Cardiology


CLINICAL CARDIOLOGY CONCEPTS
for the dog and cat

Michael R. O'Grady, DVM, MSc, Diplomate ACVIM (Cardiology)
M. Lynne O'Sullivan, DVM, DVSc, Diplomate ACVIM (Cardiology)

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.

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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.

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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.

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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.


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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

Pulmonary artery hypertension can be determined by noting an increase in the regurgitant velocity of blood flow in pulmonic valve insufficiency or by noting an increase in the regurgitant velocity of blood flow in tricuspid valve insufficiency provided pulmonic stenosis is absent and tricuspid valve insufficiency is present which is often the case.

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.

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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.

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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)

Examples to Demonstrate Diagnostic Values

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16 Acquired Disorders

  1. Cardiomyopathy of the Cat:

    You are presented with a cat which has pulmonary edema, a gallop heart rhythm, a heart murmur and weak femoral arterial pulses. The radiographs reveal generalized cardiomegaly with pulmonary edema. The electrocardiogram reveals a normal sinus rhythm and left ventricular enlargement.

    Question: Does this cat have cardiomyopathy and if so what type is it?

    Routine two-dimensional echocardiography reveals a heart typical of left ventricular concentric hypertrophy; i.e., the cavity of the left ventricle is reduced relative to the increased thickness of the interventricular septum and the left ventricular free wall. The left atrium is increased in size. The right side of the heart appears normal.

    The M-mode Echocardiogram is used to determine the thickness of the interventricular septum, the left ventricular internal dimension, and the left ventricular free wall in both diastole and systole.

    IVS-DIASTOLE 5mm

    IVS-SYSTOLE 8mm

    LVID-DIASTOLE 10mm

    LVID-SYSTOLE 5mm

    LVFW-DIASTOLE 5mm

    LVFW-SYSTOLW 8mm

    There is no real value in performing a Doppler Echocardiographic study in this individual. A Doppler examination however may show evidence of reduced left ventricular compliance (diastolic disease) and / or increased velocity of blood flow across the left ventricular outflow tract.

    Diagnosis: This is a typical case of Hypertrophic Cardiomyopathy in the cat (example). This may be due to thyrotoxicosis or it may be idiopathic.

    Note that Dilated Cardiomyopathy would have presented with the following Two-dimensional and M-mode Echocardiographic findings. The left ventricular lumen would be increased in size and the left atrium would also be increased in internal dimension (example). The M-mode examination would have revealed a normal thickness to the interventricular septum and left ventricular free wall, and an increase in the left ventricular internal dimension in diastole (>18mm) and a relative reduction in the left ventricular internal dimension in systole (example). Thus the index of left ventricular contractility, fractional shortening

    [(LVID-D - LVID-S)/LVID-D], would be reduced (normal = 25-45%).

  2. Cardiomegaly in a dog:

    A six year old German Shepherd presents to you for a sudden onset of marked lethargy and exercise intolerance. On physical examination the heart sounds are muffled and the femoral arterial pulses are very weak. Thoracic radiographs reveal marked cardiomegaly and no pleural effusion. There is no evidence for pulmonary edema. The EKG reveals low amplitude QRS complexes.

    Question: Does the very enlarged heart represent heart failure, if so what is the cause?

    The Two-dimensional Echocardiogram reveals a marked pericardial effusion. A thorough examination of the region of the right atrium indicates a mass attached to the outside wall. Although M-mode Echocardiography is ideal to assess the contractility of the left ventricle, in the presence of the pericardial effusion such assessments are erroneous.

    Some ultrasonographs enable the technician to perform ultrasound assisted biopsies. Thus a sample of the effusion can be obtained for analysis and culture with the assistance of ultrasound to guide the placement of the biopsy needle.

    Doppler Echocardiography has no real value in this type of case.

  3. The geriatric small breed dog with chronic cough:

    A 12 year old dog presents to you for a 6 month history of a chronic cough. The cough is exacerbated by excitement, exercise and stress. The cough has progressed markedly over the last month. On physical examination a thrill is palpated over the left chest and a marked systolic heart murmur is noted with a point of maximal intensity over the left cardiac apex. On thoracic radiography the heart is enlarged, the left atrium is enlarged, there is a prominent interstitial pattern in the lungs which is mainly a peribronchial pattern. The EKG reveals left ventricular enlargement and left atrial enlargement. There is no evidence of a dysrhythmia.

    Question: Is the cough due to pulmonary edema a result of heart failure?

