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Surgery of the respiratory system (Proceedings)
The upper airway distress syndrome is composed of three individual conditions that cause animals to have moderate to severe inspiratory dyspnea.
Upper airway distress syndrome
The upper airway distress syndrome is composed of three individual conditions that cause animals to have moderate to severe inspiratory dyspnea. The three conditions consist of: 1. Stenotic nares, 2. Elongated soft palate, 3. Everted lateral ventricles. These three conditions may occur together in the same animal or may occur individually. This syndrome occurs more frequently in brachycephalic animals. Dogs are more often involved. Brachycephalic cats do get stenotic nares that may have to be surgically corrected. The corrections of the elongated soft palate is performed by amputating the redundant tissue of the soft palate. The measurement of the amount of amputation is done by measuring the overlap of the soft palate to the epiglottis. The lateral ventricles are always checked when this amputation is completed to make sure that they are not everted into the airway. If they are, they are removed. The pre-operative and post-operative medications consist of antibiotics, corticosteroids, (1 mg/lb of Dexamethasone) and Lasix.
The majority of cases of collapsing trachea. are seen in toy or miniature breeds with the Chihuahua, Paneranian, Yorkshire terrier and toy and miniature Poodle having the highest incidence. This condition is seen in dogs of all ages with the average age of diagnosis being seven years. There is no sex predilection. The early signs of tracheal collapse include chronic cough or dyspnea exacerbated by mild exercise or excitement. This progresses to a more severe exercise intolerance, cyanosis and the development of a honking cough. The absence of a cough does not rule out the diagnosis of collapsed trachea. Dyspnea is seen upon inhalation with cervical tracheal collapse and upon expiration with intrathoracic collapse.
The degree of tracheal collapse can be classified into four grades. In Grade I collapse of the trachea is nearly normal with a slightly pendulous tracheal muscle and a reduction in lumen size of 25%. Grade II collapse consists of a tracheal muscle which is widened and pendulous and tracheal cartilages which are partially flattened. The tracheal lumen is reduced by 50%. In Grade III collapse the tracheal muscle is almost in contact with the nearly flat tracheal cartilages and the tracheal lumen is reduced by approximately 75%. In Grade IV collapse the tracheal lumen is essentially obliterated by the tracheal muscle lying on the dorsal surface of the tracheal cartilages.
A complete physical exam should be performed with emphasis on the respiratory system. Palpation of the cervical trachea may reveal dorsoventral flattening and incite severe coughing spasms. Lateral radiographs of an unanesthetized patient are usually diagnostic for tracheal collapse and should always be taken to evaluate the heart and lungs. It may be necessary to preoxygenate the patient and use a standing lateral position with a horizontal beam if the patient is severely affected. Fluoroscopy confirms changes in trachea diameter during respiration in the unanesthetized patient.
Endoscopy is the best technique to evaluate the trachea and bronchi prior to surgery and should be done in conjunction with anesthesia for surgery. Oxygen should be infused through the bronchoscope. Endotracheal tubes for miniature dogs are usually too small to pass an endoscope through and cannot be used. The effects of respiration and head and neck extension on the tracheal and bronchial lumen should be noted. The degree of dorsal membrane laxity, the shape of the tracheal rings and the flattening of the trachea and bronchi should be closely observed. Many patients with collapsed trachea have mild to significant right heart enlargement with mild diffuse increases in pulmonary linear interstitial densities suggestive of pulmonary fibrosis and cor pulmonale. Sane dogs have biventricular enlargement, left atrial enlargement and pulmonary edema. Peribronchial infiltrates are common. Bronchiectasis and bronchopneumonia may be present in severe cases.
The larynx should be examined to determine if it is contributing to the dyspnea and determine its functional capacity. In one study, 30% of the patients with collapsed trachea also had laryngeal paresis or paralysis. Appropriate therapy for tracheal infection is based on the results of bacterial culture-sensitivity tests and cytological findings. sheathed swabs should be used to obtain material for tracheal culture since contamination by pharyngeal organisms is likely. Cytologic samples can be collected with unsheathed swabs.
Dogs with Grade I tracheal collapse are usually managed medically. However, the use of bronchodilators, expectorants, sedatives and digitalization is considered by some to be palliative and transient. Dogs with collapses graded II through IV at any level of the trachea should be treated surgically.
