Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Two The Thorax 111
tumors, or from heart base masses. Dorsal displacement of the trachea immediately cranial to
the bifurcation may be due to right atrial, heart base, or tracheobronchial lymph node masses.
Dorsal displacement of the trachea caudal to the tracheal bifurcation occurs with left atrial
enlargement. Ventral displacement in that region may be due to esophageal enlargement,
periesophageal caudal mediastinal masses, or tracheobronchial lymph node enlargement. The
degree of ventral main stem bronchial displacement is more severe with lymph node enlargement
than with esophageal lesions. Many esophageal lesions displace both the heart and trachea,
while lymph node enlargement affects mostly the trachea and main stem bronchi. Left
ventricular enlargement will elevate the trachea at the bifurcation.
Mediastinal shifts away from the midline may be detected easily by identifying the trachea’s
position. Whether the shift is due to a mass pushing in the direction of the shift or
to the collapse of the lung lobe with shifting toward the collapsed side cannot be determined
solely from tracheal position.
Dorsal displacement of the cranial thoracic trachea may be observed on the recumbent
lateral view when pleural fluid is present. This may be due to the displacement that accompanies
a cranial mediastinal mass, but it is more often due to a shift in the position of the
lungs that occurs in the presence of pleural fluid (or even air). Compression of the tracheal
lumen indicates that a mass is present within the pleural fluid. In the absence of tracheal
compression, a mass may or may not be present.
Tracheal displacement to the left is prevented by the aorta, but it can occur when a right
aortic arch is present. Mediastinal shifts that include the heart will permit leftward tracheal
displacement. Right-sided displacement of the trachea between the thoracic inlet and
bifurcation is seen in many normal dogs and is more pronounced in brachycephalic breeds.
True right-sided displacement of the trachea also may occur with heart base masses and
esophageal enlargement. Most ventral mediastinal masses displace the trachea dorsally and
do not move it to either the right or the left.
The tracheal bifurcation may be displaced caudally. In the cat, the tracheal bifurcation
almost always is visualized at the sixth intercostal space, and any deviation caudal to this
strongly suggests the presence of a cranial mediastinal mass. In dogs, the tracheal bifurcation
occurs at the fifth or sixth intercostal space. The possibility of a cranial mediastinal
mass should be considered if there is displacement caudal to the sixth intercostal space.
Tracheobronchial lymphadenopathy, especially when associated with chronic disease,
may cause narrowing as well as displacement of the trachea and main stem bronchi. These
are reliable signs of lymphadenopathy, which can be seen despite obliteration of the lymph
node margins by a pulmonary infiltrate.
T R AC H E A L C O L L A P S E
The tracheal diameter may change very slightly with respiration and coughing in normal
animals. 204 When marked luminal narrowing occurs during normal or forced respiration,
tracheal collapse is present. Although dorsoventral narrowing is most common, congenital
side-to-side, or lateral, narrowing may occur. The extent and location of the narrowing will
vary. Narrowing of the caudal cervical trachea is more common at inspiration; at
expiration intrathoracic tracheal narrowing usually is seen (Figs. 2-87 and 2-88). The narrowing
may extend through the thoracic inlet, which probably accounts for some variation.
Left atrial enlargement may cause narrowing or collapse of the tracheal bifurcation or main
stem bronchi (Fig. 2-89).
Both inspiratory and expiratory recumbent lateral radiographs that include the entire
trachea should be obtained if tracheal collapse is suspected. Inducing a cough will accentuate
the collapse; however, obtaining a radiograph at the time of the cough may be difficult.
Fluoroscopic examination is useful in evaluating and documenting tracheal collapse.
Contrast studies using intratracheally administered barium or water-soluble iodinated
contrast agents have been described. These rarely provide additional useful information
beyond that obtained from carefully positioned, properly exposed, survey radiographs.
202,203
A lateral radiograph obtained while the animal’s head and neck are dorsiflexed is recommended
for evaluation of tracheal collapse. 204 This view accentuates the tracheal narrowing
and may be useful, especially relative to the tracheal diameter at the thoracic inlet.