Introduction

Five dogs were presented to the Veterinary Medical Teaching Hospital of Seoul National University for ovariohysterectomy, dental prophylaxis, or intracapsular lens extraction with a history of a ventriculocordectomy from April 2015 to May 2019 (Table 1). The American Society of Anesthesiologists grade of these 5 dogs varied according to the preanesthetic physical examination, CBCs, serum chemical analysis, thoracic radiographs, and underlying diseases. No remarkable, abnormal clinical signs such as exercise intolerance, respiratory distress, or stridor were found in 4 dogs. However, 1 dog (case 3) had a cough after drinking, which had started since ventriculocordectomy. No abnormalities were found during preanesthetic physical examination in any dog. An intravenous catheter was placed in a cephalic vein, and lactated Ringer’s solution or normal saline (0.45% NaCl) solution (5 to 10 mL/kg/h) was administrated.

According to the American Society of Anesthesiologists grade, dogs were premedicated with antibiotics, sedatives, and analgesics (Table 1). The size of endotracheal tube (ETT) was selected based on the lateral thoracic radiograph. The ETT had an outer diameter occupying 80% of the tracheal diameter.¹ An armored ETT was selected for dental prophylaxis and ocular surgery. Murphy type was used for laparotomy. General anesthesia was induced with propofol or alfaxalone. During intubation, a laryngeal web, suspected to be a complication of ventriculocordectomy, was observed (Figure 1). It extended across the laryngeal lumen, close to the vocal cords. The narrowed vocal fold and laryngeal inlet did not allow the insertion of the prepared ETT. Therefore, a smaller ETT (outer diameter, 66% to 89% of the prepared ETT), fitted for the laryngeal inlet with the laryngeal web and accounting for the severity of stenosis, facilitated the intubation (Table 2). The leak test with a pressure of 20 cm H₂O showed air leakage even after the high-volume, low-pressure cuff was mildly overinflated. The cuff was pulled cranially until it plugged the narrowed laryngeal lumen behind the vocal cords. The leak test used to ensure an adequate seal around the ETT cuff was reperformed, and no air leakage was confirmed. Since the proper position of the ETT was not determined, the modified cow hitch knot around the ETT was fastened after the leak test, and the string fixed to the ETT was secured behind the ears to keep the ETT in proper place.

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Endoscopic (A) and fluoroscopic (B) images of a laryngeal web (arrow) found during intubation with a laryngoscope (arrowheads) and endotracheal tube (arrowheads) in a dog of this report with a history of ventriculocordectomy. Fluoroscopy reveals the position of the endotracheal tube cuff located immediately behind the vocal cord (arrows).

Citation: Journal of the American Veterinary Medical Association 260, 12; 10.2460/javma.21.07.0350

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Anesthesia was maintained with isoflurane in 100% oxygen via the ETT. Volume-controlled ventilation (Vent V; Royal Medical) was used throughout the anesthesia period with a baseline setting of tidal volume (TV; 10 mL/kg), inspiration-to-expiration ratio (1:2), and respiratory rate (15 breaths/min), and the setting was adjusted according to the patient’s end-tidal partial pressure of carbon dioxide (PetCO₂). The heart rate, direct arterial pressure, arterial oxygen saturation, respiratory rate, PetCO₂ from capnography, end-tidal isoflurane concentration, esophageal temperature, TV, and plateau pressure from spirometry were monitored continuously with a monitor (Carescape Monitor B650 or Datex-Ohmeda AS/3; GE Healthcare) and recorded every 5 minutes. Until plateau pressure was 12 cm H₂O, the match between inspiration and expiration TV on spirometry did not show air leak during the anesthetic period of 100 to 160 minutes. The arterial blood gas analysis conducted during anesthesia in 2 cases confirmed a slight increase in PaCO₂ (Table 3). The position of the ETT cuff was confirmed through fluoroscopy before the dog recovered from anesthesia in 1 case (Figure 1). No complications related to the ETT placement were observed in the peri-anesthetic period in all 5 dogs.

Discussion

In 5 dogs with a history of ventriculocordectomy, laryngeal web with varying severities were found during intubation, necessitating intubation with a small-diameter ETT that was fitted for the laryngeal inlet. The peri-cuff air leak caused by small-diameter ETT was resolved with the noninvasive modified intubation technique of plugging—the narrowed laryngeal lumen with the inflated high-volume, low-pressure ETT cuff sealed the air leak around the ETT cuff.

A laryngeal web is a congenital membrane formed in the larynx or acquired granulation tissues and usually presents as fibrous tissues across the glottidis, blocking the larynx from the mucosal surfaces or vocal fold.² In animals, most acquired laryngeal web is caused by larynx-related surgery such as ventriculocordectomy.³ Moreover, 10 of 15 animals with a laryngeal web were found to have acquired it from oral ventriculocordectomy, whereas the rest were acquired from arytenoid lateralization, unilateral arytenoidectomy, or bilateral laryngeal sacculectomy.³ Postoperative follow-up data for 25 dogs with ventriculocordectomy have shown respiratory distress in 36%, aspiration pneumonia in 16%, or cicatrix formation in 20% of the dogs.² In this study, 1 of the 5 dogs showed a similar symptom, coughing after drinking, but the rest showed no clinical symptoms. Even in the absence of remarkable abnormal respiratory signs, the presence of the laryngeal web may be accidentally discovered at the time of intubation, making intubation impossible. Therefore, an alternative technique should be considered when the laryngeal web does not allow intubation with prepared ETT for patients with a history of larynx-related surgery.

