Imaging of acute thoracic disease (Proceedings)
Obvious thoracic disease need not be discussed here. Fulminating heart failure, pneumonia, advanced metastatic lung disease, severe pneumothorax and pleural effusion are reliably diagnosed radiographically. What I would like to share with you are the less obvious radiographic manifestations of acute cardio-respiratory disease and my approach to diagnosis. And remember, in the final assessment much can be learned from the presenting clinical signs. Does the patient have a cardiac murmur or history of heart disease? Was there an observed or suspected trauma?
Radiographic evaluation of a patient presented for acute dyspnea can be categorized into 4 basic components: Pulmonary disease, cardiac disease, disease of the pleural space, and upper airway disorders.
Presented will be cases of-Heart failure in the dog and cat
-Several cases of lung disease
-Pulmonary thromboembolism (PTE)
-Pneumothorax (and several important artifacts which mimic pneumothorax)
-Effects of positioning (VD vs. DV) on the appearance of pleural fluid
-Effects of upper airway obstruction in dogs and cats
First, the lungs. Assess the lungs for obvious pathology that could account for acute respiratory distress. Is there pulmonary consolidation (airless lung)? Interstitial lung disease (increased opacity without consolidation)? Are there abnormalities of the pulmonary blood vessels (too big, arterial-venous mismatch, hypoperfusion)? Is there striking airway oriented pathology? Are the lungs hyperinflated? Is there a notable lack of pulmonary pathology present on the radiographic study (more on this later)?
One distinction I try to make when assessing abnormal lung is to decide if cardiac failure could be responsible for the lung pathology or is the lung disease non-cardiogenic in origin? This is not always easy, but is a critical fork in our thought process. Assessment of the cardiovascular system is rather straightforward. Is the heart too large, too small? Is it misshapen? Are the pulmonary blood vessels normal for size, shape, contour, and are the arteries and veins similar in size?
Radiographic findings supporting heart failure include enlargement of the heart. In most canine cases the cardiac silhouette will be enlarged. The enlargement may be generalized or specific chamber or great vessels enlargement may be present. Further, it is critical to assess the pulmonary blood vessels for enlargement or arterial-venous mismatch, understanding that not all cases of heart failure show these alterations. In cats, differentiating between cardiogenic pulmonary disease and primary pulmonary disease can be much more challenging because cats may be in heart failure and have a normal heart size and shape on the radiographs. Understanding this potential limitation underscores the importance of an echocardiogram in many cases presented for acute respiratory disease. Generally speaking, if the lung is normal, the heart is not failing.
If heart failure is ruled our or thought unlikely, then we must investigate potential causes of the lung disease. Severity and distribution plays a key role here. Most ventrally located disease is some form of pneumonia. Dorsocaudal infiltrates may indicate a hematogenous origin, such as lung infarction from pulmonary thromboembolic disease, hematogenous sepsis, and occasionally overwhelming inhalation pneumonia from viral or fungal causes. Dorsocaudal lung disease can also indicate non-cardiogenic causes of pulmonary edema such as neurogenic disease (seizures, electrical cord shock) and vasculitis of various etiologies such as pancreatitis.
If the lungs appear normal, critical assessment of the pulmonary vascular is paramount, in particular the pulmonary arteries. Specifically, be thinking about pulmonary thromboembolism. In addition to the possible detection of small, attenuated pulmonary arteries, the regional lung parenchyma may appear reduced in opacity due to decreased blood flow.