Contributors: Fraser McConnell, Mark Rishniw, Jordi Lopez-Alvarez

 Species: Feline   |   Classification: Miscellaneous

Introduction

  • 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.

Radiographic considerations

  • The radiographic technique is particularly important in thoracic imaging   Radiography: thorax.

Positioning

  • 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.
  • The heart has a complex shape and even small degrees of rotation can make assessment of chamber size unreliable, particularly on the DV projection.

Restraint

  • Care must be taken to ensure that the stress of radiography will not cause further decompensation in animals with congestive heart failure.
  • Oxygen supplementation by a face mask may help ease the patient's respiratory distress improving the quality of the radiographs, although some cats may not tolerate this well and stress can potentially worsen the situation.
  • A DV projection can usually be taken without sedation in dyspnoeic 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 if required than to struggle with a dyspneic animal.
  • The combination of midazolam Midazolam and ketamine Ketamine is a useful sedative for cats. Low dose butorphanol Butorphanol tartrate and alfaxalone Alphaxalone, intramuscularly or intravenously, is also frequently used to sedate unstable cats.

Alpha 2 agonists should be avoided.

Exposure factors

  • 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, and using a fast film/screen Radiography: x-ray film combination.
Clinical tip:
Question: What is the normal VHS score in cats?
Answer: 7.5 +/- 0.3

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 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.
  • The normal heart size in the cat is fairly consistent and should not measure more than 2 intercostal spaces on the lateral projection   Heart: normal size - radiograph lateral  .
  • In many cases a previous radiograph is not available and to overcome this problem the technique of assessing vertebral heart score (VHS) was developed (Litster & Buchanan).
  • Obese cats often fail to inflate their lungs properly: this can imitate pulmonary edema and falsely suggest cardiac enlargement.
  • In older cats the cardiac silhouette often bulk cranially and may be parallel with the strenum. There is often a sigmoid course to the ascending aorta. These are incidental findings.
  • In fat cats, fat within the pericardium often appears as a discrete triangular bulge on the right side of the cardiac silhouette on the DV projection.

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 and 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 caudoventral, the left venticle.
  • In the DV view (clock face 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 silhouette 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 its 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 7.5 +/- 0.3. Cardiac enlargement is present when VHS is >8.1.
  • Changes in heart size are usually the result of structural changes, eg dilation Heart: dilated cardiomyopathy - radiograph (lateral) or hypertrophy   Heart: hypertrophic cardiomyopathy 02 - radiograph lateral  .

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.

  • Pericardial disease (eg PPDH) will also cause the appearance of cardiomegaly, but is less common in cats than dogs.
  • 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 Ultrasonography: heart is more useful for assessing specific chamber enlargement and in establishing the cause of heart disease.
  • Many cats with HCM Heart: hypertrophic cardiomyopathy have cardiac silhouette that is normal size.
  • Shape changes are a more sensitive indicator of cardiac disease. On the lateral projection cats with HCM often become kidney shaped due to left atrial enlargement and rounding of the cardiac apex.

Pulmonary assessment

  • The pulmonary vasculature is evaluated to assess the degree of volume overload, if any.
  • Pulmonary vasculature is not reliably or consistently altered with heart failure in cats.
  • "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 caudal lobe vessels are visualized most clearly on the DV projection  Lung: pulmonary overcirculation - radiograph DV .
    • The caudal vena cava runs parallel to the spine on the right side and must not be mistaken by a lobar vessel.
  • Left-sided 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 interstitium alveoli, resulting in an increase in the opacity of the lungs:
    • In the cat this is often patchy and generalized, often more pronounced in the periphery of the lungs Lung: pulmonary edema lateral radiograph .
    • If severe pulmonary edema is present air bronchograms may be visible.
  • The size of the caudal vena cava can be assessed if visible:
    • In normal animals  Thorax: normal 01 - radiograph lateral it often appears tapered and is usually similar size to the aorta but can vary substantially.
  • Pleural effusion Pleural effusion may occur with both left & right-sided CHF in cats  Thorax: pleural effusion 01 - radiograph lateral .