What's wrong with the ticker? (Proceedings)
When performing a complete cardiac evaluation, a minimum of two views are necessary: lateral and either VD or DV. On the lateral projection, you may either choose right or left recumbency, but make sure and always use it on serial radiographic examinations. It is important that the radiographs are not obliqued. A normal heart can appear diseased when positioning is not adequate. When you are evaluating for correct positioning, on the lateral projection, make sure the dorsal heads of the ribs should are superimposed, the forelimbs are pulled forward so that they are nor superimposed over the cranial thorax, and that the radiographic exposure is taken during full inspiration. When obtaining an orthogonal radiograph, realize that the anatomic positioning of the heart in a DV is less dependent on thoracic cavity conformation, the dorsal lung fields are hyperinflated and the vessels to the caudal lung fields are magnified, and a DV view allows increased detection of early pulmonary infiltrates. As a result of the above, a DV view is preferred, however.... a straight symmetric projection is the goal and if a VD view achieves this goal – by all means use it!! To evaluate the DV/VD view, the dorsal spinous processes of the thoracic vertebrae should be centered over the vertebral bodies and the exposure should be sufficient to define the outline of the thoracic vertebrae superimposed over the cardiac silhouette. Radiographic anatomy on the lateral view is easily divided into the cranial border which represents the margins of the right heart and the caudal border which represents the left cardiac chamber margins. The dorsal third of both the cranial and caudal border is the atria and the ventral ⅔ is the ventricle. The "waist" is the separation between the atria and ventricles. The arteries are located dorsal to the veins. The cranial pulmonary arteries and veins should be equal in size to the proximal 4th rib. The clock face analogy is commonly used to show location of important cardiac vessels and chambers on the VD/DV views. The aortic arch is located between 11 and 1o'clock, the main pulmonary artery between 1-2 o'clock, the left auricular appendage 2:30-3 o'clock, the left ventricle between 3-5 o'clock, the right ventricle between 5-9 o'clock, and the right atrium is located between 9-11 o'clock. In dogs, the cardiac apex is usually shifted approximately 30' to the left of midline. The pulmonary arteries are lateral to the veins on the DV/VD view. The caudal vena cava is to the right of midline and the descending aorta summates with the heart and extends into the abdomen to the left of the spine. Radiographic interpretation involves a systematic approach and required that abnormalities be substantiated on multiple radiographic views. Firs and foremost when you begin to evaluate for technical quality, positioning, and proper exposure....if the study is substandard....STOP AND REPEAT THE RADIOGRAPHS! To evaluate a thoracic radiograph – I use the central out approach (cardiac silhouette first, pulmonary vessels, trachea, pulmonary parenchyma, sternum, ribs, vertebrae, cervical soft tissues, and then lastly the abdomen). First, assess the position of the cardiac silhouette. The location of the cardiac silhouette is affected by non-cardiac thoracic pathology such as pulmonary disease (atelectasis from anesthesia or recumbent animal, previous lobectomy), pleural disease (fluid or air), as well as mediastinal mass lesions. All of these alter cardiac position. The cardiac silhouette can be separated from sternum due to hyperinflation of the lungs in a deep chested breed dog - don't be fooled into thinking this is only associated with pneumothorax. Congenital sternal defects (pectus carninum and excavatum) also cause the heart position to vary. Next we evaluate cardiac size. A normal heart on the lateral view is approximately ⅔ the height of the thoracic cavity. The width is between 2.5-3.5 intercostal spaces in a dog and 2.5-3 intercostal spaces in a cat. There are great breed variations in dogs however (dachshunds, yorkies, and pugs). On the DV/ VD view, the width of the heart is approximately ⅔ the width of the thoracic cavity. These rules do not apply to deep chested dogs like Dobermans or collies which have a more "up and down" heart resulting in a round cardiac silhouette on the DV/VD view. Fat in the middle mediastinum in a cat can artifactually cause the cardiac silhouette to appear enlarged. Some other facts to remember with respect to cardiac size.... young animals appear to have larger hearts relative to thoracic size, the heart is smaller on inspiration than expiration, expiration causes increased sternal contact, anemic or emaciated patients have small hearts due to hypovolemia. Cardiomegaly results when one (or more) of the cardiac chambers becomes enlarged. Left atrial