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Concern for congenital heart disease is the most common reason for evaluation of puppies with cardiac murmurs. While mild clinical signs of a congenital heart defect might not be associated with significant radiographic abnormalities, moderate or severe signs are associated with some classic abnormalities that can help solidify a diagnosis, or at least lead to a differential diagnosis that can be further evaluated using echocardiography. This article reviews an interpretation paradigm for evaluating the cardiac silhouette, pleural space, and pulmonary parenchyma and discusses typical findings seen with some of the common congenital heart defects in dogs.
Take-Home Points
- Common congenital anomalies of the cardiac silhouette can be evaluated using thoracic radiography.
- Complex congenital cardiac anomalies in dogs often require advanced imaging, including echocardiography, fluoroscopy, and gated computed tomographic angiography.
- The cardiac silhouette is a complex 3-dimensional structure that is incompletely evaluated using survey thoracic radiographs alone.
- Secondary features of left-sided and right-sided heart failure should be taken into account when reviewing survey radiographs from dogs with congenital heart disease.
Thoracic radiography is the most widely accessible imaging modality used by veterinary practitioners to assess dogs for cardiac disease. It provides a comprehensive overview of the thorax, including the extrathoracic structures, pleural space, pulmonary parenchyma, and mediastinum, in addition to the heart. Dogs with congenital heart disease can be presented at various stages of development but are often initially identified by the presence of a murmur during routine puppy examinations. Thoracic radiography can be a first-line diagnostic test for identifying cardiac abnormalities, pulmonary changes, and features of heart failure, even though the sensitivity for the detection of mild congenital heart disease is low.
Overview and Interpretation Paradigm
When evaluating thoracic radiographs, it is important to start with high-quality, well-positioned, collimated diagnostic radiographs of the thorax obtained at peak inspiration. Significant breed differences need to be considered when evaluating the cardiac silhouette on canine thoracic radiographs. For an overview of how to obtain diagnostic thoracic radiographs, see the Today’s Veterinary Practice article “Small Animal Thoracic Radiography”.
When evaluating thoracic radiographs specific to the cardiovascular system, clinicians should answer 5 questions:
- Is cardiomegaly present?
- If cardiomegaly is present, is it right sided, left sided, or generalized? If right sided or left sided, is it limited to a specific chamber (e.g., right atrium)?
- Is there enlargement of the pulmonary lobar veins, pulmonary lobar arteries, or both the pulmonary lobar arteries and veins (the last suggests pulmonary overcirculation)?
- Is there enlargement of the great vessels (aortic arch, descending thoracic aorta, main pulmonary artery on the ventrodorsal/dorsoventral [VD/DV] views, and caudal vena cava) on the lateral views?
- Is there radiographic evidence of left-sided congestive heart failure (pulmonary edema, pulmonary venous enlargement) or right-sided congestive heart failure (pleural effusion, caudal vena cava enlargement, hepatomegaly, and ascites)?
Based on the answers to these questions, a refined differential diagnosis can be created, aided by patient signalment and careful auscultation of the heart. Referral to a board-certified cardiologist for echocardiographic evaluation and possible intervention should follow the finding of radiographic findings supportive of congenital heart disease.
Radiographic Evaluation
Heart
Cardiac enlargement can be determined using several subjective and objective criteria.1-10 FIGURE 1 shows left-sided cardiomegaly as evidenced by dorsal elevation of the trachea and carina. More objective criteria include the use of the vertebral heart scale and other rules of thumb for evaluation of the cardiac silhouette. The widest point of the cardiac silhouette should measure 2.5 to 3.5 intercostal spaces in width, and the height of the cardiac silhouette should be approximately 65% of the overall internal thoracic height on lateral radiographs.2
Figure 1A. In patients with left-sided cardiomegaly, the cardiac silhouette increases in apical to basilar length, resulting in dorsal displacement of the trachea at the level of the carina so that the trachea becomes progressively parallel to the vertebral column, as shown by the difference in angle between the blue lines in (A) and (B).
Figure 1B. In patients with left-sided cardiomegaly, the cardiac silhouette increases in apical to basilar length, resulting in dorsal displacement of the trachea at the level of the carina so that the trachea becomes progressively parallel to the vertebral column, as shown by the difference in angle between the blue lines in (A) and (B).
