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Tracheal collapse (Proceedings)

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Apr 01, 2010

Cough is a common reason for dogs to be presented for veterinary care, and tracheal collapse is a common cause of chronic cough in dogs. The tracheal lumen remains open during both inspiration and expiration thanks to the support of cartilaginous rings. In the dog, rings are incomplete dorsally, forming a "C" shape where the dorsal tracheal membrane connects the edges of the C via the longitudinal tracheal muscle and connective tissue. The principal bronchi which diverge from the trachea are formed by complete cartilaginous rings. Principle bronchi rapidly divide into lobar (or secondary) bronchi. Lobar bronchi divide into segmental (or tertiary) bronchi, which in turn branch dichotomously into small bronchi. Branching continues until respiratory bronchioles are formed. The primary function of the trachea is to serve as a conduit to gas flow from the larynx to the bronchi. However, and additional vital function is the removal of particulate and infectious materials. Tracheal collapse may interfere with both conduit and protective functions of the trachea.

Tracheal collapse in dogs may involve compromise of the airway lumen due to redundant tracheal membrane, weakening of the cartilaginous rings, or both. Pathogenesis is unknown, and likely multifactorial. The lumen of the entire trachea may be affected, or only sections may be compromised. The most commonly involved section is found at the thoracic inlet where the cartilage is thinnest. Although the condition is called tracheal collapse, weakened cartilaginous support can result in bronchial collapse (typically mainstem bronchi) along with tracheal collapse.

Severity of collapse varies from minimal to near complete occlusion of the tracheal lumen. The laws of physics dictate that when the lumen of the airway is compromised by 50%, resistance to airflow does not double but rather increases by 16-fold. This means that in a dog with moderate tracheal collapse of 50%, respiratory muscles must increase generated force by 16 times to maintain ventilation.

Tracheal collapse is a dynamic condition. Pressures within the airway lumen decrease during inspiration. Because the negative pressure in the pleural space during inspiration is even greater, the walls of the intrathoracic trachea tend to be held open. Simultaneously, the walls of the cervical trachea tend to collapse since surrounding pressure in the neck is relatively positive. In health, collapse is prevented by the rigid cartilage matrix. During expiration, the reverse process occurs. During expiration the walls of the intrathoracic trachea tend to collapse while the walls of the cervical trachea are forced open by changes in relative pressure.

Clinical presentation

Collapsing trachea is primarily a disease of small dogs (eg, Yorkshire terriers, Pomeranians, Chihuahuas), although large dogs and cats are sometimes affected. Clinical signs are usually manifest as the dog reaches middle age and may become more severe over time. Often, dogs with collapsing trachea have a waxing and waning course with periodic exacerbations of clinical signs. The most consistent clinical sign is cough best described as reminiscent of a "goose honk". Cough is typically precipitated by excitement or exercise and tends to occur in paroxysms. Because the cough may be accompanied by a terminal retch and/or gagging, owners may mistake the cough for vomiting. Other clinical signs include increased respiratory rate and/or effort, exercise intolerance, respiratory distress, or collapse with cyanosis.

Abnormalities on physical examination are usually limited to the respiratory system. A dry cough is usually easily elicited on tracheal palpation, but this characteristic is not pathognomonic. An occasional finding which strongly suggests tracheal collapse is an auditory "click" heard with or without a stethoscope when the large airways collapse together, most often at the end of a cough. Because up to 30% of dogs with tracheal collapse may have laryngeal paralysis, inspiratory stridor may be detected. In severely affected dogs, palpation may reveal a flattening in the shape of the normally cylindrical cervical trachea. When present, respiratory distress may occur on either inspiration, expiration, or both phases of respiration. Cardiac murmur is a common concurrent finding in dogs with tracheal collapse, perhaps because AV valvular endocardiosis is common in the same breeds that develop tracheal collapse most often.