Contributors: Fraser McConnell, Jordi Lopez-Alvarez
Species: Canine | Classification: Miscellaneous
- Radiology plays an important part in the diagnosis and management of cardiac disease. It allows assessment of the pulmonary vasculature and lungs which is not possible with echocardiology.
- Radiology also plays an important role in the differentiation of cardiac and respiratory cough.
Follow the diagnostic tree for the Radiographic Detection of Canine Cardiomegaly Radiographic Detection of Canine Cardiomegaly.
- The radiographic technique is particularly important in thoracic imaging Radiography: thorax.
- For the investigation of cardiac disease a right lateral projection and dorsoventral (DV) projection should be taken.
- Careful positioning is important, particularly to prevent rotation of the thorax which can lead to a false impression of mild left atrial enlargement.
This effect is most marked in deep chested dogs, eg Doberman pinscher.
- The heart has a complex shape and even small degrees of rotation can make assessment of chamber size unreliable, particularly on the DV projection.
- Care must be taken to ensure that the stress of radiography will not cause further decompensation in animals with congestive heart failure.
- Oxygen supplementation via a face mask may help ease the patient's respiratory distress improving the quality of the radiographs.
- A DV projection can usually be taken without sedation in dyspneic animals.
- The lateral projection can then be taken after the patient's condition has been stabilized.
- In most cases, however, it is safer to give a low dose of sedation Sedation / sedative protocols if required than to struggle with a dyspneic animal.
- The intramuscular or intravenous combination of butorphanol Butorphanol and acepromazine Acepromazine maleate ) are generally used in cardiac patients.
Alpha 2 agonists should be avoided.
- A high kV low mA technique should be used to reduce contrast within the thorax and allow exposure times to be kept as short as possible.
- Movement blur Radiography: film faults is one of the most common problems in thoracic radiography, particularly in dyspneic patients.
- This can be minimized by using a high kV, dispensing with a grid and using a fast film/screen Radiography: X-ray film combination.
Radiological evaluation of the heart
- Evaluating every radiograph according to a set protocol will help to ensure that no significant findings are missed.
- Care should be taken to assess the entire radiograph including skeletal and extrathoracic structures.
- If abdominal structures are visible on the film these should be examined too, for instance hepatomegaly or ascites may be present which could suggest right heart failure.
- The considerable variation in thoracic conformation in dogs can make assessment of cardiac size difficult .
- The ideal method for deciding if the heart is enlarged is comparison with a previous film of the same individual.
- This may be possible if the animal has been radiographed when a murmur was first detected and before onset of clinical signs.
- Evaluation of sequential films is very useful in assessment of response to treatment or progression of cardiac failure.
- In many cases a previous radiograph is not available and to overcome this problem the technique of assessing vertebral heart score (VHS) was developed (Buchanan and Bucheler).
Evaluation of the different cardiac chambers
- Radiology for the evaluation of specific cardiac chamber size is limited, but knowing their position in the X-rays is very helpful.
- In the lateral view:
- Dorsal to the level of the caudal vena cava are the atria; dorsal to the level of the ventral chambers are the ventricles.
- The cranial chambers correspond with the right heart chambers whereas the more caudal chambers correspond with the left heart chambers, including the cardiac apex.
- This way, the heart can be divided in four quadrants; the cranio-dorsal includes the right atrium, the caudo-dorsal the left atrium, the cranio-ventral includes the right ventricle and the caudo-ventral the left ventricle.
- In the DV view (clock fact analogy):
- 11.00-1.00 o'clock: aortic arch.
- 1.00-2.00 o'clock: main pulmonary artery.
- 2.00-3.00: left auricular appendage.
- 3.00-5.00: left ventricle.
- 5.00-9.00: right ventricle.
- 9.00-11.00: right atrium.
Vertebral heart score
- This score is a ratio generated by comparing the height and width of the cardiac silhoutte with the length of the thoracic vertebra :
- On the lateral projection the height or the long axis of the heart from the apex to the base of the carina is measured.
- The width or short axis of the heart at it's widest point, perpendicular to the height measurement is determined.
- These two measurements are summed and the sum compared with the thoracic vertebrae starting at the cranial endplate of the 4th thoracic vertebral body.
When using this technique you must ensure that no abnormal vertebrae, ie hemivertebrae are included in the measurement
- The normal VHS is 9.7 +/- 1.
- A VHS greater than 11 indicates cardiac enlargement, however, a large variation is found depending on the conformation of the chest for each dog breed.
- Changes in heart size are usually the result of structural changes, eg dilation or hypertrophy.
Bradycardia and anemia may also result in enlargement of the heart on radiographs.
- Cardiac enlargement may indicate heart disease with or without failure.
Always consider the clinical signs and history when assessing the significance of alterations in heart size, eg working animals may have large hearts.
- The vertebral heart score may be dependent on breed.
- Boxer and Labrador Retriever have higher mean VHS.
- The score is more likely to be meaningful in breeds where heart disease usually equates to cardiac enlargement, eg Yorkshire Terrier and CKCS, than the Boxer where hypertrophic cardiomyopathy (concentric cardiomegaly) is common.
- Changes in cardiac shape may give an indication of the chambers or vessels involved in the disease and hence the likely etiology of the problem.
- Echocardiography Ultrasound: echocardiology is more useful for assessing specific chamber enlargement and in establishing the cause of heart disease Vertebral heart scoring video.
- The pulmonary vasculature is evaluated to assess the degree of volume overload, if any.
- "Veins are central and ventral": on the lateral projection the pulmonary veins lie ventral to the bronchus and artery, while on the DV projection the vein is central to these structures:
- The normal pulmonary artery and vein are of similar size.
- The cranial lobe vessels are best assessed on the lateral projection. In the normal animal the blood vessels (where they cross the 4th rib) should be narrower than the proximal third of this rib .
- The caudal lobe vessels are visualized most clearly on the DV projection. On this projection the blood vessel should be narrower than the 9th rib where they cross.
- The normal pulmonary artery and vein are of similar size.
- Volume overload causes the pulmonary veins to enlarge (venous congestion) due to increased left atrial pressure .
- Eventually the increase in venous pressure may result in enlargement of both pulmonary arteries and veins. As left-sided CHF develops fluid floods the alveoli resulting in an increase in the opacity of the lungs:
- The size of the caudal vena cava can be assessed if visible :
- In normal animals it often appears tapered and is usually similar size to the aorta. In the dog enlargement of the caudal vena cava to 1.5 times the width of the aorta is consistent with the presence of right heart failure.
- In right-sided CHF pleural fluid or ascites may be visible at the edge of the thoracic film.