    The Two-dimensional Echocardiogram reveals an enlarged left ventricular and left atrial cavity. Subjectively, the left ventricle appears to be strong (adequate contractility). The M-mode Echocardiogram reveals increased left ventricular internal dimensions in diastole and in systole, and normal contractility (fractional shortening = 47%).

    LVID-DIASTOLE 40mm

    LVID-SYSTOLE 21mm

    F. Shortening 47%

    Left Atrial-S 22mm

    Given that the index of contractility is normal, can we suggest that the heart is strong enough and thus not responsible for the cough? In the face of mitral valve insufficiency, we anticipate that it is easier for the left ventricle to contract. Thus the adequate contractility calculated may merely reflect that the ventricle may be ejecting blood in the wrong direction. It has been suggested that if the LVID-S is normal then the heart is probably normal in strength in spite of the mitral valve insufficiency.

    Thus as the LVID-S is normal, we believe that the left ventricle and thus the heart is not responsible for the cough. Thus therapy for pulmonary edema of cardiogenic causes is not necessary at this time.

    Doppler Echocardiography can be used to demonstrate the existence of mitral valve insufficiency. Although this confirms the existence of MI, it adds little new knowledge to this case. The Doppler examination may provide evidence of the amount of forward flow into the aorta and the amount of backward flow into the left atrium.

  4. A Doberman Pinscher with pulmonary edema:

    A six year old Doberman Pinscher presents to you for an acute onset of dyspnea and wheeze. On physical examination the lung sounds are harsh, there is a soft left apical systolic heart murmur, and there are several premature beats noted in the cardiac rhythm. Thoracic radiographs indicate mild left ventricular enlargement, left atrial enlargement, and pulmonary venous congestion and edema. The EKG indicates the infrequent uniform ventricular premature beat, otherwise the EKG is normal.

    Question: Does the dog have Dilated Cardiomyopathy?

    The Two-dimensional Echocardiogram reveals left ventricular enlargement and left atrial enlargement. The contractility appears subjectively to be reduced. The M-mode Echocardiogram reveals an increase in the left ventricular internal dimension in diastole and systole and an increase in the left atrial internal dimension in systole.

    LVID-DIASTOLE 52mm

    LVID-SYSTOLE 46mm

    F. Shortening 11.5%

    For the Doberman the LVID-D should not be more than 40mm. Thus we have documented the presence of a global reduction in contractility and marked left ventricular enlargement. This is typical of Dilated Cardiomyopathy.

    Doppler Echocardiography would reveal mitral valvular insufficiency and reduced stroke volume. Although this information is useful, we were able to deduce these findings without the application of Doppler Echocardiography.

    See more information on Doberman Heart Disease. (Dilated Cardiomyopathy)

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17 Congenital Heart Disorders

  1. Patent Ductus Arteriosus:

    A 3 month old Sheltie female pup presents for vaccination. The physical examination reveals a continuous heart murmur. This finding alone is usually sufficient to indicate the existence of Patent Ductus Arteriosus. The EKG reveals marked left ventricular and left atrial enlargement. Thoracic radiographs reveal left ventricular and left atrial enlargement and pulmonary venous congestion and pulmonary edema.

    Question: Is there any use for Echocardiography in this disorder?

    Echocardiography is useful to:

    1. confirm the presence of Patent Ductus Arteriosus,
    2. help rule out the presence of other concurrent disorders.

    The Two-dimensional examination reveals enlargement of the left ventricle and left atrium. It is still uncertain as to how frequently the ductus arteriosus itself can be visualized. The M-mode Echocardiographic study does not increase our understanding of this disorder.

    The Doppler Echocardiographic study reveals the presence of turbulence in the main pulmonary artery which occurs in both systole and diastole. This picture of turbulence in the main pulmonary artery is similar to that seen with pulmonic stenosis, however with pulmonic stenosis this pattern of turbulence is noted only in systole. Furthermore, the Doppler Echocardiographic examination is ideal to identify the co-existence of other congenital cardiac disorders.

  2. Aortic Stenosis:

    A 4 month old male Newfoundland presents to you for vaccination. A left basilar systolic heart murmur is noted that radiates well to the right chest. Thoracic radiographs reveal a normal cardiac silhouette and no evidence of pulmonary vascular or interstitial disease. The EKG is normal.

    Echocardiography is of outstanding value to confirm a presumptive diagnosis of aortic stenosis or subaortic stenosis. In this disorder, short of cardiac catheterization, there is no other method available to confirm the existence of aortic stenosis.

