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M-Mode Echocardiography
Symposium on Cardiopulmonary Diagnostic Techniques
M-Mode Echocardiography Basic Principles
john D. Bonagura, D.V.M., M.S.*
The clinical application of ultrasound has expanded the diagnostic capabilities of the veterinarian and has provided a noninvasive method of cardiac imaging. Echocardiography employs the piezoelectric crystal or ultrasound transducer to generate high-frequency sound waves (1 to 7 MHz). Ultrasound, when transmitted through the thorax, can penetrate the heart and subsequently be collected and processed to delineate cardiac structures. Such imaging is possible because ultrasound behaves according to the laws of optics and will reflect (echo) from the interface of cardiac tissues of different acoustic densities. Accordingly, the hand-held transducer can be directed to transmit ultrasound, collect the reflected sound waves, and dispense the resultant signal to the echocardiograph for spatial arrangement and display. If the transducer is maintained in a constant position during the cardiac cycle, the phasic motion of cardiac structures can be recorded. The resultant record is termed a motion, or M-mode, echocardiogram. The echocardiogram is recorded on paper, with the horizontal axis and electrocardiogram serving as a time reference. The cardiac structures are discerned by observing their typical motion and the relative vertical distance of each structure from the transducer artifact. Since the Y axis of the M-mode echogram is calibrated in millimeters, the thickness, size, and excursions of the cardiac chambers and valves can also be determined. Echocardiography is useful in the evaluation of patients with congenital or acquired heart diseases and can be employed to estimate left ventricular function (Table 1). Among the cardiac lesions readily identified by ultrasonography are pericardial effusion, valvular vegetations, atrial and ventricular dilatation and hypertrophy, and abnormal cardiac motion. Since the Mmode echogram affords only an "icepick" view of the heart, certain lesions cannot be imaged. Among these are atrioventricular valvular insufficiency, *Diplomate, American College of Veterinary Internal Medicine (Internal Medicine and Cardiology); Assistant Professor, Department of Veterinary Clinical Sciences, The Ohio State University Veterinary Teaching Hospital, Columbus, Ohio
Veterinary Clinics of North America: Small Animal Practice-Yo!. 13, No. 2, May 1983
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Table 1. PARAMETER
Echocardiographic Indices of Left Ventricular Function* MEASUREMENTS REQUIRED
CALCULATION
Left ventricular diastolic dimension (LVDD), left ventricular systolic dimension (LVSD)
Fractional shortening (%) LVDD- LVSD
Velocity of circumferential fiber shortening (VcF) (circumferences/sec)
LVDD, LVSD, left ventricular ejection time (ET)
VcF (circumferences/sec) LVDD- LVSD
Ejection fraction (EF)
End diastolic volume (EDV), LVDD, end systolic volume (ESV), LVSD
EDV = LVDD 3 ESV = LVSD 3 EDV- ESV EF (%) EDV
Fractional shortening (FS, or % .lD)
INTERPRETATION
Decreased contractility decreases FS
LVDD Decreased contractility decreases VcF
LVDD x ET
Pre-ejection period (PEP) Left ventricular ejection time (LVET)
Phases of ventricular systole, preejection period, ET
PEP LVET
Decreased contractility decreases EF
Decreased contractility increases ratio of PEP to LVET
*Ejection phase indices and systolic time intervals are difficult to interpret in the presence of significant mitral valve insufficiency.
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Figure l. The echocardiographic examination. The transducer is held gently over the heart against the ventral right hemithorax. The tranducer is angulated in order to record the standard echocardiographic images. The patient is awake and restrained in left lateral recumbency. An electrocardiogram is recorded for purposes of timing.
