Clinical Approach

Accurate diagnosis requires integration of clinical signs with medical history, physical examination, and imaging findings, since many conditions contribute to coughing and dyspnea. Coughing is a sudden expiratory effort resulting in noise with expulsion of air. The coughing reflex is initiated throughout the upper and lower respiratory system (pharynx, larynx, tracheobronchial tree, and small airways). Dyspnea or respiratory distress refers to difficult or labored breathing. Severity may be judged by assessing breathing effort, respiratory rate, rhythm, and character.

Thoracic radiography is the most useful first-line diagnostic test since it portrays the cardiac silhouette, airways, and lung parenchyma.

Imaging the Chest, Neck, and Head

Radiographs help confirm or exclude clinical impressions, support or reject specific diagnoses, and provide important information not otherwise suspected. They help to screen for cardiopulmonary, systemic, and metabolic disorders and assist to formulate initial treatments. Repeat radiographs (using the same radiographic technique and positioning as in initial exposures) supply useful comparative data. Cross-sectional imaging of the chest, neck and head with CT, magnetic resonance imaging, or ultrasonography delineates mediastinal, hilar, and pleural pathologies and identifies pharyngeal and nasal lesions.

Computerized Tomography

CT can demonstrate various lung disorders: lung cancer, pneumonia, emphysema, bronchiectasis, inflammation or other pleural diseases, and diffuse interstitial lung disease. A CT angiogram evaluates arteries, veins, and cardiac structures. CT is capable of identifying intra- and extracavitary masses. CT uses shorter anesthesia time vs. MRI and is less liable to motion artifact associated with MRI.

Magnetic Resonance Imaging (MRI)

MRI produces detailed images of organs, soft tissue, bone, and internal structures. It can assess masses including pulmonary neoplasia which cannot be assessed adequately with other imaging modalities (typically CT). It helps determine tumor size, extent, and the degree of metastasis, assess cardiac anatomy and function and its structures, determine blood flow dynamics, display lymph nodes, assess vascular and lymphatic malformations, and assess extracardiac abnormalities (vertebrae, ribs, sternum, chest wall lesions). Magnetic resonance angiography (MRA) is helpful to assess vasculature. Disadvantages of MRI include its longer anesthesia time vs. CT and challenges in cardiac monitoring.

Thoracic Radiography

Good-quality chest films are essential for accurate diagnosis and effective management.

Radiographic Technique

Films should be exposed at peak inspiration using a high kVp/low mAs technique. Poorly inflated lungs will appear increased in density (i.e., 'whiter'). Breed conformation, state of respiration, obesity, relative state of hydration, stage of cardiac cycle, positioning errors and effusions alter radiographic appearances. Overexposure causes loss of important information; underexposure causes overinterpretation of lung fields.

The patient should be correctly positioned (superimpose the spine and sternum on the VD/ DV and adjust the animal in the lateral view so that the sternum and spine are equidistant to the table top, the costochondral junctions and ribs are superimposed, the front legs are drawn forward). Align with the primary beam centered approximately at the 5th–6th intercostals. Oblique views will greatly distort the cardiac silhouette. Avoid motion artifact by using short exposure times (< 1/30th sec). The ventrodorsal (VD) radiograph is advantageous when pleural effusion is present, since free fluid gravitates along the paravertebral gutters and does not superimpose over the heart, as occurs with the DV view.

Radiographic Interpretation

Thoracic Wall

The chest wall includes the spine, ribs, sternum and related soft tissues, and is framed by the caudal cervical vertebrae cranially, and diaphragm caudally. Evaluate symmetry in both views (altered by pectus excavatum, scoliosis, trauma).

Mediastinum

These are potential spaces between cranial and caudal pleural cavities. In the cranial mediastinum lie the heart, ascending aorta, main pulmonary artery, cranial vena cava, thoracic duct, nerves, trachea, esophagus, lymph nodes, and thymus. In the caudal mediastinum are the posterior vena cava, trachea, descending aorta, nerves, and lymph nodes. Because the mediastinum communicates with fascial planes of the neck and retroperitoneal space, pneumomediastinum may result in contrast and thus, visualization, of mediastinal structures, as well as subcutaneous edema or pneumoretroperitoneum. Widened cranial mediastinum may result from lymphadenopathy, thymoma, megaesophagus, neoplasia, or abundant mediastinal fat.