Using a clock-face analogy (FIGURE 2), different chamber enlargement patterns can be seen depending on the congenital heart defect present.2 These focal changes in size and shape are then used to characterize the cardiac chambers that are enlarged (TABLE 1). Dogs with mild congenital defects can have normal thoracic radiographs. The changes described in this article are typical of dogs with moderate to severe defects.
Figure 2. (A) Clock-face representation of cardiac chamber location on right lateral radiographs.
Figure 2. (B) Clock-face representation of cardiac chamber location on ventrodorsal radiographs.
Vasculature
Important changes of the great vessels should be included in the interpretation paradigm. Enlargement of these structures is usually secondary to turbulent blood flow and poststenotic dilation. Enlargement of the caudal vena cava is usually secondary to congestive heart failure from right-sided disease.
Changes in the pulmonary vasculature should also be evaluated. On a lateral radiograph, the cranial lobar pulmonary vessels should not exceed the width of the proximal portion of the fourth rib near the origin at the thoracic vertebra. On ventrodorsal or dorsoventral images, the caudal lobar pulmonary vessels should not be wider than the summation shadow formed as they cross the ninth rib. In other words, the caudal lobar vessels should form square summation shadows with the ninth rib and not horizontal rectangles (in which case they are enlarged).2 Patients with left-sided congestive heart failure can have enlarged pulmonary veins. Enlargement of both pulmonary arteries and veins, known as pulmonary overcirculation, is a common feature of left-to-right intra- and extracardiac shunts (e.g., atrial septal defects, ventricular septal defects [VSDs], patent ductus arteriosus [PDA]).2
Other Structures
Evaluation for congestive heart failure should be part of the interpretation paradigm. Radiographic evidence of left-sided heart failure includes a perihilar to caudodorsal distribution of an unstructured interstitial to alveolar pulmonary pattern as well as enlarged pulmonary veins, although the latter is not always present, especially if diuretics were administered prior to radiography.2 Radiographic features of right-sided heart failure include the presence of an enlarged caudal vena cava, hepatomegaly, ascites, and occasionally pleural effusion.2
The above aspects should be assessed together to create a logical radiographic interpretation that will help in creating an appropriate differential diagnosis of a congenital cardiac anomaly. In most cases, radiographic findings should be consistent with a single congenital cardiac defect. Therefore, for example, features of left-sided cardiomegaly along with an enlarged vena cava, hepatomegaly, ascites, or pleural effusion would be unexpected. In this scenario, a complex congenital anomaly or combination of congenital anomalies should be considered.
Common Cardiac Anomalies
The following 5 defects are the most common congenital cardiac anomalies in dogs (TABLE 2). They may occur in isolation or in combination with other defects. When they are combined with other defects, diagnosis becomes challenging without advanced imaging such as echocardiography or cardiac gated computed tomography angiography.
Left-to-Right Patent Ductus Arteriosus
In a dog with a left-to-right shunting PDA, physical examination reveals a continuous cardiac murmur best auscultated at the base of the heart on the left side, with bounding femoral pulses.6,7
The underlying pathophysiology of a left-to-right PDA involves blood flow through the ductus arteriosus into the pulmonary trunk that overloads the pulmonary vasculature and, subsequently, the left heart.2-5 This results in left-sided cardiomegaly (left atrial, left auricular, and left ventricular enlargement), enlargement of the pulmonary arteries and veins (pulmonary overcirculation), enlargement of the main pulmonary artery, and the presence of ductus diverticulum (enlargement of the descending thoracic aorta at the fourth intercostal space on the left side of the thorax) (FIGURES 3 AND 4). Concurrent left-sided congestive heart failure would be supported by the presence of pulmonary edema.
Figure 3. (A) Left lateral projection of a dog with a left-to-right patent ductus arteriosus.
Figure 3. (B) Same view, with changes characteristic of enlargement of the left cardiac chambers highlighted. A small soft tissue bulge is present along the caudodorsal aspect of the cardiac silhouette in the region of the left atrium (green), and the cardiac silhouette is elongated. Additionally, the pulmonary veins (purple) are congested and larger than their corresponding arteries (red).
Figure 4. (A) Ventrodorsal projection of the dog in FIGURE 3.
Figure 4. (B) Same view, highlighted to demonstrate enlargement of the aortic arch (yellow line and arrow) and main pulmonary artery (green). The left atrium is mildly enlarged (pink). The pulmonary veins (purple) are larger than their corresponding arteries (red).
