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Initially it is important to be able to identify radiographic signs of cardiac chamber enlargement. The left atrium on the lateral view when enlarged causes a change in shape of the dorsocadual aspect of the cardiac silhouette.
Initially it is important to be able to identify radiographic signs of cardiac chamber enlargement. The left atrium on the lateral view when enlarged causes a change in shape of the dorsocadual aspect of the cardiac silhouette. The enlarged left atrium causes there to be a slight concavity on the caudal margins of the heart. Elevation of the left caudal lobe bronchus above the right on the lateral view is also indicative of left atrial dilation. On the VD or DV view, left atrial enlargement causes the normally acute angle separating the mainstem bronchi to be lost as the bronchi diverge. A double opacity sign is visualized due to summation of the enlarged fluid filled left atrium with the left ventricle. A dilated left auricular appendage can be seen as focal bulge in the cardiac border from 2-3 oclock on the VD view.
Left ventricular dilation is most often caused by volume overload. There is elevation of the entire intrathoracic trachea on the lateral view with narrowing of the angel between the trachea and thoracic vertebrae. Elongation of the cardiac silhouette on the ventrodorsal view is also noted.
Radiographic enlargement of the right atrium is very uncommonly seen. On the lateral view it is difficult to differentiate from other cardiovascular enlargements as can be caused by dilation of the aortic arch or main pulmonary artery in the 9-12 o'clock region of the cardiac silhouette. On the VD view, a bulge in the 9-11oclock position of the cardiac silhouette is noted.
Right ventricular enlargement on the lateral view can show increased sternal contact with the cardiac silhouette measuring greater than 3 intercostal spaces in width. However this is related to conformation as some breeds normally have hearts that are 4 intercostal spaces in width. On the VD view, an enlarged right ventricle is seen as a reverse D.
Once you have finished evaluating the cardiac silhouette, you should look at the pulmonary vessels for evidence of major vessel enlargement. Remember, there is incomplete evaluation of heart without evaluation of pulmonary vessels! The caudal vena cava size varies depending on the phase of the respiratory and cardiac cycle and therefore conclusions to size should be made if the caudal vena cava is consistently enlarged on multiple radiographs. The caudal vena cava is considered enlarged if it is greater than 1.5 times the size of the descending aorta on the lateral view. The aortic arch on the ventrodorsal view when enlarged in dogs causes widening of the caudal aspect of the cranial mediastinum. In cats, the aortic arch becomes "knoblike" and appears as a mass between the left 3-5th intercostal spaces on the ventrodorsal view. The descending aorta is followed down the left side of the vertebra on the VD view and should be smooth. Dilation is not common but a focal bulge in the descending aorta is seen with patent ductus venosus. The descending aorta of geriatric cats is serpentine in course. This finding is clinically insignificant. Main pulmonary artery enlargement on the lateral view is not usually recognized as it is superimposed with the right atrium and ascending aorta. On the ventrodorsal view, a focal bulge in the 1 o'clock position of the cardiac silhouette represents an MPA bulge.
The peripheral pulmonary vessels are evaluated after the major vessels have been interrogated.
On the lateral view, the cranial lobar arteries are dorsal and the veins are ventral. The right cranial lung lobe vessels are best seen in a left laterally recumbent view. The caudal lobar vessels are superimposed on the lateral view and therefore difficult to definitively visualize. On the ventrodorsal view, the caudal lobar arteries and veins easily visualized. On the VD view, the artery is lateral while the vein is axial. Arteries should be equal in size to their corresponding veins. The normal size of the cranial artery on the lateral view is not larger than lower (less magnified) 4th rib. On the ventrodorsal view, the caudal artery should not be greater than the width of the ninth rib. Where the rib crosses the artery, a square is made if they are similar in size.
The following is a list of radiographic changes seen in the peripheral pulmonary vasculature and the corresponding diseases which result in these changes.
- Pulmonary artery enlargement is seen with heartworms and pulmonary hypertension.
- Pulmonary vein enlargement is common with mitral insufficiency (most common), cardiomyopathies (DCM, RCM, HCM), mass or thrombus in the left atrium
- Pulmonary artery and vein enlargement can be due to iatrogenic intravenous fluid overload.
- Pulmonary artery and vein small in size with septicemia, hypovolemia, and shock. When the vessels are small the lungs appear more hyperlucent.
The following is a list of common acquired cardiac diseases and the expected changes identified radiographically.
- Mitral insufficiency is the most common cause of acquired heart disease in small dogs. Left atrial enlargement, left ventricular enlargement, distended pulmonary veins, and pulmonary edema (when in left heart failure) are expected.
- The severity of radiographic changes with heartworm disease depend on the duration of infection, number of worms present, location of the worms, rate and degree of cardiac compensation, and die off of adult worms. Right ventricular hypertrophy, dilation of main pulmonary artery, parenchymal pulmonary artery enlargement, truncation, and or tortuosity; focal to multifocal alveolar pulmonary infiltrates, and hepatomegaly are visualized.
- Cardiomyopathy: There are three types of cardiomyopathy. Dilated cardiomyopathy (DCM) is most common in dogs. Hypertrophic cardiomyopathy (HCM) is more common in cats.
- DCM changes are due to weak and dysfunctional myocardial contractility. Most common in Dobermans and Boxers. The radiographic changes can be subtle and the radiographs can even be normal. Generalized cardiomegaly, left atrial enlargement, pulmonary venous dilation, pleural effusion/hepatomegaly, ascites due to right heart failure, and pulmonary edema seen.
- HCM causes the left ventricular lumen to become small in size due to the thickening of the heart muscle itself. Left atrial and left ventricular enlargment, pulmonary artery dilation, and sometimes pulmonary edema.