Thoracic radiographs are essential in the evaluation of animals with suspected heart disease. Changes in the cardiac silhouette and lung fields provide direct information about the heart size, the condition of the lungs and volume load in the circulation. Thoracic radiography may facilitate or confirm a diagnosis suspected on the basis of physical examination, help in assessing the severity of known heart disease, or aid in the evaluation of the efficacy of treatment.
Good quality radiographs are required for accurate diagnosis and assessment. Attention to detail during the making of the radiograph is important. Accurate positioning is essential The use of rare earth intensifying screens and a high kilovoltage (kV) technique will help to minimize movement artefact. Exposure should be made at full inspiration so that the lung fields are fully expanded and aerated. Both right lateral and dorsoventral (DV) rather than ventrodorsal projections should be made. These projections are preferred since the heart lies in a more consistent position.
Radiographs should be examined carefully and systematically for the following criteria.
Size of the cardiac silhouette
Wide variations in shape and size of the heart occur in different breeds of dogs, for example in deep chested breeds the heart is tall, narrow and upright whereas in barrel-chested breeds it is almost globular. On the lateral radiograph, the craniocaudal width of the normal heart is between 2.5 and 3,5 intercostal spaces and its height is approximately two-thirds the height of the thoracic cavity. The trachea normally diverges from the thoracic spine except in the case of barrel-chested breeds in which ii may run parallel to the spine. On the dorsoventral projection, the normal heart should occupy no more than two-thirds the width of the thoracic cavity with the apex to the left of the midline.
The cat shows far less variation in size, although the angle of the long axis does vary. The width of the normal feline heart taken perpendicular to the long axis on the lateral projections should be less than 2,25 intercostal spaces.
The size of the cardiac silhouette is increased in congestive heart failure, pericardial effusion and peritoneopericardial diaphragmatic hernia. The cardiac silhouette may be decreased in size (microcardia) due to hypovolaemia.
Assessment of the pulmonary vasculalure
The pulmonary arteries and veins should be of similar size and run on either side of the bronchi. The arteries are dorsal and the veins ventral on the lateral projection and the diameter of the cranial lobe vessels should be smaller than the proximal third of the fourth rib. On the dorsoventral projection the right and left caudal lobe vessels can be identified. The arteries are lateral to the veins. They should be of equal size and should not exceed the diameter of the ninth rib where they cross. The pulmonary vessels should be assessed for signs of tortuosity, pruning and loss of margination. Conditions which increase the size of the pulmonary vessels include left-to-right shunting (for example patent ductus arteriosus, ventricular septal defect), congestive heart failure, heartworm disease and iatrogenic fluid overload. The pulmonary vessels may be reduced in right-to-left shunting (for example tetralogy of Fallot), severe pulmonic stenosis, hypovolaemia and pulmonary thrombocmbolism.
Assessment of the pulmonary patterns
Pulmonary congestion and oedema may be seen predominantly in the dorsocaudal lung fields in chronic left-sided cardiac failuren In acute left-sided failure, alveolar oedema is seen more diffusely throughout the lung fields. In cats, pulmonary oedema often appears as an interstitial rather than alveolar pattern.
Assessment of the caudal vena cava
The normal caudal vena cava is usually parallel sided and of similar size to the aorta. On the lateral projection, it slopes slightly cranioventrally before merging with the caudal outline of the heart. Enlargement of the caudal vena cava and liver is evidence of right-sided heart failure. Pleural effusion and ascites can also be seen in some cases.
Assessment of the great vessels
Post-stenotic dilatation of the great vessels can be seen in some cases of congenital subaortic or pulmonic stenosis and results in filling of the cranial cardiac waist.