Figure 4. (C) With progressive enlargement of the left cardiac chambers, enlargement of the left auricle also becomes radiographically apparent (purple), resulting in a “three knuckles” sign (concomitant enlargement of the descending thoracic aorta [ductus diverticulum] (yellow), main pulmonary artery (green), and left auricle).
Subaortic Stenosis
Dogs with subaortic stenosis are usually presented for murmur evaluation or syncope. The murmur usually has an ejection quality and is heard best over the left heart base. Femoral pulse quality is weak when the obstruction is severe.7
The underlying pathophysiology of subaortic stenosis involves pressure overload of the left ventricle due to partial obstruction of the left ventricular outflow tract.2 This results in concentric hypertrophy of the left ventricle.
The radiographic features of subaortic stenosis include elongation of the left ventricle (particularly on a ventrodorsal radiograph) and enlargement of the aortic arch secondary to poststenotic dilation. Left atrial enlargement can occur secondary to concurrent mitral regurgitation, especially if concurrent mitral valve dysplasia is present (FIGURES 5 AND 6).
Figure 5. (A) Left lateral projection of a dog with severe subaortic stenosis.
Figure 5. (B) Same view, with characteristic findings highlighted. A large bulge is seen at the heart base in the region of the aorta due to poststenotic dilation (red). The ventricle is increased in apical to basilar length, demonstrating left ventriculomegaly (white arrow).
Figure 6. (A) Ventrodorsal projection of the dog in FIGURE 5.
Figure 6. (B) Same view. A large bulge is seen at the heart base in the region of the aorta due to poststenotic dilation (red).
Pulmonic Stenosis
Dogs with pulmonic stenosis are often presented for evaluation of a murmur, syncope, or exercise intolerance. The murmur is usually an ejection-type murmur (crescendo–decrescendo) that is loudest at the left dorsal heart base due to the proximity of the pulmonary trunk in this region.7 Unlike subaortic stenosis, the femoral pulse quality should be normal.
The underlying pathophysiology of pulmonic stenosis involves pressure overload of the right ventricle due to partial obstruction of the right ventricular outflow tract.2,7,8 The stenosis can be supravalvular, valvular (most common in dogs), or subvalvular. Due to the increased resistance associated with valve stenosis, the right ventricle becomes concentrically hypertrophied with gradual loss of compliance and reduced size of the right ventricular lumen.
Radiographically, pulmonary stenosis can be recognized by the presence of a large bulge in the region of the pulmonary trunk due to poststenotic dilation and rounding of the right ventricle indicating right ventriculomegaly (FIGURES 7 AND 8). Right atrial enlargement can develop secondary to tricuspid valve insufficiency, especially if concurrent tricuspid valve dysplasia is present. Some dogs eventually develop right-sided heart failure, recognizable radiographically by enlargement of the caudal vena cava, hepatomegaly, and the presence of ascites and/or pleural effusion. The pulmonary vasculature may appear small from undercirculation in dogs with severe pulmonic stenosis.2,7
Figure 7. (A) Right lateral radiographic projection of a dog with severe pulmonic stenosis.
Figure 7. (B) Same view. A large bulge is seen at the heart base in the region of the main pulmonary artery due to poststenotic dilation (blue).
Figure 7. (C) Fluoroscopic selective angiogram from a dog with pulmonic stenosis. The positive contrast medium was injected into the right ventricle. There is narrowing of the right ventricular outflow tract (arrow) with poststenotic dilation of the main pulmonary artery (asterisk).
Figure 8. (A) Ventrodorsal projection of a dog with severe pulmonic stenosis.
Figure 8. (B) Same view. A large bulge is seen at the 1- to
2-o’clock position in the region of the main pulmonary artery due to poststenotic dilation (blue).
Left-to-Right Ventricular Septal Defects
Dogs with VSDs are often identified during routine examination when a murmur is auscultated. The murmur of an uncomplicated, perimembranous VSD is holosystolic and has a right parasternal intensity due to the direction of the shunt flow.7
The underlying pathophysiology of a left-to-right VSD involves overcirculation of the left side of the heart, the right ventricular outflow tract, and the pulmonary circulation. The position of the shunt within the right ventricle influences the presence or absence of right ventricular enlargement (eccentric hypertrophy).
