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.

Pulmonary Vasculature

The pulmonary arteries can be identified arising from the main pulmonary artery on the lateral projection. They are course dorsal to the lobar bronchi, while the pulmonary veins are ventral to the bronchi. The cranial and middle lung lobe arteries and veins can usually be differentiated on the lateral radiograph, but the caudal lobar vessels cannot, as they overlap one another. On the DV or VD radiographs, the lobar arteries are lateral to the major bronchi, the pulmonary veins are medial. The DV view is the best view to assess the caudal pulmonary artery and vein pairs. Tertiary branches of the pulmonary vasculature can often be seen in the larger breeds of small animal species and in the large animal species.

Rule-of thumb for pulmonary blood vessel size (unreliable, but I know you are keen to know)

Cranial pulmonary artery and veins should be no larger than the width of the 4th rib on the lateral view. The caudal artery and vein pair should be no larger than the width of the 9th rib.

The Pleural Space

If the lungs are normal radiographically, we can place primary lung disease and heart failure low on the differential list. From here, assess the pleural space. Pleural space is assessed critically for retraction of lung lobes away from the parietal pleura, as with a pneumothorax or pleural effusion. Is there evidence of pleural effusion or pneumothorax? Pneumothorax should be easily seen with high quality radiographs. A small (mild) pneumothorax is usually not life threatening, but always be asking, "why did the pneumothorax occur? Pitfalls in misinterpretation of a pneumothorax include "heart elevated off the sternum" in a left lateral recumbent view, in which this sign alone is misconstrued as a pneumothorax when in reality it is simply the apex of the heart shifted away from midline. In some cats that are hyperinflated, the heart can be "elevated" from the sternum as the lungs become so inflated that they literally cradle the heart ventrally. Another commonly misinterpreted radiographic sign is the presence of an overlying skin fold, mimicking a pneumothorax.

Pleural fluid is usually easily detected. However, it can be difficult to diagnose in two specific circumstances. The first is when there is a small amount of effusion present, the second is when a ventrodorsal (VD) view is obtained, which can easily "hide" free fluid and therefore a proper diagnosis is missed.

Specific Comments Regarding the Pleural Space

The pleural space is in general only considered a potential space. That is, under normal circumstances, there is very little actual volume and pleural space contains only a few mils of pleural fluid for lubrication. Abnormalities of the pleural space most commonly involve abnormal accumulations of air or fluid, or the presence of mass lesions.

Pneumothorax

A pneumothorax is defined as free air trapped within the pleural space. Major causes for pneumothorax include the following.

Injury or disease of one or more lung lobes, resulting in intrapulmonary gas escaping into the pleural space. Ruptured congenital pulmonary bullae can cause a spontaneous pneumothorax.

Injury to the trachea or bronchi, in which the mediastinum is also ruptured. This will result in pneumothorax, as well as pneumomediastinum, or air trapping within the potential space of the mediastinum produced by the parietal pleural reflections.

Penetrating chest wounds which lead to an open pneumothorax, or a pneumothorax in which a direct conduit between the pleural space and the external environment is present.

Rarely, the intrathoracic esophagus can be damaged with ingested air accumulating within the pleural space. Disruption of the surrounding mediastinal margins is also required for this rare cause of pneumothorax to occur.

Radiographic Findings

When air fills the pleural space, the visceral pleura of the lung lobes is retracted away from the thoracic wall. Radiographically, the lung lobe margins can be identified peripherally. Lung contours consistent with the demarcations of the various lung lobes are often seen. When a pneumothorax is severe, significant lung volume reduction occurs, and this is associated with an increase in pulmonary opacity.

In mild or moderate pneumothorax, the lung margins are most easily detected in the caudodorsal region on a lateral projection, and in the caudolateral recesses of the thorax on a DV or VD projection. Sometimes fissures representing the demarcation between lung lobes are also evident. A second radiographic finding that is commonly present with pneumothorax is an elevation of the cardiac margin away from the sternum. This is associated with a loss of pulmonary volume and resulting passive displacement of the heart away from the sternum. Other radiographic findings that may be seen secondary to pneumothorax are:

     • Pulmonary lesions that may be the source of the escaping air.

     • Tracheal or esophageal lesions that may be the source of pleural air.

     • Marked pulmonary atelectasis with associated increased pulmonary opacity.

     • A shift of the mediastinum and cardiac silhouette toward the side of the thorax with the greatest degree of pulmonary volume loss.

     • A shift of the mediastinum away from the side of greatest atelectasis indicates a tension pneumothorax, that is, an increase in intrapleural pressure exceeding the atmosphere. This is a life threatening form of pneumothorax that must be recognized and treated immediately.

     • A loss of bronchial or vascular pulmonary markings in the periphery of the thoracic space.

Illustration depicting a unilateral pneumothorax and pneumomediastinum.