Four basic methods have been devised to surgically correct a collapsed trachea. Dorsal tracheal membrane plication has been used with reasonable success in patients with collapses of Grades I or II. In this method the dorsal aspect of the trachea is exposed and the lax tracheal membrane is plicated with a 3-0 or 4-0 monofilament nonabsorbable interrupted horizontal mattress sutures. This prevents the redundant membrane from sagging into the tracheal lumen and obstructing air flow. Use of the plication technique in severe tracheal collapse causes severe narrowing of the trachea as the tips of the dorsoventrally flattened rings are drawn toward each other to assume a shape. With complete dorsoventral flattening of the rings, approximately 25% of the existing circumference is lost in the plication technique. This results in approximately a 44% decrease in the potential cross-sectional area of the collapsed trachea. Therefore plication should be reserved for those patients with reasonable cartilage development and tracheal stenosis caused by dorsal membrane laxity (Grades I-II).
The use of internal stents as a method of long-term internal support of a weak trachea has not been successful. These stents tend to become dislodged, become obstructive or be coughed out. Internal tracheal support as an emergency method for maintaining an airway can be achieved with an endotracheal tube placed through a tracheostomy site. The tube should be passed distally to a point that relieves respiratory distress. A Montgomery T-tube made of soft Silastic tubing may be used to support the trachea and larynx post-surgically. The tube is slightly smaller than the internal diameter of the trachea and is a non-cuffed tube placed through a tracheostomy site. Mucosal damage is prevented by a good fit, soft consistency, light weight and good stability. The tube is seldom changed and is easily cleaned while in place.
The external ring prosthesis is currently the technique of choice because tracheal diameter can be restored without significantly interfering with the blood or nervous supply or the mucociliary apparatus. The rings are placed on the external surface of the trachea and help maintain a normal conformation during coughing and respiration. The split rings are placed 1-3 rings apart to provide flexibility needed for tracheal movement. These rings are implanted without interfering with the vascular and nervous supply of the larynx, trachea or bronchi. Polyproylene syringe case and Teflon tubing have been the source of the prosthetic rings of varying widths (4.8-9.5 mm wide). The internal diameters are selected according to the size of the trachea around which they are placed. These rings have been adequate for the trachea, but are too coarse for the mainstem bronchi. A flexible porous polypropylene is used for these small airways Prosthetic rings are made by cutting a 3 cc syringe case into cylinders approximately 5-8 mm wide. A longitudinal 2-3 mm gap is cut in each ring to facilitate placement around the trachea. 4-6 holes of 2 mm diameter are drilled through the rings for suture placement. The edges of the rings should be smoothed to minimize tissue reaction.
The patient is positioned in dorsal recumbency and a ventral midline skin incision is made from the larynx to the sternal manubrium. Subcutaneous tissues and the sternohyoid muscles are separated and retracted laterally to expose the collapsed trachea. Branches of the thyroid arteries and recurrent laryngeal nerves are bluntly dissected from the trachea only in the areas of ring placement. The ring is carefully positioned between the trachea and the recurrent laryngeal nerves. The trachea is sutured to the ring with a 3-0 or 4-0 absorbable suture. Each suture is placed around a tracheal cartilage or through the dorsal tracheal membrane before it is passed through a hole in the ring. Three to four sutures per ring are placed around the cartilage and at least one suture is placed through the tracheal membrane. Sutures enter the tracheal lumen so care must be taken to avoid passing sutures through the endotracheal tube cuff. Rings should be placed 5-10 mm apart. Five to six prosthetic rings are usually required to reinforce the trachea. Following placement of the final ring the surgical site is flushed with warm saline solution. The sternohyoid and subcutaneous tissues are closed with absorbable sutures. The skin is apposed with a nonabsorbable suture.
Post-operative care includes antibiotics for five to ten days and corticosteroids for at least three days. The dog frequently has a persistent cough during the healing phase. Post-operative complications include hemorrhage at suture sites, slight to moderate peritracheal swelling and bruising of the skin around the incision. Recurrent laryngeal nerve damage may cause laryngeal spasm and respiratory distress.
Post-operatively, coughing and dyspnea due to tracheal collapse are significantly improved. A persistent cough post-operatively should be cause for re-examination as there may be concurrent pulmonary or cardiac disease. The patient with severe tracheal collapse (Grades III or IV), chronic lung disease and cor pulmonale should improve following surgery. Most dogs cough less, breathe easier, are more active and have fewer episodes of tracheobronchitis.