Page 316 - Small Animal Internal Medicine, 6th Edition

288 PART II Respiratory System Disorders

Only the inner wall of the trachea should be visible. Vis­ be narrowed in the cervical region during inspiration. In
ibility of the outer wall of the trachea is suggestive of pneu­ animals with intrathoracic tracheal collapse, the air stripe
VetBooks.ir momediastinum. The trachea normally has a uniform would be narrowed within the thorax during expiration.
In reality, a diagnosis of tracheal collapse may be missed
diameter and is straight, deviating ventrally from the verte­
bral bodies on lateral views as it progresses toward the carina.
tial is insufficient for a dog lying on the radiology table to
It may appear elevated near the carina if the heart is enlarged simply because the intra­ to extra­airway pressure differen­
or if pleural effusion is present. Flexion or extension of the create visible narrowing of the trachea. Further, in the cervi­
neck may cause bowing of the trachea. On VD views, the cal trachea, a soft tissue opacity extending along the dorsal
trachea may deviate to the right of midline in some dogs. margin of the trachea may represent either abnormal sagging
The tracheal cartilage becomes calcified in some older dogs of the dorsal tracheal membrane or overlying esophagus (or
and chondrodystrophic breeds. other soft tissue). Fluoroscopy, available primarily through
The overall size and continuity of the tracheal lumen referral centers, provides a more sensitive assessment of tra­
should also be evaluated. The normal tracheal lumen is cheal collapse.
nearly as wide as the laryngeal lumen. Hypoplastic tracheas
are most often found in English Bulldogs and have a lumen LUNGS
less than half the normal size (Fig. 20.1). The ratio of tracheal The clinician must be careful not to overinterpret lung
diameter to thoracic inlet diameter (TD:TI) can be used to abnormalities on thoracic radiographs. A definitive diagno­
more objectively define tracheal size in these patients. If sis is not possible in most animals, and microscopic exami­
identified in dogs less than 1 year of age, the hypoplasia may nation of pulmonary specimens, further evaluation of the
partially resolve with maturity (Clarke et al., 2011). Stric­ heart, or testing for specific diseases is necessary. The lungs
tures and fractured cartilage rings can cause an abrupt, local­ are examined for the possible presence of four major abnor­
ized narrowing of the air stripe. mal patterns: vascular, bronchial, alveolar, and interstitial.
Mass lesions in the tissues adjacent to the trachea can Mass lesions are considered with the interstitial patterns.
compress the trachea, causing a more gradual, localized Lung lobe consolidation, atelectasis, pulmonary cysts, and
narrowing of the air stripe. The air contrast of the trachea lung lobe torsions are other potential abnormalities. The dis­
sometimes allows foreign bodies or masses to be visualized tribution of lesions within the lungs is also noted. Diseases
within the trachea. Most foreign bodies lodge at the level of airway origin, such as bronchopneumonia and aspiration
of the carina or within the bronchi. The inability to radio­ pneumonia, typically have more severe radiographic disease
graphically identify a foreign body does not rule out the affecting the gravity­dependent lung lobes (right middle and
diagnosis, however. cranial, and/or left cranial lobes). Diseases originating from
The radiographic diagnosis of tracheobronchomalacia the vasculature or lymphatics, such as metastatic neoplasia
(tracheal collapse) can be challenging, and radiographic and systemic mycoses, may affect the caudal lung lobes more
signs should be interpreted with some caution. In theory, severely. Animals in severe respiratory distress localized to
the diagnosis should be straightforward. In animals with the lungs by history and physical examination and normal
extrathoracic tracheal collapse, the tracheal air stripe would thoracic radiograph usually have thromboembolic disease or
have suffered a very recent insult to the lungs, such as trauma
or aspiration (Box 20.1).

Vascular Pattern
The pulmonary vasculature is assessed by evaluating the
vessels to the cranial lung lobes on the lateral view and the
vessels to the caudal lung lobes on the VD or DV view. Nor­
mally, the blood vessels should taper gradually from the left
atrium (pulmonary vein) or the right ventricle (pulmonary
arteries) toward the periphery of the lungs. Companion
arteries and veins should be similar in size. Arteries and
veins have a consistent relationship with each other and with
the associated bronchus. On lateral radiographs, the pulmo­
nary artery is dorsal and the pulmonary vein is ventral to the
bronchus. On VD or DV radiographs, the pulmonary artery
is lateral and the pulmonary vein is medial to the bronchus.
Vessels that are pointed directly toward or away from the
X­ray beam are “end­on” and appear as circular nodules.
They are distinguished from lesions by their association with
FIG 20.1
Lateral radiograph of a Bulldog with a hypoplastic trachea. a linear vessel and adjacent bronchus.
The tracheal lumen (narrow arrows) is less than half the size Abnormal vascular patterns generally involve an increase
of the larynx (broad arrows). or decrease in the size of arteries or veins (Box 20.2). The