal diameter) endotracheal tube was inserted into the trachea and advanced to the level of the thoracic inlet. Administration of 100% oxygen via a circle system attached to the endotracheal tube was instituted. Because of the size of the tracheal defect and the severity of the associated clinical signs, a decision was made to perform surgical correction of the tracheal tear via a ventral midline approach.

To maintain anesthesia during the surgical procedure, propofol was administered as a constant rate infusion (0.1 mg/kg/min, IV). A bolus of fentanyl (0.5 μg/kg [0.23 μg/lb], IV) was administered, and a constant rate infusion of fentanyl (0.2 μg/kg/min [0.09 μg/lb/min], IV) was initiated to provide analgesia. Lactated Ringer's solution was administered (5 mL/kg/h [2.3 mL/lb/h], IV) throughout the surgical procedure.

Additional monitoring during the surgical procedure included measurement of rectal temperature and end-tidal partial pressure of CO₂. Oxygen saturation as measured by use of pulse oximetry ranged from 83% to 85%, most likely because of inefficient alveolar ventilation and gas exchange as a result of oxygen escaping through the tracheal defect into the surrounding tissues. After the initial surgical incision, assessment of the patient revealed increased jaw tone, eye movement, and an increased respiratory rate. These signs were interpreted as a lack of sufficient anesthesia or analgesia for the surgical procedure, and the fentanyl infusion rate was increased (0.3 μg/kg/min [0.14 μg/lb/min], IV).

The cervical portion of the trachea was explored, and the dorsal membrane defect was located and isolated. To facilitate surgical repair, the endotracheal tube was withdrawn to a position cranial to the defect. The defect was sutured in a simple interrupted pattern, and the uncuffed endotracheal tube was then removed and replaced with a 4-mm (internal diameter) cuffed endotracheal tube with the tip placed just cranial to the repaired defect (Figure 2). The cuff was inflated until no gas could be detected escaping from around the endotracheal tube when a positive pressure of 15 cm H₂O was applied via compression of the rebreathing bag on the anesthesia machine. Oxygen saturation increased to 98% after the tracheal defect was closed and the endotracheal tube was replaced.

Photograph of the tracheal tear (between arrows) in the cat in Figure 1 following surgical correction. The tear was at the area where the trachealis muscle attaches to the cartilaginous tracheal rings.

Citation: Journal of the American Veterinary Medical Association 234, 12; 10.2460/javma.234.12.1539

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Photograph of the tracheal tear (between arrows) in the cat in Figure 1 following surgical correction. The tear was at the area where the trachealis muscle attaches to the cartilaginous tracheal rings.

Citation: Journal of the American Veterinary Medical Association 234, 12; 10.2460/javma.234.12.1539

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Photograph of the tracheal tear (between arrows) in the cat in Figure 1 following surgical correction. The tear was at the area where the trachealis muscle attaches to the cartilaginous tracheal rings.

Citation: Journal of the American Veterinary Medical Association 234, 12; 10.2460/javma.234.12.1539

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The subcutaneous tissue and the skin were closed in a routine manner. The infusions of propofol and fentanyl were discontinued, and the cat was extubated 15 minutes later. Postoperative pain was managed with buprenorphine (0.02 mg/kg [0.009 mg/lb], IV, q 8 h) and meloxicam (0.1 mg/kg, SC, once), and the cat was transferred to the intensive care unit for postoperative observation. The cat was discharged to the owner 3 days after surgery. Follow-up communication with the owner revealed that the cat's subcutaneous emphysema had resolved and that the cat was not having any complications.

Discussion

Progressive subcutaneous emphysema in a cat that has recently undergone a procedure requiring endotracheal intubation is most often a result of tracheal rupture or a tracheal tear. Tracheal rupture has most often been associated with routine dental prophylaxis,^1–3 although it has been reported following other procedures in cats, including ovariohysterectomy, onychectomy, oral mass removal, and anesthesia for radiography.¹

Clinical signs associated with tracheal rupture in cats include subcutaneous emphysema, dyspnea, anorexia, lethargy, respiratory stridor, vomiting, dehydration, hemoptysis, and signs of depression.¹ Additionally, pneumomediastinum, pneumothorax, and pneumoretroperitoneum have been reported in cats with tracheal rupture.¹ Subcutaneous emphysema is the most common clinical finding and has been reported to occur in 100% of cases.^1,2 The clinical signs associated with tracheal rupture are a result of air escaping from the tracheal defect and dissecting between and along tissue planes. The deep cervical fascial planes are in direct communication with the mediastinum, which explains the development of pneumomediastinum, and the mediastinum communicates with the retroperitoneal space, which explains the development of pneumoretroperitoneum. Pneumothorax can occur when there is a pressure gradient between the mediastinum and pleural space that is high enough to rupture the mediastinum.