Pleural Space

This potential space located between the parietal pleura and visceral (pulmonary) pleura is occupied by the lungs. Pleural thickening may allow visualization of pleural fissures. Diseases which increase pleural space opacity include pleural masses and effusions. Occasionally, effusion is loculated or trapped and involves the region of a cranial lung lobe or right middle lung lobe. Small volumes of free pleural effusion may cause blunting (rounding) of the costophrenic angles, accentuation of pleural fissure lines, and might be best visualized on the DV projection. Chronic effusions may cause pleural fibrosis. Pneumothorax decreases pleural space opacity. Overinflation can mimic pneumothorax.

Diaphragm

Altered diaphragmatic symmetry may occur with diaphragmatic or peritoneopericardial hernia. Diaphragmatic hernia and pleural effusion may obscure the diaphragmatic border.

Abnormalities in Cardiac Size and Shape

The cardiac silhouette is affected by breed and body conformation as well as disease condition. Overestimating heart size is common with barrel-chested dogs. In deep-chested dogs, the cardiac silhouette is 'tall.' In obesity, pericardial fat can cause the cardiac silhouette to appear larger. The cardiac silhouette may assume a more horizontal position in geriatric cats and barrel-shaped dogs. Pleural effusions may obscure the cardiac silhouette. Cardiomegaly usually results from congenital or acquired lesions causing volume overload (e.g., valvular insufficiency or shunts), pressure overload (e.g., valvular stenosis), myocardial disease (e.g., cardiomyopathy), pericardial disease, or respiratory conditions (e.g., cor-pulmonale).

Radiographic Lung Patterns

Lung patterns can be graded as follows:

 Alveolar lung patterns - indicate alveolar collapse or filling with blood, pus, or water. Typical findings include 1) patchy, poorly defined, increased densities with fluffy, indistinct margins which tend to coalesce, 2) air bronchograms and 3) silhouetting of pulmonary vessels and bronchial walls by lung alveoli and interstitium containing fluid. Alveolar patterns are typically fluffy and indistinct, and coalesce. Cranioventral distribution is most associated with bronchopneumonia; perihilar distribution (in dogs) is most associated with CHF. Noncardiogenic edema usually occurs in dorsocaudal lung fields. Diffuse or patchy alveolar distribution may be seen with bronchopneumonia, pulmonary edema, hemorrhage (often lobar), and atelectasis.

 Interstitial lung patterns - one form includes increased nodular densities having distinct, well-defined margins (e.g., neoplasia, chronic granulomas). The second causes nonspecific, localized or generalized interstitial "grayness" (e.g., pulmonary edema, pulmonary fibrosis, some neoplasia, interstitial pneumonia or hemorrhage); vasculature and bronchi are blurred.

 Bronchial patterns - result when bronchial walls become more opaque when thickened or surrounded by fluid or cellular infiltrate. Bronchial disease may progress to bronchiectasis that appears as thin-walled, cylindrical or saccular bronchial dilatation with enlarged bronchial lumens that lose their distal tapering; emphysema appears as saccular or coalescing airways.

Radiographic Vascular Patterns

Cranial lung lobe vessels assessed from the lateral projection show that arteries are dorsal and veins are ventral to related bronchi. Caudal lobar vessels assessed from the VD or DV view show arteries are lateral and veins are medial to associated bronchi. Normally, arteries and veins are approximately the same size.

 Hypervascularity refers to arteries and/or veins which may be enlarged together in states of increased pulmonary blood flow (left-to-right shunts), high output states (thyrotoxicosis, severe anemia, fluid overload), left-sided CHF from severe mitral insufficiency or canine dilated cardiomyopathy (i.e., chronic pulmonary venous dilatation with secondary pulmonary hypertension).

 Increased pulmonary artery size and shape suggest pulmonary hypertension (usually dirofilariasis; occasionally, right-to-left shunts, idiopathic pulmonary hypertension). Pulmonary venous congestion is associated with left-sided CHF.

 Hypovascularity (hypoperfusion or under-circulation) creates thin arteries, veins and radiolucent interstitium and may accompany low cardiac output (shock, dehydration, caval syndrome, cardiac tamponade, acute blood loss, restrictive pericarditis, hypoadrenocorticism, severe myocardial failure), or right to left shunts.

Esophagus

A small amount of gas is often present in the mid-thoracic esophagus. Caudal thoracic esophagus may be visualized in left-lateral recumbency as a soft tissue or fluid-filled structure. Aerophagia or anesthesia can result in a gas-filled, distended esophagus.

Trachea

Collapsing trachea (often also referred to as large airway disease) is a dynamic condition best demonstrated using fluoroscopy.

Spinal Vertebrae

Older dogs often display narrowing of the thoracic intervertebral disc spaces.

Sternebrae

Pectus excavatum causes cardiac shift (VD or DV view). Sternebrae malformations may accompany other congenital anomalies (e.g., peritoneopericardial diaphragmatic hernia).