When encountering the laryngeal web, several methods to secure the airway have been suggested in human medicine.⁴ First, when resistance is felt during intubation, intubation should be stopped without excessive trials, and a smaller ETT should be used. When a smaller-sized ETT is unavailable or the airway is completely closed, the use of an anesthetic mask or tracheostomy is recommended. Any additional granulation tissue formation due to stimulation of intubation trials should be followed up.⁴ Among the various suggestions, a noninvasive method was first tried. However, dogs have various muzzle shapes, which might interrupt a proper sealing by a single standard anesthetic mask. Therefore, intubation with a smaller-sized ETT was attempted.

The relatively small cuff volume of the smaller ETT, however, might not be able to completely block the tracheal lumen, which in turn leads to anesthetic gas leakage. This gas leakage during anesthesia causes an inadequate depth of anesthesia and contamination of the surrounding area. Moreover, aspiration of gastric contents or secretions cannot be prevented.⁵

To prevent the gas leakage while using a small-diameter ETT, the cuff may be inflated excessively⁶ over 25 to 35 mmHg. If a laryngeal lumen is plugged by an overinflated cuff with excessive pressure, it would result in tracheitis, necrosis of the tracheal mucosa, or tearing of the tracheal membrane.⁶ Additionally, the excessive pressure of the cuff narrows the lumen of ETT, leading to a poor supply of oxygen and respiratory anesthetics to patients.⁷ Another way to prevent leaking while using a small-diameter ETT is to pack the larynx with gauze. This method reduces the amount of air leak but is not completely preventable, but strong packing can cause injury of the pharyngeal mucosa and possible swallowing of the gauze if it is not taken out before the extubation.⁵ Therefore, compared to the packing methods when only a small-diameter ETT is available, the noninvasive plugging technique used in these cases reduces air leak while reducing the burden of laryngeal mucous membranes due to a laryngeal web.

When a relatively large amount of air is introduced by the ventilator via a narrow diameter of a small ETT, high resistance in breathing should be considered. According to an in vitro experiment, when given a flow of 5 L/min, the 6.0- and 2.5-mm inner diameters of ETT had the pressure of 3.1 and 81.2 cm H₂O/L/s, respectively. The increase of resistance due to the reduction in diameter was pronounced, especially when the inner diameter was < 4.0 mm. These presented cases were for small animals, and ETT with a diameter of 4.0 mm or less was used. However, since the in vitro experiment was conducted on the atmosphere without the loss of kinetic energy, a significantly lower pressure may be expected in vivo.⁸ In fact, in an in vivo experiment in humans, a large volume of ventilation support was provided via a small-diameter ETT, and high pressure found at the patient end of the ETT did not necessarily lead to a corresponding high intratracheal pressure during the inspiration period.⁹ Although the positive tracheal pressure was identified at the end of the expiration period, it was still within the limit of therapeutic positive end-expiratory pressure, except in extreme cases. In all presented cases, neither abnormal PaCO₂ in arterial blood gas analysis was found in 2 cases and it was less than the criteria of hypoventilation in the anesthetized patient (PaCO₂ or PetCO₂ > 65 mmHg) and would be adjustable with ventilator control.¹⁰

Although the cuff is needed to be closely contacted with the laryngeal web for sealing, pulling the cuff cranially hard is unnecessary. When positive-pressure ventilation is performed, positive pressure in the trachea would be created and might push the ETT out closer to the mucous membrane around the laryngeal lumen. Thus, the modified cow hitch was adopted as an effective method to keep ETT in place without the risk of untying the knot.¹¹ The last knot of the modified cow hitch knot was fastened after no air leakage was observed and proper position was confirmed.

The laryngeal web in these cases was complete; therefore, it was possible to be blocked with the cuff of a small ETT. However, there is a possibility of the laryngeal web not being complete. This could lead to a situation in which the laryngeal web itself allows gas passage, and thus plugging the visible aperture into the trachea is not sufficient, although it may be better than having the cuff in its normal position. This noninvasive plugging technique may not always resolve the peri-cuff leak induced by the laryngeal web, and it is necessary to recognize that other possible methods may be needed.

This case report described the noninvasive modified intubation technique of plugging the narrowed laryngeal lumen with the inflated ETT cuff sealed air leak around the cuff without complications in all 5 dogs. Therefore, when it is unavoidable to use a relatively small-diameter ETT due to a laryngeal web, this could be an adequate and applicable method to sufficiently secure the TV without high ventilation pressure and high flow resistance.