enlargement is suspected on the lateral view when there is dorsal elevation of the caudal portion of the trachea and carina, the right and left mainstem bronchi are no longer superimposed (the left bronchus will be more dorsal than the right), ultimately this results in straightening of the caudal cardiac silhouette. On the DV/VD view in a dog, a "double opacity" of the atrial body over the caudal aspect of the cardiac silhouette is seen. The body of left atrium causes lateral bowing of the mainstem bronchi (aka "bow legged cowboy"). In a cat, enlargement of the cardiac margin at the 2-3 o'clock position (Indentation of the caudal cardiac waist). There are numerous causes of left atrial enlargement. Mitral insufficiency from valvular endocardiosis, cardiomyopathy, congenital heart diseases (mitral valve dysplasia, PDA-patent ductus arteriosus, VSD -ventricular septal defects, ASD-atrial septal defects) and left ventricular failure. Tracheobronchial lymph node enlargement and a pulmonary mass adjacent to the cardiac hilus can also cause increased opacity overlying the heart and need to be differentiated from left atrial enlargement. Left ventricular enlargement on the lateral view shows loss of the caudal waist, the caudal cardiac margin is straighter and more vertical than normal, there is dorsal elevation of the intrathoracic trachea, carina, and mainstem bronchi, and the angle between the thoracic spine axis and trachea is diminished to the point of being parallel. On a DV/VD view, rounding and enlargement of the left ventricular margin, rounding of the cardiac apex conformation, and shift of the cardiac apex to the right. Etiologies of left ventricular enlargement include mitral insufficiency, cardiomyopathy, congenital heart diseases (PDA-patent ductus arteriosus, VSD -ventricular septal defects, AS-aortic stenosis), high output cardiac diseases (fluid overload, chronic anemia, peripheral arteriovenous fistula, obesity, and chronic renal disease), hyperthyroidism, and hypertension. Right atrial enlargement is identified on the lateral view by elevation of the trachea, accentuation of the cranial waist, enlargement of the more dorsal margin of the cranial cardiac silhouette. On the DV/VD view, enlargement of the cardiac margin at the 9-11 o'clock position is seen. Causes of right atrial enlargement include right heart failure, tricuspid insufficiency, cardiomyopathy, right atrial neoplasia (such as hemangiosarcoma), other diseases that can mimic right atrial enlargement are cranial mediastinal masses, heart base tumor (common in brachycephalic breeds), tracheobronchial lymph node enlargement, superimposition of the aortic arch or main pulmonary artery, and right cranial or middle lobar pulmonary alveolar consolidation or mass lesion. Right ventricular enlargement appears on the lateral view as increased sternal contact (considered a subjective radiographic sign), elevation of the cardiac apex from the sternum (remember this is normal on a left lateral view), increased cardiac width (rounding of the cardiac silhouette), and disproportionate enlargement of the cranial portion of the cardiac silhouette. On the DV/VD view, right ventricular enlargement appears as enlargement of the cardiac silhouette at the 6- 11 o'clock position, a reverse "D" appearance of the cardiac silhouette (which is caused by the the enlargement and rounded conformation of the right margin causing the left margin by comparison to appear more straight in shape), and shifting of the cardiac apex to the left. Causes of right ventricular enlargement are secondary to left heart failure, tricuspid insufficiency, cardiomyopathy, cor pulmonale, heartworms, congenital heart diseases (PS-pulmonic stenosis, PDA -patent ductus arteriosus, VSD-ventricular septal defect, TOF -tetralogy of Fallot, and tricuspid valve dysplasia. Microcardia (overall small cardiac silhouette) appears as decreased size of the cardiac silhouette with respect to the thoracic cavity size and can be caused by Addison's disease, hypovolemic shock, pericarditis, and tension pneumothorax. Now we need to evaluate the great vessels. Enlargement of the aortic arch and aorta on the lateral view is seen as widening of the dorsal aspect of the cardiac silhouette as well as enlargement of the craniodorsal cardiac margin. On the DV/VD view, widening of the caudal portion of the cranial mediastinum between the 11 and 1 o'clock position. The causes of aortic enlargement are PDA – descending aorta (at 1 o'clock position), aortic stenosis – post stenotic enlargement of the ascending aorta (at 11 o'clock position), and aortic aneurysm (which is uncommon). Other diseases that need to be differentiated from an aortic bulge are aortic knob in geriatric cats, cranial mediastinal mass, thymus ("sail sign" in young dogs), cranial mediastinal fat, and this can also be a normal variation in some dogs.