Cardiology

Tracheal Collapse: Stents

Tracheal collapse is a common disorder of the trachea in dogs. It is a progressive, degenerative disease of the cartilage rings in which hypocellularity and decreased glycosaminoglycan and calcium content leads to a dynamic tracheal collapse during respiration. This is a condition of predominantly middle-age, small and toy-breed dogs.

The collapse can affect any part of the trachea (cervical, intrathoracic) and even the main stem bronchi. It usually affects the entire trachea to various extents.

The etiology remains unclear: genetic, nutritional, neurological, inflammatory hypotheses have been advocated. This disease affects particularly Toy Poodles, Yorkshire Terriers, Chihuahuas, Pomeranians, Maltese, Pugs. The prevalence reaches 2.7% in these breeds. The range of onset of symptoms is: 0.25 to 15y, usually around 6-8 year of age.

Clinical signs range from a mild, intermittent "honking: cough to severe respiratory distress from dynamic upper-airway obstruction. Thirty percent of affected dogs have concomitant laryngeal paralysis.

Symptoms	% Cases (n=80)
Cough	72% (n=26)
Honking	67% (n=24)
Gagging / Retching	50% (n=18)
Dyspnea	67% (n=24)
Cyanosis	22% (n=8)
Inspiratory Stridor/Expiratory Stertor	44% (n=16)
Wheezing	52% (n=19)
Exercise Intolerance	42% (n=15)

The diagnosis relies on documentation of abnormal narrowing of trachea during inspiration (for cervical collapse), expiration (for intrathoracic tracheal and bronchial collapse), or during cough. Diagnostic procedures include:

• X rays (beware that longus colli superposition at thoracic inlet can lead to false positive diagnosis of TC). Inspiratory and expiratory X rays are needed.
• Positive and negative pressure X rays (+ and - 30 cm H2O). This mimics forced inspiration and expiration and greatly enhance the visualization of tracheal collapse, and allows proper measurement of the grade of collapse and of the maximal tracheal diameter (for stent measurement)
• Fluoroscopy (this allows to see what happen during cough)
• Tracheo-bronchoscopy. This allows to directly visualize the collapse at rest, and to obtain a BALF for analysis.

The tracheal collapse needs to be graded (grade 0: no collapse, grade 1: bowing of the dorsal membrane, grade 2: up to 50% reduction of the tracheal lumen, grade 3: 50 to 75% reductions, grade 4: > 75%).

Treatment:

Medical:

The medical treatment is symptomatic. It includes antitussive (butorphenol, hydrocodone), bronchodilators (theophyline, terbutaline), corticosteroids, antibiotics, anticholinergics (Lomotil), and weight control. Acceptable control of the symptoms can be achieved in 70% of the cases.

Surgical:

Surgical procedures can also be considered in severe cases (grade 4 collapse), or cases not responding to therapy. These surgical procedures apply mostly to cervical tracheal collapse. Various procedures have been used. They include: plication of dorsal membrane, extraluminal rings: total ring prosthesis (TRP), polypropylene spiral ring (PSP), and extralumnal stents. Complications can occur with these procedures and include narrowing of the tracheal lumen (mostly with the placation of the dorsal membrane), and disruption of tracheal vascular supply and recurrent laryngeal nerve damage. In one retrospective study of 90 cases, complication occurred in 37% of dogs: coughing (66%), dyspnea (42%), and laryngeal paralysis (30%) (Only 2/10 dogs who developed laryngeal paralysis had evidence of laryngeal dysfunction prior to surgery), 17 dogs (18%) required permanent tracheostomy, 10 within 24 hr of surgery. The degree of improvement observed was significant (65% improvement, and 22% return to normal). Other authors report success rate varying between 20 and 80%.

The major limitation of surgical approaches has been the impossibility to deal effectively with intrathoracic tracheal collapse. Surgery has now been replaced by the placement of intratracheal stents.

Tracheal stents:

Various types of stents have been tried. In the recent years, an intratracheal stent designed specifically for dogs had become available: the Vet Stent-Trachea© from Infiniti MedicalTM. This stent comes in various sizes. It is self-expanding, reconstrainable, flexible and atraumatic. This stent is made of a thermal-shape-memory nickel-titanium alloy (Nitinol or NiTi). These stents expands as they get heated by the tracheal lumen (Marmen effect). The Nitinol alloy has a good compatibility with the tracheal lumen. They can be placed under fluoroscopy. We have placed Vet Stents (Infiniti Medical) in more than 50 patients so far. Here are a few lessons we learned from our experience.

When is a tracheal stent indicated?

Endotracheal stenting is justified when the symptoms of breathing difficulties are important (dyspnea, honking, wheezing, cyanosis, severe exercise intolerance, syncopes). These patients usually have grade 3 or 4 collapse. Cough is Not an indication for tracheal stenting. It has to be managed medically.