Pleural Effusion

Pleural effusion, or an accumulation of fluid within the pleural space, can result from a wide variety of diseases and the character of pleural fluid may be that of a transudate, an exudate, or frank blood. Some of the more common causes of pleural effusion are heart failure, chylothorax (leakage of lymphatic fluid into the pleural space), pyothorax, and hemothorax.

Radiographic Findings

Radiographically, pleural effusion causes an overall increase in opacity within the thoracic cavity. Fluid accumulation within the pleural space displaces lung volume, with resultant pulmonary volume loss and increased pulmonary opacity. Generally, aerated lung lobe margins can be identified within the pleural fluid. Often lobar margins are well defined and separated by the fluid. When a moderate volume of effusion is present, positioning the patient in dorsal recumbency for a ventrodorsal (VD) thoracic image provides a better evaluation of remaining aerated lung and better visualization of the heart. This is because the lungs "float" to cradle the heart. HOWEVER, a VD view can mask the presence of small or even moderate pleural effusions. On a dorsoventral view (DV), the lungs "FLOAT AWAY" from the heart, thus the fluid silhouettes with the heart margin. It is this silhouetting of the heart margin (or border effacement) that allows detection of smaller pleural effusions on a DV view when compared to a VD view. The DV view also allows a more nature and consistent placement of the heart, thus allowing easier assessment for size and shape. The pleural fluid can also cause complete collapse of lung lobes.

Thoracic Radiographic Signs of Upper Airway Obstruction

If the lungs are normal, the heart is normal, and the pleural space is normal, then we must consider abnormalities of the upper respiratory tract, such as laryngeal and nasal disease. While imaging of the upper airway will be necessary, there are several radiographic signs of the thorax that can indicate an upper airway disease process.

Making thoracic radiographs of dyspneic small animals is common, as differentiating upper from lower airway disease is not always intuitive. The reader should be aware of several different thoracic radiographic presentations possible as a result of upper airway disease. In practice, some cases of upper airway disease may be made by recognition of thoracic radiographic abnormalities. Of course, in many cases of upper airway disease, the thoracic radiographs may be normal.

One of the most common thoracic radiographic abnormalities noted in small animals with upper airway obstruction is underinflation of the lungs and a resultant increase in cardiothoracic volume, mimicking cardiac enlargement. The increased lung opacity is due to incomplete aeration of the lungs because the upper airway lesion prevents proper aeration of the lung. The cardiac silhouette appears enlarged because of the relatively small thoracic volume. The diaphragm may be displaced cranially, with a large degree of cardiac overlap, creating an even stronger impression of cardiac enlargement. Other common findings include a dilated intrathoracic trachea and main stem bronchi and inward deviation of the intercostal muscles. These latter findings occur in dogs (not cats) and are a result of increased intrathoracic pressure generated against an occluded upper airway. The more severe the obstruction, the more pronounced the thoracic radiographic findings may be.

In cats, thoracic radiographs taken at inspiration may show dorsal displacement of the sternum. This mimics the appearance of pectus excavatum and results from large negative intrathoracic pressure generated and relatively pliable costal arches. The diaphragm may be markedly displaced. This appearance also occurs in puppies and is sometimes seen in smaller breed dogs.

Curiously, over-inflation of the lungs and a resultant increased thoracic volume can occur in some patients with upper airway obstruction. Air is trapped in the lungs because it cannot adequately pass the upper airway obstruction. This is particularly true for cats and small breed dogs. The authors have not observed this finding in large breed dogs, however.

Severe upper airway obstruction causes aerophagia. Aerophagia is recognized radiographically by gas within the esophagus, stomach, and small intestinal tract. On occasion, the radiographic findings can be quite dramatic. Severe gas distension of the esophagus (megaesophagus) may be seen, displacing the trachea, cardiac silhouette and caudal vena cava ventrally. The stomach will likely contain a large amount of gas. Hyperinflation of the lungs may occur in cats and small breed dogs.

Lastly, it must be recognized that severe upper airway obstruction, especially if acute, may cause pulmonary edema. This is due to capillary leakage as a result of exceedingly high intrathoracic pressures generated during the dyspnea. It is similar in part to near drowning.

Summary of thoracic manifestations of upper airway disease

Dogs

     • Underinflation of lung (and therefore increased lung opacity, can be severe, can mask lung pathology).

     • Cranial displacement of ribs and diaphragm.

     • Dilated trachea (due to tremendous increase in negative intrathoracic pressure generated as intercostal muscles and diaphragm work against the obstruction).

     • ncreased cardiothoracic ratio (heart appears relatively larger).

Cats

     • Often increased lung/thoracic volume.

     • Lungs may be hyperinflated (DDx: asthma).

     • Indentation of intercostal muscles, diaphragm attachment ("tenting").

     • Sternum displaced dorsally (DV compression).

     • Severe aerophagia, "megaesophagus".

     • PUPPIES are similar-soft, compliant ribs.