ventricular or atrial septal defects , and regional dysfunction of the ventricles. Some of these limitations are obviated by the recognition of associated cardiac changes (such as dilatation) or the use of intravascular contrast agents that outline blood flow. The development of newer imaging techniques , such as real-time cardiac sector scanning and Doppler echocardiography, has expanded the diagnostic capabilities of cardiac ultrasound. Yet, in spite of the aforementioned limitations, the properly interpreted Mmode echocardiogram remains a powerful diagnostic test in the evaluation of patients with heart disease. Technical Considerations The equipment needed for M-mode echocardiography includes the echocardiograph, an electrocardiograph, the ultrasonic transducer, a coupling gel, and a method of recording the echocardiographic display (such as a strip chart recorder). Ancillary equipment is needed for simultaneous phonocardiography, apexcardiography, and hemodynamic and pressure determinations. Selection of the transducer depends principally on the size of the patient, as a shorter wavelength offers less penetration but greater resolution and a more detailed image. For cats and small dogs, a 5- or 7MHz unfocused transducer is used. A 3.5-MHz medium-focused transducer is appropriate for imaging dogs weighing 15 kg or more. Examination of horses and cattle requires a transducer that emits a wavelength of 2. 25 or 1.6 MHz and an echocardiograph that has been adjusted to image to a depth of up to 35 em. The small animal patient is generally positioned in left lateral recumbency (Fig. 1). Some animals are examined more effectively in a 30 to 45° left lateral oblique position, which can be maintained with foam wedges of the type used for radiography. Occasionally, the standing position is necessary to obtain a satisfactory echogram.
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The transducer is placed at the lower right sternal border between the second to fourth intercostal spaces and is coupled to the thorax with an ultrasonic gel (see Fig. 1). Gentle pressure is applied as the transducer is angulated caudomedially in order to obtain an image. Suitable adjustments are made in the depth of field, gain, time-gain compensation, and reject controls in order to delineate the image. The technical aspects of ultrasound imaging have been described elsewhere. 7 By changing the transducer angle or location, the standard positions of Feigenbaum7 can be recorded (Fig. 2). With little exception, echocardiograms from animals are qualitatively similar to those from humans. 1• 5 • 11· 14- 16
Normal Findings The identification of cardiac structures requires both a preconceived notion of the expected echocardiographic image and a knowledge of the changes that occur in disease. The echobeam initially traverses the right ventricle; therefore, this structure is closest to the transducer artifact. The right ventricular dimension is normally quite small, since the ultrasonic beam usually passes through the tip of this crescent-shaped chamber (Fig. 3B). The ventricular septum is the next structure encountered, followed by the left ventric~lar lum~n and posteriorleftventricularwalL The ventricular walls are particularly thiek at the level of the papillary m:uscles (Fig. 3A). Ventricular dimensions are measured just below the mitral valve (Fig. 3B). If the transducer is angled dorsally, the mitral apparatus appears. The normal mitral valve is imaged as two structures: the anterior (cranial) and posterior (caudal) leaflets and associated chordae tendineae of the valve. The anterior mitral valve leaflet has a wider excursion and inscribes a typical "M" shape. The posterior mitral valve leaflet inscribes a "W," or the mirror image of the anterior mitral valve leaflet. Normal mitral valve motion has been designated as follows: C, point of mitral valve systolic closure; D, end of systolic mitral closure; E, maximum early diastolic separation of the anterior and posterior mitral valve leaflets; F, semiclosure of the valve; and A, mitral opening as the result of atrial activation (Fig. 3C to E). The mitral valve normally may appear very thick during systole when the annulus and valve move toward the transducer and the echobeam strikes the apparatus in a tangential manner. The tricuspid apparatus has similar motion and can be imaged by angulating the transducer slightly craniad (Fig. 3D); however, the lateral and septal leaflets are more readily observed when there is right ventricular dilatation. If the transducer is angulated dorsally and slightly cranially, the aorticleft atrial position is imaged (Fig. 3F and G). On a smooth sweep from the mitral position, the anterior aspect of the aortic root blends with the ventricular septum, and the posterior part of the root is continuous with the anterior mitral valve (see Fig. 2). The aortic root is recognized as two parallel lines that move toward the transducer in systole. The left atrium is caudal to the aorta and is contiguous 'with the posterior wall of the aortic root. Aortic motion results from both the systolic ejection of blood and from the filling of the left atrium. One or two aortic valvules are usually visible. These structures open toward the wall of the aorta during systole and close in the center of the aortic root during diastole. The left atrial size is
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Figure 2. A and B, Schematic illustration of the standard echocardiographic positions as described by Feigenbaum. 7 Position one traverses the right ventricle, ventricular septum, and left ventricle at the level of the papillary muscles. As the transducer is angulated dorsally, the chordae tendineae and cusps of the posterior and anterior mitral valve are imaged (positions 2 and 3). In the most dorsal position, the aortic root and the left atrium are recorded. The approximate motion mode (M-mode) echocardiograms obtained as the transducer is directed through these different positions are illustrated below. (Modified from Feigenbaum, H.: Echocardiography. Edition 3. Philadelphia, Lea & Febiger, 1981.)