Which stent size?

Accurate sizing of the stent is key to success. If the stent is too small, it may migrate in the trachea. If it is too wide, it will not expand fully and become too long. To assess that, the best is to obtain X rays under positive pressure (30 cm H20) to accurately evaluate the maximal tracheal diameter, and negative pressure (-30 cm H20) to better appreciate the location and the full length of the collapse, and hence the length of the stent. Accurate measurements require calibration with either an esophageal catheter or Orthoplan©. This procedure was followed by a trans-tracheal wash.

The nominal stent diameter needs to be 20% larger than the maximal tracheal diameter. The length has to take in account the length of the collapse and the length that the stent will have once deployed and constrained in the narrower trachea. This can be predicted by consulting the lengthening charts on the Infiniti Medical website.

Stent placement

The placement of the Vet Stent® was done under general anesthesia using fluoroscopy for guidance. The distal extremity of the stent is placed at the level of the 4th rib. The proximal extremity of the stent should not be cranial to the cranial edge of C3. A negative pressure X-Ray was performed post stent placement to assess the presence of absence of residual collapse. In the cases where the residual collapse cranial to the stent was deemed problematic (> 2cm long), bronchoscopy was used to pull the stent cranially.

Results

All dogs received concomitant medical management with prednisone (1 mg/kg/day tapered over 6 months), bronchodilators (Theo-LA 10 mg/kg q12, salbutamol), antibiotics (Clavamox +/- baytril for 21d) and cough suppressants (Hydrocodone or Torbutrol 0.25 mg/kg q8-12

Degree of improvement	% of cases
Very satisfactory with resolutions of all symptoms	25%
Acceptable (improvement of dyspnea and exercise tolerance) with lingering cough requiring medical Rx	53%
Very satisfactory with resolutions of all symptoms	25%
Poor due to severe persistent cough affecting quality of life	5%
Very satisfactory with resolutions of all symptoms	25%
Lost to follow up	5%
Perioperative mortality	7%

Residual tracheal collapse: 24%

The 2 main causes would be the initial fear of crossing the thoracic inlet with the stent, inaccurate prediction of the final length of the deployed stent, and limitations due to available stent sizes. We now stent the entire trachea, and have far less issues with residual collapse.

A second stent had to be placed in 3 dogs where the residual collapse continued to be symptomatic.

Acute dyspnea 24-48 hours post stent placement: 8%

dogs developed severe dyspnea in the 48 hours post stent placement leading to ARDS and death in 4 of them. The cause of this is unclear but presumed to be secondary to airway collapse and swelling distal to the stent. We have also seen dogs develop ARDS due to prolonged severe dyspnea before we could place a stent, indicating that this complication may be due to the severity of the disease itself, or concomitant laryngeal collapse.

Cough: 69%

Cough was frequently present post stent and required medical management. The cough proved to be severe and intractable in 6 dogs.

Stent migration: 1%

This was due to the fact that the diameter of the stent chosen was too small. The diameter needs to be 120% of the maximal tracheal diameter on positive pressure XR.

Stent rupture: 6%

The rupture affected the distal or the proximal extremity of the stent and was clearly associated with ongoing cough; this was resolved by placement of another stent inside the first one.

Steroid responsive granulomas: 6%

These granulomas are usually steroid responsive.

Intractable cough due to bronchial collapse: 5%

Concomitant medical treatment

Dogs receiving a stent need also medical treatment management, to reduce the cough and the mucus accumulation which may still occur after the stent placement. This include prednisone tapered over 6 months), bronchodilators, antibiotics and cough suppressants.

Another complication is stent migration. This usually results from choosing a stent too small. This happened to us in 2 cases (13%). This required pulling the stent out using endoscopy and replacing it by a bigger one.

Granulomas obstructing the proximal extremity of the stent are a rarer complication (1 case in our series). They occur 2-3 weeks post stent placement. Fortunately, they are steroid responsive.

Stent fracture is also another possible complication. We have not witnessed it so far. Aggressive cough management is very important to minimize fatigue of the stent.

What have our results been?

All the symptoms of dyspnea (stridor, wheezing, honking, cyanosis, exercise intolerance, suffocation attacks) disappeared after the stent placement. Impact on cough was variable: 75% of the dogs were still coughing 2-3 weeks post stent placement. This percentage dropped to 37.5% after 6 months.

At this stage of our experience, we can conclude that the Vet Stents are effective in improving the dyspnea symptoms of severe tracheal collapse. The procedure is fairly rapid, but requires a careful choice of the stent. Complications include mucus plugs, pneumonias, stent migration and granulomas. Medical treatment remains necessary before and after the stent placement, especially to control the cough and lower airway disease.