Thoracic radiography is an important part of the diagnostic work up in dogs and cats with respiratory distress. The radiographic examination can help to differentiate upper versus lower airway obstruction as well as ruling out other causes of respiratory problems. Altered airway dynamics have particular radiographic features which will be discussed in this lecture with case examples.
Radiographic Examination Technique
Dogs and cats with respiratory distress that exhibit stridor require radiographic examination of the cervical region, including the larynx, in addition to the orthogonal views of the thorax. Radiographs are performed without sedation in most cases when respiratory distress is present. Furthermore, patients should not be excessively stressed to obtain radiographs. Ideally, lateral and VD or DV projections of the thorax are required. The same is true for the cervical trachea. Dorsoventral projections of the thorax are usually better tolerated in distressed patients compared with the ventrodorsal one. The craniocaudal tangential view of the thoracic inlet can be used to show the tracheal lumen in cross section in order to appreciate its shape and extent of collapse. The fluoroscopic examination, if available, is very useful for demonstrating the dynamic nature of airway collapse during inspiration and expiration. If not available, static lateral radiographs during inspiration, expiration, or during coughing episodes can be performed to demonstrate the changes in luminal diameter of the extrathoracic and intrathoracic trachea.
Dynamic Changes with Inspiration and Expiration
The tracheal lumen diameter should not change during inspiration and expiration in a normal animal. Changes in the diameter during inspiration and expiration can occur if weakening of the tracheal rings or luminal obstruction are present or if excessive respiratory effort occurs during the radiographic examination. Incidental narrowing of the trachea is common and involves the soft tissue opacity at the dorsal border of the trachea due to the trachealis muscle. This finding is rarely associated with clinical signs. When the airways are weakened or obstructed due to disease, the changes in pressure within and around the airways can lead to obstruction either during inspiration or expiration. Due to positive atmospheric pressure surrounding the cervical trachea compared with the negative pressure within the thorax, inspiratory tracheal collapse is seen in the cervical portion of the trachea. Expiratory collapse occurs in the intrathoracic portion due to the decreasing negative pressure on the airways during expiration.
Upper Airway Obstruction
Obstructions between the larynx and carina lead to upper airway obstruction. Foreign bodies or masses can lead to obstruction, however tracheal collapse is more common. Increased inspiratory efforts create enough negative pressure within the trachea to create a collapse if the walls of the trachea are weakened. Inspiratory radiographs demonstrate collapse of the cervical portion of the trachea. Expiratory views show collapse of the intrathoracic portion of the trachea, with a normal cervical portion. Clinically significant collapse is diagnosed when 80% or more of the lumen is collapsed. The dorsal border of the trachea is typically displaced ventrally and leads to the collapse. This is the site of the open end of the C-shaped cartilage where the trachealis muscle is located. Upper airway obstruction at the larynx and cervical trachea can lead to the following radiographic signs depending on the severity: reduced lung volume, cranial displacement of the diaphragm, enlarged cardiac silhouette, dorsal displacement of the xyphoid and craniodorsal displacement of the costal arch.
Lower Airway Obstruction
Luminal obstruction, narrowing, constriction and collapse of the bronchi lead to obstruction of in- and outflow of air. Obstruction may be intraluminal, extramural or intramural. Obstructions can also be complete or incomplete. Causes include intraluminal foreign bodies, mucous plugs, exudates, allergic bronchitis and asthma. Extraluminal compression is caused by an enlarged left atrium, perihilar lymphadenopathy, pulmonary masses or congenital hypoplasia. Dynamic obstructions occur also, as with upper airway disease. This is due to a one-way valve effect. Air trapping follows and over-inflation of the lung results. A number of radiographic features can be detected. The lung fields will appear hyperlucent and the pulmonary vessels will be small, as will the heart. The diaphragm will be caudally displaced and possibly flattened and the rib spacing will be increased. Bronchial constriction can create air trapping, such as in allergic bronchitis and asthma. In cats, a common finding is collapse or atelectasis of the right middle lung lobe. This lobe is easily affected by airway diseases due to its high surface area to volume ratio compared with other, larger lobes. This should not be confused with aspiration pneumonia which is not associated with a decreased lung volume. Complete blockage of a major lobar bronchus or multiple smaller airways can result in atelectasis which appears as a lobar alveolar pattern and small lung volume. Bronchiectasis is an end-stage, chronic, irreversible bronchial disease that leads to peripherally enlarged, saccular or cylindrical dilations of the bronchi that appear as large, tortuous air-filled tubular structures extending to the periphery of the lung. The changes are usually generalized.