Cardiology Logo
Indications in Small Animals
1. What is Echocardiography?

Echocardiography refers to the imaging of the heart with ultrasound. This is the same technology which is used so extensively in man to examine the heart as well the abdominal viscera and the pregnant uterus. Echocardiography may be divided into three types based on the instrumentation and application. These are M-mode, Two-dimensional, and Doppler Echocardiography.

2. What is M-mode Echocardiography?

This is the oldest form of echocardiography. It was introduced into Veterinary Medicine in the late 1970s. It is more specifically known as the Time Motion Mode.

This view of the heart provides information about a very narrow core of tissue. The dimensions of this narrow core of cardiac tissue can be readily obtained over numerous cardiac beats in this format. Although this form of echocardiography was the first to be described, it is still best suited to measure the thickness of the ventricular walls and the dimensions of the internal cavity of the ventricles. From these parameters indices of the ventricular contractility can be readily determined. Of these indices, fractional shortening [FS] (left ventricular internal dimension in diastole [LVID-D] minus left ventricular internal dimension in systole [LVID-S] all divided by the left ventricular internal dimension in diastole [LVID-D] is the most common. The normal value for FS in the dog and cat is 25 to 45 %. Other indices of contractility are under consideration as to their utility, such as:

  1. the mitral valve "E" point to septal separation [EPSS],
  2. the ratio of the LVID-D to the left ventricular free wall dimension in diastole [LVFW-D],
  3. the ratio of the LVID-S to the left ventricular free wall dimension in systole [LVFW-S].

As well as these derived parameters there are numerous other indices that have been proposed to assess the performance of the heart. The dimensions of the internal diameter of the ascending aorta and the left atrium can also be determined. As well, M-mode Echocardiography is ideal to closely examine for abnormalities in the motion of the valve leaflets (such as fluttering of the anterior leaflet of the mitral valve in diastole associated with aortic valve insufficiency or premature closure of the aortic valve associated with left ventricular concentric hypertrophy) which may suggest a specific cardiac morphologic or physiologic abnormality.

Even though M-mode Echocardiography represents the oldest form of this imaging modality, M-mode Echocardiography will continue to have an important role to play in the assessment of cardiac disorders a result of its great ability to readily and repeatedly quantitate intracardiac linear dimensions.

It must be noted that M-mode Echocardiography requires considerable expertise to perform properly. The most technically demanding feature is to identify the exact positions in the heart that fulfill the criteria established to obtain the specific parameters to be measured. Furthermore, the specific location for calaper placement to properly measure the desired parameters has been rigidly established. For adequate interlaboratory comparison of data, the established criteria must be rigidly adhered to.

3. What is Two-dimensional Echocardiography?

This form of echocardiography provides a wide overall view of the heart. It is ideal to assess the relative size of the four intracardiac chambers and the relationship of the thickness of the cardiac walls to the size of the chambers. Thus, for individuals with regional disorders such as intracardiac tumors, this mode of echocardiography is indispensible. Two-dimensional Echocardiography is also well suited to identify pericardial effusion. This mode of imaging may allow the identification of intracardiac shunts, prolapse of an atrio-ventricular valve leaflet, the obstruction of outflow of blood or the restriction to motion of a valve.

4. What is Doppler Echocardiography?

This is the newest form of echocardiography. Doppler Echocardiography is an additional and supplemental part of an echocardiographic examination. Doppler involves the detection of blood flow. During an echocardiographic examination, the Doppler examination detects blood flow throughout the heart.

The location of normal and abnormal blood flow as well as the velocity of blood flow can be readily detected. The velocity of blood flow is displayed as a histogram of peak velocities over time.

5. How does Doppler Echocardiography assist with the identification of cardiac disease?

The Doppler Echocardiographic examination enables the specific identification of valvular heart disease; ie, valvular insufficiency and valvular stenosis. As well, the Doppler examination enables the detection of intracardiac shunts; ie, atrial septal defects and ventricular septal defects.

The Doppler examination also provides evidence of abnormal diastolic function, and reductions in cardiac output.

6. How is the Doppler Echocardiographic examination different from the routine echocardiographic examination?

Although a Doppler examination can be performed independent from a two-dimensional examination, most Doppler echocardiography is performed simultaneously with a two-dimensional examination.

A routine two-dimensional echocardiographic examination provides spacial information about cardiac chamber size and wall thickness. Thus, with a two-dimensional examination the size of the left and right ventricle in systole and diastole can be measured, particularly with the aid of M-mode echocardiography, the size of the left atrium can be measured, and indices of contractility can be derived. And so we tend to infer the existence of valvular disease based on changes in the volume of the chambers or the thickness of the walls. Sometimes two-dimensional echocardiography may reveal the existence of reduced valve leaflet excursion as in stenotic valvular disease, or prolapse of valve leaflets as in valvular insufficiency. However, as opposed to such indirect evidence, Doppler echocardiography provides direct evidence for the existence of valvular disease.

7. Can Doppler Echocardiography provide insight into the severity of valvular heart disease?

One of the greatest assets of Doppler echocardiography is its ability to assess the severity of the valvular disorder. In the case of stenotic valvular disease, as the stenosis progresses the peak velocity of blood flow across the stenotic lesion increases. Thus stenotic disorders can be readily classified as to severity with the determination of the Doppler velocity of blood flow.

