Radiography, echocardiography, and electrocardiography (Proceedings)

In this session we will review thoracic radiology and echocardiography with an emphasis on normal and abnormal anatomic features.

Indications

Modern imaging technology (MRI, CT, echocardiography) has dramatically improved the veterinarian's ability to diagnose cardiac and thoracic disease. However, it has not diminished the need for thoracic radiographs. Advanced imaging has not replaced thoracic radiography and thoracic radiography does not replace advanced imaging. Thoracic radiography continues to be one of the most practical and best tests for identifying parenchymal disease (congestive heart failure, pneumonia, heartworm and Neoplasia).

Remember that dyspneic animals can be extremely fragile. In some cases, it may be reasonable to treat based on suspicion of the disease for 12-24 hours to try to stabilize the patients before taking radiographs.

Limitations

Remember that thoracic radiographs can not indicate cardiac or pulmonary function. Additionally, while they may be fairly specific for cardiac enlargement, they are not very sensitive, or said another way- if significant cardiac enlargement is present it is likely real, if it is absent, it does not rule out cardiac disease. Significant breed variations are known and can complicate the interpretation of films even in normal patients. Finally it can be very difficult to interpret right ventricular enlargement patterns.

Technique for interpretation

In order to avoid misinterpretation it is ideal to examine every thoracic radiograph in a systemic fashion.

Technical quality

In almost all cases 2 views should be obtained. An exception to this would be a recheck radiograph taken within 24 hours after a thoracocentesis to determine the efficacy of a pleurocentesis. In this case it might be reasonable to just retake the lateral film.

Although different radiologists use different approaches, we generally evaluate the right lateral and a dorsoventral (DV) view. The use of a ventrodorsal approach is also acceptable, but may be more risky in animal with dyspnea or significant pleural fluid.

     1. Thoracic space

The thoracic space should be evaluated for both pneumothorax and pleural fluid. Sometimes the most sensitive detection of pleural fluid is observed on a DV view in the costodiaphragmatic angles.

     2. Cardiac anatomy

The cardiac silhouette should be evaluated for enlargement. If enlargement is identified, it is extremely important to then determine the specific chamber (s) enlarged. Unfortunately, breed and species differences can sometimes mislead and suggest the appearance of general cardiac enlargement even if it is not present.

The Vertebral Heart Score was developed to help reduce the confusion observed with these differences. The Vertebral Heart Score (VHS) is a method of normalizing cardiac size to body length or conformation by relating cardiac size to vertebral body (JAVMA 1995;206:194-199, Buchanan et al). The procedure is as follows:

          • Measure the long axis of the heart from the ventral border of the left mainstem bronchus to the most distal ventral contour of the cardiac apex using calipers or a note card. Reposition the calipers or note card to the cranial edge of the fourth thoracic vertebrae and measure the number of vertebrae caudally to the nearest 1/10 of a vertebrae.

          • In the central third of the heart, measure the maximal width or short axis of the heart at the angle that is perpendicular to the previously measured long axis. Reposition the measuring device over the thoracic spine at the cranial aspect of the fourth vertebrae and record the number of vertebrae caudally to within the nearest 1/10 of a vertebra.

          • Add the 2 measurements together to equal the VHS.

          • The average VHS is 9.7, most dogs are less than 10.5. Most cats are less than 8.

The Vertebral Heart Score is not very useful in cats since many cats with myocardial disease, including hypertrophic cardiomyopathy, can have normal VH scores and have very abnormal hearts.

Perhaps one of the best uses of the VHS is to study progression of known cardiac disease. For example, the VHS may be determined in cases where radiographs will be taken annually or every 6 months in patients with cardiac disease to help identify subtle size changes or different interpretations between doctors.

After evaluating for global cardiac enlargement, the specific chambers of the heart should be evaluated for enlargement. Generally we use a clock face analogy.

Dorsoventral view

12:00- 1:00 aorta

1:00-2:00 pulmonary artery

2:00-3:00 left auricle

2:00-5:00 left ventricle

5:00-9:00 right ventricle

9:00-11:00 right atrium

Lateral view

12:00- 3:00 left atrium

3:00-5:00 left ventricle

5:00-9:00 right ventricle

9:00-11:00 right atrium, pulmonary artery or aorta

     3. Vasculature

On the lateral view, the vessels are best evaluated in the cranial lobes where they are arranged dorsal to ventral as artery, bronchus and vein. On the dorsoventral view the vessels are best evaluated in the caudal lobes as they are arranged lateral to medial as artery, bronchus and vein. Pulmonary artery and vein are generally of equal size and have clear sharp edges. A rule of thumb is that the average vessel size is approx. 75% of the diameter of the proximal 1/3 of the fourth rib on the lateral view.

