Page 315 - Small Animal Internal Medicine, 6th Edition

CHAPTER 20
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Diagnostic Tests for the

Lower Respiratory Tract

THORACIC RADIOGRAPHY The dog or cat should be restrained adequately to
prevent movement, and a short exposure time used. Radio­
GENERAL PRINCIPLES graphs should be taken during maximum inspiration. Fully
Thoracic radiographs play an integral role in the diagnostic expanded lungs provide the greatest air contrast for soft tissue
evaluation of dogs and cats with clinical signs related to the opacities, and motion is minimized during this phase of the
lower respiratory tract. They are also indicated for the evalu­ respiratory cycle. Radiographic indications of maximum
ation of animals with vague, nonspecific signs of disease to inspiration include widening of the angle between the dia­
detect occult pulmonary disease. Thoracic radiographs can phragm and the vertebral column (representing maximal
be helpful in localizing disease processes, narrowing and expansion of caudal lung lobes); a lucent region in front of
prioritizing the differential diagnoses, determining the the heart shadow (representing maximal expansion of the
extent of disease involvement, and monitoring the progres­ cranial lung lobes); flattening of the diaphragm; minimal
sion of disease and response to treatment. contact between the heart and the diaphragm; and a well­
A minimum of two views of the thorax should be taken delineated, nearly horizontal vena cava. Radiographs of the
in all dogs and cats, but three views will improve the detec­ lungs obtained during phases of respiration other than peak
tion of lesions. Right lateral and ventrodorsal (VD) views inspiration are difficult to interpret. Incomplete expansion of
are standard. The addition of the left lateral view improves the lungs can cause increased pulmonary opacities to be seen
the sensitivity of radiographs in the detection of disease that appear pathologic, resulting in misdiagnosis.
of the right middle lung lobe, single nodules or metastatic Animals that are panting should be allowed to calm down
disease, and other subtle changes. The side of the lung away before thoracic radiographs are obtained. It may be neces­
from the table is more aerated, thereby providing more sary to sedate some animals.
contrast for soft tissue opacities, and is slightly magni­ All structures of the thorax should be evaluated system­
fied compared with the side against the table. Dorsoven­ atically in every animal to enhance diagnostic accuracy.
tral (DV) views are taken to evaluate the dorsal pulmonary Extrapulmonary abnormalities may develop secondary to
arteries in animals with suspected heartworm disease, pul­ pulmonary disease and may be the only radiographic finding
monary thromboembolism, or pulmonary hypertension to (e.g., subcutaneous emphysema after tracheal laceration).
enhance contrast of the dorsally oriented vessels. Patients Conversely, pulmonary disease may occur secondary to
in respiratory distress are evaluated with DV, rather than other evident thoracic diseases, such as mitral valve insuffi­
VD, views to minimize stress. Horizontal beam lateral ciency, megaesophagus, and neoplasia of the body wall.
radiographs with the animal standing can be used to eval­
uate animals with suspected cavitary lesions or pleural TRACHEA
effusion. The trachea and, in young animals, the thymus are rec­
Careful technique is essential to ensure that thoracic ognizable in the cranial mediastinum. Radiographs of
radiographs are obtained that yield useful information. the cervical trachea are obtained from dogs and cats with
Poor technique can lead to underinterpretation or overinter­ suspected upper airway obstruction or primary tracheal
pretation of abnormalities. Appropriate exposure settings disease, including tracheal collapse (tracheal malacia) in
should be used and the settings recorded so that the same dogs. During evaluation of the trachea, it is important to
technique can be used when future images of the patient obtain radiographs of the cervical portion during inspiration
are obtained; this allows for more critical comparison of and of the thorax during both inspiration and expiration to
progression of disease. Radiographs should be interpreted identify dynamic changes in luminal diameter. As previously
using a large, high­resolution monitor and dim ambient discussed, avoid over­reading lung lesions in the expiratory
lighting. exposures.

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