Introduction

Thoracic radiographs are the imaging method of choice in the coughing or dyspneic dog or cat, but abnormalities of the pulmonary parenchyma are inherently difficult to interpret due to the overlap in radiographic appearance of different diseases. The question of heart vs. lung disease is a common one in patients with clinical signs of cough or dyspnea and in the following we will go over some interpretation criteria that help with this decision.

Importantly, even though we will mostly talk about thoracic radiographs in this presentation, radiographic findings have to be interpreted in light of clinical findings. Abnormalities in heart rate, heart rhythm, and presence of a heart murmur are important information to take into account. It is equally important to assess clinical findings over the course of the disease and after treatment has been initiated based on the primary differential diagnosis to determine if adequate treatment response is present.

Radiographic Findings of Heart Disease

Radiographic findings that point to heart disease as the primary cause of cough or dyspnea in a dog include changes in the cardiac silhouette, pulmonary vasculature, and pulmonary parenchyma. Cough and dyspnea is caused by left sided heart disease or heart dysfunction. Increased size of the left heart results in a taller cardiac silhouette. In a lateral radiograph this usually results in dorsal displacement of the trachea. In a ventrodorsal (VD) or dorsoventral (DV) projection the cardiac silhouette may be elongated as well but could also have a rounded appearance only if the dog is deep chested and the heart is in a very upright position. Left atrial enlargement results in a convex bulge in the caudodorsal contour of the cardiac silhouette in the lateral views. The main stem bronchi are splayed in the VD or DV projections and there may be a bulge visible in the left lateral aspect of the contour of the cardiac silhouette approximately at 2-3 O’clock. Dorsal deviation of the main stem bronchi by an enlarged left atrium may result in bronchial compression and cough, particularly if a component of bronchomalacia and bronchial collapse is present. Left atrial enlargement is typically pronounced in dogs in left sided congestive heart failure and a lack of left atrial enlargement should prompt the clinician interpreting the radiograph to consider other causes for the cough.

Evaluation of the pulmonary vasculature should always go hand in hand with evaluation of the cardiac silhouette. It is especially important in dogs with cough or dyspnea where left-sided congestive heart failure is considered. In presence of left heart failure there is volume overload of the left atrium and subsequent congestion of the pulmonary veins trying to return the blood volume from the pulmonary circulation into the heart. Enlargement of the pulmonary veins is, therefore, an excellent radiographic sign of left heart dysfunction. Pulmonary venous dilation, however, may not be present if a patient has been treated with diuretics prior to obtaining the radiographs. Additionally, very mild pulmonary venous dilation may be difficult to recognize radiographically so a lack of pulmonary venous dilation should not be used to completely rule out heart failure.

Lastly, there are pulmonary changes associated with left sided congestive heart failure. Cardiogenic edema transitions from the interstitial space to the alveolar space and, therefore, has the potential to create variations of pulmonary patterns. Once there is pulmonary venous congestion with associated pressure increase in the vessels, fluid may be leaking into the interstitial space around the vessels. The lymphatics initially compensate by increased drainage of the interstitial space but ultimately may become overwhelmed, resulting in fluid buildup in the interstitial space. The interstitial fluid will cause opening of the tight junctions between alveolar wall cells and fluid will flow into the alveolar space. It, therefore, makes sense that pulmonary patterns can vary from interstitial to alveolar, depending on the stage of the disease. To make things more complicated the cardiogenic interstitial edema is present around the bronchial walls as well which can give the appearance of a peri-bronchial infiltrate and a radiographic bronchial or bronchointerstitial pattern. This pattern mostly occurs in large breed dogs with cardiogenic pulmonary edema. The distribution of pulmonary changes is often more helpful than the pattern itself - the pulmonary changes associated with heart failure tend to be located in the perihilar area or in large breed dogs in the caudodorsal lungs.