    Although routine Two-dimensional Echocardiography may give us clues to the existence of aortic stenosis, in many cases this test is inconclusive. In severe cases of aortic stenosis, the Two-dimensional exam may reveal left ventricular concentric hypertrophy and a discrete subvalvular lesion, in the case of subaortic stenosis. The M-mode Echocardiographic examination may reveal a clue to the presence of co-existent aortic valve insufficiency (diastolic fluttering of the anterior leaflet of the mitral valve) premature closure of one cusp of the aortic valve. As well this modality should indicate evidence of left ventricular concentric hypertrophy.

    The Doppler Echocardiographic study yields data which usually definitively establishes the diagnosis of aortic stenosis and provides evidence of the severity of the disorder. Doppler Echocardiography determines the velocity of blood flow as it exits the left ventricle. The normal maximal velocity of blood flow exiting the left ventricle is approximately 1.5 meters per second. Thus velocities detected in excess of 1.5 m/s suggest stenosis of the column of blood flow (especially velocities in excess of 2.0 m/s). In addition, we have noted that aortic valve insufficiency occurs in a large percentage of cases of subaortic stenosis. It appears that this aortic valve insufficiency does not significantly hemodynamically embarrass the performance of left ventricle, however it does frequently serve as a useful marker to the presence of concurrent subaortic stenosis. As the subaortic stenosis progresses the maximal velocity of blood flow across the stenotic region increases. Severe stenosis is characterized by a velocity of blood flow of 5 m/s or greater.

    In this dog the Doppler study revealed a velocity of blood flow exiting the left ventricle of 3.5 m/s and the presence of aortic valve insufficiency.

  3. Pulmonic Stenosis:

    A 3 month old Samoyed presents to you for vaccination. On physical examination a left basilar systolic heart murmur is detected. Thoracic radiographs appear normal. The EKG also is normal. Once again Echocardiography is ideal to confirm the diagnosis of this congenital cardiac disorder. As for aortic stenosis, Doppler Echocardiography is the specific mode of echocardiography which offers the most information to establish the diagnosis of pulmonic stenosis and address the severity of the disorder.

    Routine Two-dimensional Echocardiography nevertheless provides much insight into the disorder. As the severity of the pulmonic stenosis increases, one will observe an increase in the thickness of the interventricular septum and right ventricular free wall. The moderator band of the right ventricle also increases in size. The region of the pulmonic valve is noted to be narrowed and the motion of the pulmonic valve is seen to be restricted (reduced amplitude of excursion). The main pulmonary artery is usually noted to be enlarged. On the M-mode examination the measured thickness of the interventricular septum is increased as well as that of the right ventricular free wall. The motion of the interventricular septum is noted to be abnormal demonstrating a flat motion in systole.

    The Doppler Echocardiographic examination reveals an accelerated velocity of blood flow across the pulmonic valve in systole. The normal maximal antegrade velocity of blood flow across the pulmonic valve should be no more than 1.2 meters per second. Thus peak velocities of blood flow detected in excess of 1.2 m/s (especially velocities in excess of 2.0 m/s) across the pulmonic orifice in systole suggest pulmonic stenosis. As the severity of the stenosis increases the measured maximal velocity of blood flow detected increases. Velocities in excess of 5 m/s indicate a severe degree of stenosis.

    The Doppler study in this dog revealed a velocity of blood flow across the pulmonic valve of 2.5 m/s and pulmonic valve insufficiency.

  4. Other congenital disorders:

    Routine Two-dimensional Echocardiography and Doppler echocardiography are useful to detect intracardiac shunts and dysplasia of the mitral and tricuspid valves.

Normal M-Mode Parameters

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18 Normal M-Mode Echocardiographic Parameters

A. For the cat:

X +/- 2SD
LVID-D (mm) 13 3.5
LVID-S (mm) 8.5 3.5
F.S. (%) 35 10
IVS-D (mm) 3.5 2
IVS-S (mm) 5.0 1.5
LVFW-D (mm) 3 2
LVFW-S (mm) 5.5 1.5
AO (mm) 9 3
LA (mm) 9 3

B. For the dog

  1. Parameters that are independent of body weight:

    X +/- 2SD
    F.S. (%) 33 12
    EPSS 3.25 2.7

  2. Other parameters varied with body weight and thus normograms must be utilized to determine individual values based on body weight.