Enlarged pulmonary arteries may suggest pulmonary hypertension (heartworm disease, chronic pulmonary disease, thromboembolic disease).

Enlarged pulmonary veins may suggest pulmonary venous congestion with left heart failure.

Enlarged pulmonary arteries and veins may suggest over circulation, i.e.intravenous fluid overload or a left to right shunt (PDA, VSD, etc).

     4. Parenchymal changes

Parenchymal patterns are generally divided into interstitial, bronchial and alveolar patterns.

Interstitial patterns are generally described as "ground glass" and result from the accumulation of fluid in the connective tissue framework of the lung. This may be associated with aging changes, cardiogenic pulmonary edema, noncardiogenic pulmonary edema, interstitial pneumonia, lymphoma or diffuse metastatic disease.

Bronchial patterns are associated with increased lymphatic flow in the peribronchial tissue as well as accumulation of fluid in the bronchial walls and peribronchial tissue. This pattern is characterized by the "doughnut" appearance of thickened bronchi in cross section. This may be associated with feline asthma, canine bronchitis, allergic pneumonitis or pulmonary infiltrates with eosinophils.

Alvelolar patterns are associated with the filling of the alveoli with fluid so that the fluid silhouettes the remaining air filled bronchi which appear as air bronchograms. This may be observed with severe heart failure, bronchopneumonia, pulmonary contusion, hemorrhage or other noncardiogenic edema.

It should be remembered that radiographs are good for suggesting cardiac enlargement and excellent for detecting vascular and parenchymal changes associated with heart failure. They are not specific for types of cardiac disease, therefore an echocardiogram is still needed to determine the type of cardiac disease that initiated the problem. Secondly, echocardiographs may tell us about the specific type of cardiac disease, but they can not tell us about the status of congestive heart failure. Thus radiographs and echocardiography are BOTH needed in many cases, one does not replace the other.

Echocardiography

Echocardiography is not a particularly difficult technique, but it is very user dependent!!! It is similar in many ways to learning a sport or musical instrument in that it takes great dedication to practice to become skilled! Practice is crucial for the echocardiographer to develop the ability to obtain all of the images as well as to develop the ability to identify normal anatomical variations from disease states.

Equipment

There are many different types of machines. We will not discuss types of machines today, but minimally, the machine should come with multiple probes of different frequencies, a electrocardiogram that will run concurrently, printing ability and calculation packages to allow calculation of fractional shortening, pressure and shunt gradients, etc.

Transducers

High frequency probes (7.5 MHz) produce sound waves with a shorter wavelength which allows better reflection from smaller structures. This allows better detail and resolution but less tissue penetration than a lower frequency (3.5 MHz). So a small animal like a cat or puppy would be best approached with a 7.5 MHz, but a larger animal like a Doberman pinscher should be approached with a 3.5 MHz.

Types of echocardiography

M-Mode and two-dimensional echocardiography are used to evaluate cardiac chamber size anatomy and motion. The best images are obtained when the ultrasound beam is perpendicular to the heart structures. Doppler echocardiography is best to evaluate direction and speed of blood flow. Doppler information is most accurate when the Doppler waves are parallel to the blood flow,

Two dimensional (2D echocardiography)

Many cardiologists start with the 2D echo to begin the examination. The 2D study creates a pie or fan shaped image to display anatomic features and should be used to evaluate both the Long axis and Short Axis views.

M-Mode echocardiography

After the 2D study is completed, M-Mode images should be obtained from the short axis at the level of the left atrium, mitral valve, and left ventricle. The M-Mode evaluates a single "ice pick" beam over time. This is the mode generally used to evaluate the size and function of the ventricle. Measurements are made from leading edge to leading edge. After the wall thickness and ventricular lumen sizes are determined, the fractional shortening (FS%), an index of contractility, can be determined from the formula = Left ventricular dimension at diastole- left ventricular dimension at systole/left ventricular dimension at diastole.

Doppler echocardiograpy

Doppler echocardiography is used to evaluate blood flow direction and speed. It can be used to identify an insufficient valve as well as a congenital shunt. Color flow may be used to detect the direction of the valve, and Pulsed is generally used to detect flow velocity. The velocity is important since it can be used to identify the pressure gradient across an stenotic valve or shunt using the formula 4x (velocity)2 = Pressure gradient. It should be kept in mind that the accuracy of Doppler is best when the beam is parallel to flow and the lowest MHz probe should be used.

In most evaluations, all three forms of echocardiography should be performed since they support each other and the study usually begins with a 2D study, followed by M- Mode and then Doppler.

However, in some cases, one can find sufficient information with just 2D or M-Mode. For example, 2D will be sufficient to diagnose pericardial effusion and cardiac tamponade. However, we may want 2D, M- mode and Doppler to evaluate the cat with hypertrophic cardiomyopathy.