The areas of the trachea most commonly injured in cats with tracheal rupture are the junctions of the tracheal rings and the trachealis muscle on the dorsolateral aspect of the trachea.¹ Postulated causes of tracheal rupture in cats include overinflation of an endotracheal tube cuff; changes in patient body position that are performed without disconnecting the endotracheal tube from the breathing circuit Y piece, resulting in twisting of the endotracheal tube within the trachea; traumatic intubation; and removal of the endotracheal tube prior to deflation of the cuff.¹ However, in a study² performed on cadavers, only overinflation of the endotracheal tube cuff could produce lesions.

A presumptive diagnosis of tracheal rupture can be made on the basis of a history of previous tracheal intubation for anesthesia in combination with typical clinical signs and physical examination findings. Definitive diagnosis requires direct examination of the lesion via tracheoscopy, advanced imaging techniques, or exploratory surgery. In most cats with tracheal rupture, surgical repair is not necessary, with medical management adequate for cats with only mild to moderate signs of dyspnea and subcutaneous emphysema. These cases are managed with strict cage rest, monitoring of respiratory function, administration of sedatives, and supplemental oxygen administration when indicated. Surgical correction is recommended for cats with severe dyspnea, respiratory distress, cyanosis, and progressive subcutaneous emphysema.³

Anesthetic management of patients with tracheal injuries and patients requiring surgery of the pharynx, larynx, or trachea can be challenging. As with the cat described in the present report, use of a total IV anesthetic technique, instead of an inhalant anesthetic technique, can be a better approach for several reasons. First, because of the potential for escape of anesthetic vapors into the environment, an inhalant anesthetic technique could potentially result in unacceptable exposure of anesthetic and surgical staff to waste gases during the procedure. Additionally, inhalant anesthetic gas could be trapped in subcutaneous air, resulting in a continued escape of waste anesthetic gas, leading to additional exposure of hospital personnel and pet owners. Second, with the escape of anesthetic gas from the respiratory system, maintaining a consistent alveolar concentration of the inhalant anesthetic could be difficult, resulting in an unstable anesthetic plane. Third, endotracheal intubation or an inflated endotracheal cuff may interfere with surgical correction of the lesion.

Total IV anesthesia refers to the induction and maintenance of anesthesia and analgesia by the use of IV drug administration alone. Various protocols have been described, but all generally involve administration of drugs to achieve hypnosis, muscle relaxation, and immobility. The most readily available drug for this procedure is propofol. Analgesia can be provided by administration of additional drugs such as ketamine, an α₂-adreonoceptor agonist, or an opioid. Drugs can be administered as a constant rate or variable rate infusion or as intermittent boluses. Most often, a bolus is administered initially to induce a surgical plane of anesthesia, with additional drug administered as necessary to maintain a plasma concentration that will result in a consistent plane of anesthesia or sufficient analgesia.

Drugs that are best suited for total IV anesthetic protocols have minimal tissue accumulation and rapid metabolism and result in desired effects following administration of small volumes. Because of breed and individual differences in drug pharmacokinetics and differences in the amount of external stimulation during the anesthetic procedure, no single protocol is appropriate for all patients. A common approach would be to use a combination of drugs, with one administered as a constant rate infusion and a second administered as a variable rate infusion, with the rate adjusted on the basis of patient needs.

For animals undergoing total IV anesthesia, loading doses and maintenance doses for administration as constant rate infusions can be readily calculated with standard formulas, with loading dose equal to the desired plasma concentration times the volume of distribution and maintenance dose equal to the desired plasma concentration times total body clearance. In addition, recommended loading and maintenance doses have been published.^4,5 For the cat described in the present report, we chose a combination of propofol and fentanyl for total IV anesthesia. The recommend dose of propofol for administration as a constant rate infusion in cats ranges from 0.1 to 0.3 mg/kg/min (0.045 to 0.14 mg/lb/min),^4,5 and the recommend dose of fentanyl in cats ranges from 0.05 to 0.3 μg/kg/min (0.023 to 0.14 μg/lb/min).^4,5 However, pharmacokinetic studies evaluating administration of propofol and fentanyl as constant rate infusions in cats are lacking. Other drugs used for total IV anesthesia in cats include alfentanil, sufentanil, remifentanil, morphine, ketamine, and lidocaine. Recommended doses of these drugs have been published.⁴

In summary, subcutaneous emphysema in a cat following a procedure that involved endotracheal intubation is consistent with a primary tracheal lesion. Most cases can be diagnosed on the basis of history and clinical signs and managed with medical treatment alone. Definitive diagnosis and surgical correction should be pursued in animals that have severe dyspnea or cyanosis and in animals in which the condition progressively worsens. Anesthesia for endoscopy, advanced imaging, and surgical correction of tracheal lesions can be obtained with the use of total IV anesthesia protocols.