| Radiographic pattern | Differential diagnoses | Sampling technique |
| Interstitial | Viral pneumonia Rickettsial pneumonia Protozoal pneumonia Hemorrhage Vasculitis Pulmonary fibrosis Neoplasia Early pulmonary edema Aspiration pneumonia | Fine‐needle aspirate Lung biopsy Bronchoscopy Tracheal wash |
| Bronchial | Chronic bronchitis Feline bronchitis/asthma Bronchiectasis Parasitic bronchitis Early bronchopneumonia | Tracheal wash Bronchoscopy |
| Alveolar | Bronchopneumonia Aspiration pneumonia Fungal pneumonia Hemorrhage Pulmonary edema Neoplasia Non‐cardiogenic pulmonary edema | Tracheal wash Bronchoscopy |
| Consolidation | Neoplasia Lung lobe torsion Consolidating pneumonia Granuloma Bronchial obstruction Feline bronchitis Foreign body inhalation | Bronchoscopy Fine‐needle aspirate Tracheal wash |
| Vascular | Congenital heart disease Congestive heart failure Heartworm disease Pulmonary hypertension Pulmonary thromboembolism | Echocardiography |
| Effusion | Hydrothorax Pyothorax Hemothorax Chylothorax Neoplasia Diaphragmatic hernia | Thoracocentesis |
Orthogonal views are always recommended for evaluation of thoracic contents, and assessment of the thorax is improved by obtaining both left and right lateral views, as well as a dorsoventral or ventrodorsal image. Lateral views provide an optimized view of the lung closest to the radiographic unit, therefore a left lateral projection would be more likely to identify infiltrates in the right middle lung lobe in a patient with aspiration pneumonia. A right lateral projection might be preferred to investigate airway collapse at the left cranial lobar bronchus (cranial and caudal segments). The dorsoventral view provides better imaging of the cardiac silhouette and pulmonary vessels, although the ventrodorsal view allows better assessment of small volumes of pleural effusion, as well as infiltrates in the ventral or lateral portions of the lung. Importantly, attempts should be made to confirm the presence of pulmonary nodules on both a lateral and an orthogonal view.
In some patients, cervical radiographs can provide valuable information on the extrathoracic respiratory tract and its potential role in thoracic disease. Loss of the nasal air column from the nasal cavity into the nasopharynx, elongation or thickening of the soft palate, the suggestion of laryngeal edema or mass, air in the laryngeal saccules, or caudal retraction of the larynx are clues to the presence of an upper airway obstructive lesion that could be contributing to disordered breathing or a lower respiratory tract process.
Ultrasound
Ultrasound of the larynx can be used to evaluate patients for laryngeal paralysis or mass lesions, and cervical tracheal collapse can also be identified with ultrasound. However, these studies can be technically challenging because soft tissues are adjacent to air‐filled structures, which causes marked attenuation of the ultrasound beam. However, valuable information can be gained by an experienced ultrasonographer.
Thoracic ultrasound is highly useful in detecting pleural fluid and evaluating parenchymal mass lesions, and has also proven effective in evaluating pulmonary infiltrates, particularly in the emergency situation when the clinician needs to distinguish congestive heart failure from a respiratory condition. The presence of fluid in the interstitial–alveolar space in association with pulmonary edema results in the artifact of comet tails, referred to as B‐lines. In dogs and cats with pulmonary edema due to congestive heart failure, there is an increase in the number of detectable B‐lines visible in multiple lung fields (Lisciandro et al. 2017; Ward et al. 2017). Because this technique relies on attenuation of the ultrasound beam by thickening of the interstitial–alveolar junctions, other diseases that result in this process and extend to the lung periphery could lead to the generation of B‐lines on ultrasound. Therefore, the use of lung ultrasound in large numbers of patients with respiratory conditions such as interstitial lung disease, pneumonia, and non‐cardiogenic pulmonary edema requires additional study. Nonetheless, it appears that ultrasound of the lung could be used to evaluate patients for fluid overload or congestive failure and assess response to therapy. This technique would likely be less stressful to the patient than recheck radiographs.
Fluoroscopy
Airway fluoroscopy uses air as the contrast medium to evaluate dynamic changes in the luminal diameter of the airways during normal breathing and during cough. Fluoroscopy is typically performed in lateral recumbency. It most readily recognizes cervical tracheal collapse because of the large airway diameter, but can also provide information on lobar bronchial collapse. A fluoroscopic study performed in right lateral recumbency will highlight the left lung lobes, while a study in left lateral recumbency will examine the right lobar bronchi, although right lateral recumbency appears to be used most commonly. Dorsoventral positioning would likely be required to identify collapse of the accessory lobar bronchus. Fluoroscopy has poorer resolution than radiographs and the lower image quality can hamper interpretation.
Given the intricate relationship between neural pathways of the respiratory and digestive tracts as well as their close anatomic association, videofluoroscopic swallowing studies have a role in investigating the underlying etiology of some respiratory diseases, as well as detecting risk factors for disease in other cases. Esophageal dysfunction and gastroesophageal reflux are common and can lead to chronic recurrent pneumonia or pneumonitis. Increasing evidence indicates that laryngeal paralysis in the geriatric Labrador is accompanied by esophageal dysfunction as part of a generalized neuropathy, and swallowing studies can help assess the risk for lower respiratory complications in affected dogs. Weak pharyngeal contractions in other breeds of dogs also can predispose to aspiration events. Interestingly, videofluoroscopy has revealed that brachycephalic breeds commonly display pharyngeal collapse (Pollard et al. 2018), likely due to anatomic issues and pressure fluctuations associated with respiratory effort. In addition to perpetuating aspiration injury to the lower respiratory tract, nasopharyngeal aspiration events can lead to nasal discharge and might be a cause for upper respiratory tract inflammation.
Computed Tomography
Imaging the respiratory tract with CT provides superior detail in comparison to standard radiography because of the removal of superimposed structures and the ability to reformat images in three dimensions. When a helical CT is available, image acquisition is very fast; however, anesthesia is employed in many instances to obtain optimal images. Some facilities perform CT in awake or sedated patients using a plexiglass holding chamber (VetMousetrap™; Oliveira et al. 2011). Motion artifact can obscure detail, particularly when respirations interfere with the evaluation of thoracic structures. It is a clinical decision whether the risks of anesthesia outweigh the value of images obtained, but often an intervention is planned after CT (e.g., endoscopy or surgery) which will also require anesthesia. For the thorax, the animal is typically ventilated several times and a breath hold at 15 cm water (H2O) is employed to expand the chest during the 10–30 seconds of image acquisition.
Airway Sampling
Transoral Tracheal Wash
A transoral tracheal wash is appropriate for use in large and small dogs or in cats. A sterile endotracheal tube and a sterile polypropylene or open‐ended red rubber catheter (3.5–8 French) are needed. Prior to doing a tracheal wash, the