Due to the altered blood flow through the shunt and overcirculation, rounding of the cranial and right lateral margins of the cardiac silhouette may be observed on radiographs (FIGURES 9 AND 10). Enlargement of the great vessels is not apparent. Pulmonary overcirculation is present throughout the lung fields with equal enlargement of the pulmonary arteries and veins. Chambers and vessels that accommodate shunt flow will become enlarged and, therefore, enlargement of the pulmonary arteries, pulmonary veins, left atrium, and left ventricle can be seen, with variable right ventricular enlargement depending on the location of the VSD.2,7
Figure 9. (A) Left lateral radiographic projection of a dog with a left-to-right shunting ventricular septal defect.
Figure 9. (B). Same view. The cranial margin of the cardiac silhouette is rounded (white arrowheads) due to altered blood flow dynamics through the septal defect and overcirculation of the right ventricle.
Figure 10. (A) Left lateral radiographic projection of the dog in FIGURE 9.
Figure 10. (B) Same view. The right lateral margin of the cardiac silhouette is rounded (white arrowheads) and there is rounding of the cardiac apex (yellow) due to morphologic distortion of the heart from overcirculation of the right ventricle.
Tricuspid Dysplasia
Dogs with severe tricuspid dysplasia can present in right-sided heart failure with a systolic right-sided murmur.7,9 Enlargement of the jugular veins and ascites are common features of dogs with right-sided congestive heart failure.
The underlying pathophysiology of tricuspid dysplasia involves volume overload of the right atrium and right ventricle due to regurgitation. The tricuspid valve is malformed and cannot close properly, resulting in regurgitation of blood into the right atrium and gradual eccentric enlargement of the right atrium and ventricle. The chordae tendineae may also be malformed and shortened, further restricting closure of the tricuspid valve.
Tricuspid valve dysplasia is suspected in dogs with right atrial and ventricular enlargement on radiographs (FIGURE 11). Subsequent caudal vena cava and hepatic enlargement and the presence of peritoneal effusion support a diagnosis of secondary right-sided heart failure.
Figure 11. (A) Right lateral projection of a dog with tricuspid valve dysplasia. The cranial and right lateral margins of the cardiac silhouette are rounded in association with the presence of right atrial and ventricular enlargement.
Figure 11. (B) Ventrodorsal projection of a dog with tricuspid valve dysplasia. The cranial and right lateral margins of the cardiac silhouette are rounded in association with the presence of right atrial and ventricular enlargement (white arrows). Associated morphological changes result in subsequent leftward displacement of the cardiac apex (blue arrow).
Summary
A solid grasp of anatomy, physiology, and pathophysiology, along with clinical findings (signalment, murmur grade and quality, pulses, jugular distention, features of congestive heart failure), is key to understanding the radiographic features of congenital heart disease in dogs. It is important to remember that dogs with mild congenital heart disease might not show changes in the cardiac silhouette and pulmonary vasculature on thoracic radiographs and that a continuum of changes can be seen, depending on the severity of the congenital heart defect present. It is also important to be sure that the radiographic findings are consistent with the suspected congenital heart disease. Additionally, radiographs should always be evaluated for correct positioning and technique to aid proper interpretation.
The interpretation paradigm presented in this article is intended to facilitate complete evaluation of thoracic radiographs for abnormalities that might indicate the presence of congenital heart disease. Although survey radiography may not provide a definitive diagnosis and lacks sensitivity for the detection of mild congenital heart disease, it is a valuable initial diagnostic test for the evaluation of heart murmurs in dogs, providing an opportunity to assess for changes consistent with isolated congenital defects. While thoracic radiography is a test of choice for detection of congestive heart failure, echocardiography is required to obtain a definitive diagnosis of congenital heart disease, and dogs with complex (multiple) congenital defects often require advanced imaging, including 3- and 4-dimensional imaging (computed tomography angiography with electrocardiographic gating).
References
- Thrall DE. Principles of radiographic interpretation of the thorax. In: Thrall DE, ed. Textbook of Veterinary Diagnostic Radiology. 7th ed. Elsevier; 2018:568-582.
- Bahr R. Canine and feline cardiovascular system. In: Thrall DE, ed. Textbook of Veterinary Diagnostic Radiology. 7th ed. Elsevier;
2018:684-709. - Lamb CR, Boswood A, Volkman A, Connolly DJ. Assessment of survey radiography as a method for diagnosis of congenital cardiac disease in dogs. J Small Anim Pract. 2001:42(11):541-545.
