Lower airway obstructions result in expiratory dyspnea with auscultable expiratory wheezes. Causes of such obstructions include bronchoconstriction/spasms, inflammation of the bronchial walls (feline asthma, chronic bronchitis), intraluminal exudative/mucoid debris, intrathoracic tracheal collapse and bronchomalacia, or bronchial compression secondary to left atrial enlargement.
Thoracic wall disorders (eg, flail chest, “sucking” chest wounds) can lead to paradoxical respiration with the affected area of the thorax collapsing inward on inspiration and forced outward on expiration.
Pleural space disorders, such as pleural effusion or pneumothorax, can lead to rapid, shallow breathing patterns with inspiratory distress.
Pulmonary parenchymal disorders, such as pneumonia (Figure 1), edema (cardiogenic or noncardiogenic), pulmonary contusions, interstitial lung disease, and neoplastic or fungal infiltration (Figure 2), can lead to both inspiratory and expiratory difficulty. The presence of heart murmurs, gallops, or arrhythmias may suggest underlying heart disease but not necessarily congestive heart failure.
Congestive heart failure can lead to activation of the sympathetic nervous system, which almost invariably results in tachycardia. Primary respiratory disease often induces a vagal response leading to normal sinus rhythms, sinus arrhythmias, or sinus bradycardias.
Pulmonary vascular disorders are most commonly pulmonary thromboembolism and heartworm disease. Clinical signs of pulmonary vascular diseases are variable and include hemoptysis, coughing, dyspnea, and syncope. Other clinical signs may be noted and attributed to the predisposing disease process.1 Split heart sounds may be heard due to concurrent pulmonary hypertension.
Severe abdominal distension can impair diaphragmatic contraction, leading to inspiratory distress that is typically characterized by a slow and exaggerated pattern. Differentials include ascites, gastric dilatation-volvulus, organomegaly, and pregnancy.
Figure 2: Right lateral (A) and ventrodorsal (B) radiographic projections of the thorax of a dog presenting for progressive coughing and acute dyspnea. A diffuse structured interstitial lung pattern is present. Primary differentials would include metastatic neoplasia and fungal disease.