The normal peak velocity of blood flow across the four valves in the heart has been determined for the dog:

  • Tricuspid valve = 0.9 meters/sec
  • Mitral valve = 1.1 meters/sec
  • Pulmonic valve = 1.2 meters/sec
  • Aortic valve = 1.5 meters/sec

The ability of Doppler echocardiography to quantitate the severity of valvular insufficiency has been more ambiguous. It would appear that Doppler echocardiography may only be useful to provide a semi-quantitative assessment of the severity of valvular insufficiency.

8. How does Doppler Echocardiogrpahy detect atrio-ventricular valvular insufficiency?

Doppler echocardiography employed to detect atrio-ventricular valvular insufficiency is primarily performed from the left parasternal position. In the Doppler (spectral display graphic display of the peak velocity histograms), the turbulent signal is negative and occurs in systole.

A lesion is classified as mild if the insufficiency signal is detected only in the region close to the atrio-ventricular valves. A lesion is considered to be of moderate severity if the signal is detected into the mid-region of the affected atrium. And finally, the insufficiency is classified as severe if the turbulent signal of atrio-ventricular valve insufficiency is detected at the base of the affected atrium or in the inlet vessels to the affected atrium.

The incidence of tricuspid valve insufficiency, as determined by Doppler echocardiography, has been reported to be approximately 50% in normal dogs.

9. How does Doppler Echocardiography detect semilunar valve insufficiency?

The pulmonic valve may be interrogated from the right and left parasternal positions with Doppler echocardiography. The aortic valve is examined from the left parasternal position with Doppler. In the Doppler spectral display, the turbulent signal of semilunar valvular insufficiency occurs as a positive signal and in diastole.

Mild lesions result in the detection of turbulence only in the region of the outflow tract proximal to the semilunar valves. With moderate to severe lesions the turbulent signal is detected deep into the chamber of the ventricle.

Pulmonic valve insufficiency as detected by Doppler echocardiography occurs in 25 to 70% of normal dogs.

10. How does Doppler Echocardiography detect atrio-ventricular valve stenosis?

The Doppler examination is performed from the left parasternal position. The Doppler spectral display reveals an increased peak velocity that is positive and occurs in diastole.

11. How does Doppler Echocardiography detect semilunar valve stenosis?

The pulmonic valve may be examined from the left or right parasternal position. The aortic valve is examined from the left parasternal position. Stenosis is present when the peak velocity of blood flow detected across the outflow tract is increased. The Doppler spectral display reveals a negative, systolic signal that is increased.

As the severity of the stenosis progresses, the peak velocity of blood flow across the stenosis increases.

12. How does Doppler Echocardiography shed light on the presence of diastolic disease of the ventricle?

Diastolic disease of the ventricle is commonly one of the earliest signs of heart disease. Diastolic disease of the left ventricle is demonstrated by the presence of left atrial enlargement and later pulmonary edema. Diastolic disease of the right ventricle is indicated by the presence of right atrial enlargement and later jugular distention or ascites.

The presence of increased velocity of blood flow across the atrio-ventricular valve that normally occurs in late diastole and is associated with atrial contraction suggests the presence of reduced ventricular compliance (the Doppler E wave wave).

13. What other disorders can be detected with Doppler Echocardiography?

A ventricular septal defect can be detected with Doppler echocardiography. With a left to right shunt, the peak velocity of blood flow is inversely related to the severity of the defect.

Left to right patent ductus arteriosus can be diagnosed by detecting a continuous turbulent signal in the main pulmonary artery.

Cases of reduced cardiac output are associated with reduced velocities of blood flow across the aortic valve.

14. Is Echocardiography an invasive procedure?

This procedure is a completely non-invasive diagnostic test in all its various modalities. In the vast majoriity of cases even sedation is not required as an aid to perform this examination.

15. What are the Standard Transthoracic Echocardiography Views?
There are four standard parasternal locations used in a typical echocardiogram.
(From Thomas et al, JVIM 1993, 7(4) 247-252)

Right parasternal long axis views:

  • Right parasternal long-axis four-chamber view (2a).
  • Right parasternal long-axis left ventricular outflow tract view (2b).
  • Right parasternal long-axis view of the left ventricular inflow and outflow tracts (2c).

Right parasternal short axis views:

  • Right parasternal short-axis view at the level of the papillary muscles (3.2)
  • Right parasternal short-axis view at the level of the chordae tendinae (3.3)
  • Right parasternal short-axis view at the level of the mitral valve (3.4)
  • Right parasternal short-axis view at the level of the aortic valve (3.5)
  • Right parasternal short-axis view at the level of the pulmonary arteries (3.6)

Left caudal parasternal views:

  • Left caudal parasternal four-chamber view (5a)
  • Left caudal parasternal five-chamber view (5b)

Left cranial parasternal views:

  • Left cranial parasternal long-axis view of the aorta (6a)
  • Left cranial parasternal long-axis view of the left ventricle showing the tricuspid valve (6b)
  • Left cranial parasternal long-axis view of the pulmonary artery (6c)
  • Left cranial parasternal short-axis view of the aorta showing the right ventricular inflow and outflow tracts (7)