- Ackerman N, Burk R, Hahn AW, Hayes HM. Patent ductus arteriosus in the dog: a retrospective study of radiographic, epidemiologic, and clinical findings. Am J Vet Res. 1978;39(11):1805-1810.
- Van Israël N, French AT, Dukes-McEwan J, Corcoran BM. Review of left-to-right shunting patent ductus arteriosus and short term outcome in 98 dogs. J Small Anim Pract. 2002;43(9):395-400. doi:10.1111/j.1748-5827.2002.tb00090.x
- Ware WA, Bonagura JD. Congenital cardiac shunts. In: Cardiovascular Disease in Companion Animals: Dog, Cat and Horse. 2nd ed. CRC Press; 2022:435-472.
- O’Grady MR, Holmberg DL, Miller CW, Cockshutt JR. Canine congenital aortic stenosis: a review of the literature and commentary. Can Vet J. 1989;30(10):811-815.
- Fingland RB, Bonagura JD, Myer CW. Pulmonic stenosis in the dog: 29 cases (1975-1984). JAVMA. 1986;189(2):218-226.
- Hoffmann G, Amberger CN, Seiler G, Lombard CW. Tricuspid
valve dysplasia in fifteen dogs. Schweiz Arch Tierheilkd. 2000;142(5):268-277. - Stickle RL, Anderson LK. Diagnosis of common congenital heart anomalies in the dog using survey and nonselective contrast radiography. Vet Radiol. 1987;28(1):6-12. doi:10.1111/j.1740-8261.1987.tb01715.x
CE Quiz
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Topic Overview
This article reviews the radiographic features of 5 common congenital heart defects in dogs and reviews an interpretation paradigm for evaluating the cardiac silhouette, pleural space, and pulmonary parenchyma in dogs presenting with clinical signs of heart disease.
Learning Objectives
After reading this article, participants should be able to describe the interpretation paradigm of the cardiac silhouette and associated thoracic structures for review of congenital heart disease in dogs. They should also be able to identify common radiographic changes associated with common congenital heart defects in dogs.
1. Undercirculation of the pulmonary vasculature may be seen in dogs with which congenital heart defect?
a. Pulmonic stenosis
b. Subaortic stenosis
c. Left-to-right ventricular septal defect
d. Left-to-right patent ductus arteriosus
2. The underlying pathophysiology in a dog with a left-to-right shunting patent ductus arteriosus is
a. Right-sided cardiac volume overload
b. Left-sided cardiac volume overload
c. Right-sided cardiac pressure overload
d. Left-sided cardiac pressure overload
3. Enlargement of the left auricle is located at the ___ o’clock position on a canine VD/DV radiograph.
a. 1
b. 2
c. 3
d. 4
4. The predominant radiographic abnormality seen in dogs with subaortic stenosis is enlargement of the
a. Left atrium
b. Left ventricle
c. Aortic arch
d. Left auricle
5. Ductus diverticulum in a dog with a left-to-right shunting PDA is best seen on the ______ view.
a. Right lateral
b. VD/DV
c. Left lateral
d. Cannot be seen radiographically
6. In a dog with a left-to-right PDA, the blood flows from the ___________ to the ____________ through the patent ductus.
a. Main pulmonary artery; descending aorta
b. Descending aorta; main pulmonary artery
c. Main pulmonary artery; aortic arch
d. Aortic arch; main pulmonary artery
7. Which great vessel becomes enlarged in dogs with tricuspid dysplasia?
a. Caudal vena cava
b. Main pulmonary artery
c. Azygos vein
d. Caudal lobar pulmonary veins
8. In dogs with left-to-right VSDs, the underlying pathophysiology is overcirculation of the left heart and pulmonary circulation; however, enlargement of the _____________ can be noted radiographically.
a. Right atrium
b. Right ventricle
c. Main pulmonary artery
d. Descending thoracic aorta
9. A dog presents with the following radiographic changes: right atrial enlargement, normal pulmonary vasculature, and pleural effusion. The most likely congenital heart defect is:
a. Left-to-right PDA
b. Pulmonic stenosis
c. Tricuspid dysplasia
d. Left-to-right VSD
10. The key radiographic finding for a dog with a left-to-right shunting PDA is enlargement of the
a. ductus diverticulum of the descending thoracic aorta
b. main pulmonary artery
c. left auricle
d. pulmonary veins