Before we can start begin to review abnormalities in the lungs lets first do a review of normal anatomy. In the lungs there
are three main structures, the interstitium, the airways, and vessels. To differentiate between the vessels and the bronchi,
remember that bronchi converge together toward the lung periphery as they taper, where as pulmonary vessels diverge. On the
lateral view, the artery is always dorsal, the bronchus is in the middle, and the vein is ventral. To evaluate size of the
vessels, in left lateral recumbency compare the right cranial lobar artery and vein. They should be the same diameter and
each vessel should not exceed the smallest diameter of the right 4th rib. On the VD view, the artery is lateral, the bronchus
again is in the middle and the vein is axial. As on the lateral view with the cranial lobar vessels, the caudal lobar pulmonary
artery and vein are of similar size and in the dog the diameter of these two vessels should not exceed the diameter of the
9th rib. In the cat, we compare these vessels to the10th rib.
We need to initially look at a film and determine if it is of adequate quality to interpret. Initially we evaluate to determine
if the radiograph was exposed during the inspiratory phase of respiration. The area of the lung that is caudal to the heart,
cranial to the diaphragm and ventral to the caudal vena cava should be large in size on an inspiratory film. The lumbodiaphragmatic
angle (where the caudal dorsal lung extends adjacent to the lumbar spine) is caudal to T12 on inspiratory films. Make sure
the film is adequately exposed, as underexposure may simulate disease (non-structured interstitial pattern) while overexposure
may mask disease.
Now we are onto patterns of pulmonary disease. There are three main patterns: alveolar, bronchial, and interstitial. Let us
begin with alveolar. Alveolar patterns represent infiltration of lung to the point of filling of the alveoli. The roentgen
signs are replacement of air with soft tissue opacity and air bronchograms. Air bronchograms represent air within all (or
portions of) a bronchial lumen and the adjacent lung tissue (alveoli) is of abnormal fluid opacity. In long axis these appear
tree-like, while end-on they look like "holes". You are unable to see the outside walls of the vessels or bronchi, and this
is what differentiates air bronchograms from doughnuts seen with bronchial disease. The soft tissue infiltrates can be blood,
pus, or water. We determine the likely etiology of the infiltrates depending on the distribution. Localized alveolar lung
disease can be seen with pneumonia, hemorrhage (due to trauma), atelectasis, neoplasia, infarction, and torsion. Disseminated
alveolar pattern can be seen with severe inflammatory lung disease, severe edema, hemorrhage (due to bleeding disorder), near
drowning, or smoke inhalation. Edema has many etiologies, cardiogenic (concurrent cardiac enlargement is present), neurogenic
(from electric cord bite, following head trauma, post ictal, and from strangulation injury or smoke inhalation. The location
of alveolar infiltrates can also help us to determine etiology. Asymmetric lung infiltrate is common with hemorrhage (contusion
associated with trauma), inflammatory, atypical pneumonia, foreign body pneumonia, bronchoesophageal fistula, and thromboembolism
(often the history includes predisposing renal or adrenal disease). Consolidation of a lung lobe (with infiltrates) will have
no loss of volume and no shift of the mediastinum. By comparison atelectasis will have loss of volume and shift of the mediastinum
toward the alveolar infiltrates. With respect to atelectasis....if a patient is recumbent for at least 30 minutes or longer
atelectasis will occur. If an animal is under anesthesia, positive pressure ventilation is necessary, as atelectasis will
obscure visualization of pulmonary parenchyma. Accidental intubation into a single bronchus also results in atelectasis.
The histories also give clues to the etiology of the pulmonary infiltrates. Pneumonitis can occur from circulating pancreatic
enzymes with pancreatitis, re-expansion pulmonary edema following chronic lung collapse (esp in cats), sepsis with secondary
acute respiratory distress syndrome (ARDS), and pulmonary hemorrhage from disseminated intravascular coagulopathy (DIC).
Bronchial is the next main pulmonary pattern to discuss. Bronchial disease is caused by infiltration of airways by inflammatory
cells, edema, neoplastic cells, or mineral. Chronic bronchitis can be due to allergic, parasitic, or inflammatory etiologies.
Edema creating thickened bronchi is actually a peribronchial infiltrate and is caused by cardiac disease. Neoplasia such as
bronchoalveolar carcinoma has a primary bronchial pattern yet can also have other patterns present. Mineralization of the
bronchi can be due to age or Cushing's disease. The bronchi are increased in visualization when mineralized, but not increased
in thickness. Roentgen signs of bronchial disease include tram lines and donuts.