Radiographic Findings of Pulmonary Parenchymal Disease

Pulmonary parenchymal or large airway disease is the main alternative differential diagnosis in the coughing dog. Increased pulmonary opacity in absence of cardiac and pulmonary vascular changes are the main findings that lead to suspected lung disease. To further characterize the type of lung disease, pulmonary patterns are often the intuitive choice. The concept of pulmonary pattern recognition was developed to aid in generating the most likely differential diagnosis based on the assumption that different disease types affect different compartments within the lungs. There is, however, a large degree of overlap between the radiographic pulmonary patterns in many types of lung disease and focusing on pulmonary patterns only is not the most successful way of interpreting radiographs. The distribution of the abnormal lung patterns as well as other radiographic findings are often more helpful than the pulmonary patterns alone.

Alveolar pulmonary pattern is characterized by marked increased pulmonary opacity, loss of visibility of vascular structures in the affected segment, presence of air bronchograms, and if only on lung lobe is affected, presence of a lobar sign. Pneumonia is one of the most common causes of alveolar pulmonary pattern besides heart failure. Contrary to heart failure, alveolar patterns with pneumonia are typically ventrally distributed with a preference to the cranial and right middle lung lobes. Ventral alveolar patterns can also be caused by atelectasis which is differentiated from acute pneumonia by evidence of volume loss of the affected lung lobe causing a midline shift to the same side. Pleural effusion is rarely seen with pneumonia on dogs and presence of effusion should point to a different disease process such as trauma, neoplasia, or primary pleural space disease with secondary involvement of the lungs.

Bronchial pulmonary patterns can be difficult to recognize as they do not result in overall increased pulmonary opacity but there is also a tendency to over- interpret the normal structure of the lung parenchyma as bronchial pattern. Signs of advanced bronchial disease include bronchiectasis which is characterized by lack of normal tapering of the bronchial lumen towards the periphery of a bronchus. Presence of bronchiectasis is often a sign that there is chronicity to the bronchial disease. Chondromalacia leading to airway collapse can be recognized by paying close attention to airway diameter between radiographic projection particularly if they were obtained during slightly different phases of respiration. In a normal dog there should be minimal variance between airway diameter in in- vs. expiratory radiographs.

Interstitial lung disease is characterized by diffuse increase in pulmonary opacity without loss of visibility of vascular structures or bronchial walls. Interstitial pattern is the least useful of the pulmonary patterns. Most disease processes have an interstitial component, and as outlined above when describing the pathogenesis of cardiogenic edema, many disease processes start out as or resolve as interstitial pattern.

Overlapping Disease Patterns

There are cases where it is very difficult if not impossible to decide of the radiographic findings are most likely due to cardiac or pulmonary disease. One of the more difficult diseases to recognize radiographically are pulmonary infiltrates secondary to pulmonary hypertension. Dogs with moderate to severe pulmonary hypertension can present with diffuse patchy alveolar infiltrates consistent with non-cardiogenic edema. The clinical presentation may include acute dyspnea and syncope and often heart murmurs suggestive of valvular insufficiency are present. The clinical and radiographic findings may lead to an initial misdiagnosis of congestive heart failure or pneumonia whereas the dogs improve both clinically and radiographically once therapy with sildenafil is instituted.

Atypical appearance of heart disease can occur if there is acute chorda tendinea rupture, or in cases of severe cardiomegaly, left atrial rupture with subsequent acute pericardial effusion. Pulmonary thromboembolism is another disease with a large variation in radiographic appearance, ranging from normal appearing lungs to hyperlucency or patchy alveolar pattern. These dogs typically present with marked dyspnea.

References

1.  Kellihan HB, Waller KR, Pinkos A, Steinberg H, Bates ML. Acute resolution of pulmonary alveolar infiltrates in 10 dogs with pulmonary hypertension treated with sildenafil citrate: 2005–2014. J Vet Cardiol. 2015;17(3):182–191.

2.  J Vet Intern Med. 2008;21(2):258–264.