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Figure 3 (Continued). D, Echogram obtained at the mitral valve level. Both AMV and PMV are evident and inscribe the typical M and W configurations. Parts of the tricuspid valve (TV) apparatus are imaged in the right ventricle. E, Normal AMV motion. The systolic (C to D) and diastolic excursions of the AMV are evident. Note the normal recording of multiple mitral echoes during systole . The C point occurs coincident with the first heart sound at the onset of mitral closure. F, E chogram recorded at the aortic root-left atrial position. The aortic root is evident as two parallel lines that move toward the transducer during systole. The aortic valve (AV) is imaged and closes at the time of the aortic incisura and the second heart sound (arrows). During systole, the valvulae move towards the walls of the aorta (arrowhead) and achieve maximal separation during rapid ejection of blood into the aortic root (dotted line). G, Measurements obtained from the aortic root position. The left atrium (LA) is noted to fill during ventricular systole. The amplitude (a) of aortic motion correlates to ventricular stroke volume and atrial filling. The normal ratio of the LA to aortic root dimension (AO) is approximately 0.8 to 1.2 in the dog.
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determined at its greatest dimension during ventricular systole (Fig. 3G). The posterior wall motion of the left atrium is attenuated and can thus be distinguished from the left ventricular wall. The ratio of the left atrial dimension to the aortic root dimension differs between species, with a normal ratio of approximately 0. 8 to l. 2 in the dog and cat and approximately 0.4 to 0.8 in the horse and cow. Intracardiac dimensions and ventricular wall thickness vary with the size of the animal and are best indexed to body weight or body surface area. Valve motion and the per cent of ventricular shortening normalized to the initial cavity dimension (fractional shortening) are relatively constant between species, though slightly higher in the cat. Other normal findings include concordant motion of the ventricular septum and left ventricular wall (toward each other during systole), and a ratio of septal thickness to left ventricular wall thickness of approximately 1.0 in normal small animals. Special echocardiographic contrast studies can be performed using the patient's blood, saline solution, carbon dioxide, or indocyanine green dye. 3 · 9 • 10· 18 When injected into the heart or vascular system, these agents induce microcavitation, forming echo-dense "targets" that are readily imaged in the heart (see Fig. 6). These targets tend to follow the path of blood flow, and thus permit the detection of intracardiac shunts and mitral regurgitation. I have experienced the greatest success with an equal mixture of patient blood and green dye, using a dose of 3 to 6 ml of green dye for most small animal patients.
ECHOCARDIOGRAPHIC EVALUATION OF HEART DISEASE Congenital Heart Disease M-mode echocardiography is helpful in the diagnosis of congenital heart disease. Although it is difficult to directly identifY some congenital cardiac lesions, the echogram still provides information about the responses of the heart. For example, right ventricular or left ventricular hypertrophy can be identified with pulmonic or aortic stenosis. Paradoxical (toward the transducer) systolic septal motion and increased right ventricular dimensions are noted in right ventricular volume overloading due to atrial septal defect, tricuspid valve dysplasia, and severe pulmonic valve stenosis with secondary tricuspid regurgitation. Left ventricular dilatation, as occurs with patent ductus arteriosus, aortic regurgitation, mitral dysplasia, and ventricular septal defect, can be determined. Finally, echo-contrast studies can identify intracardiac shunting. Pulmonic Valve Stenosis. The pulmonic valve is difficult to image and the dysplastic valve may not be apparent. However, the response to this pressure overload is usually evident with right ventricular hypertrophy, increased right ventricular internal dimension, and paradoxical septal motion imaged in most cases (Fig. 4). The left atrium and left ventricle may be smaller than normal owing to reduced right ventricular output. The tricuspid valve may be prominent with enlargement of the right ventricular cavity.
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Figure 4. Echocardiogram obtained from a four-month-old Great Dane with pulmonic stenosis and mild tricuspid valve dysplasia. Salient features include hypertrophy of the right ventricular wall (RVW), increased right ventricular internal dimension (RV), which is approximately equal to the left ventricular dimension (LV), and paradoxical movement of the ventricular septum (S) during systole. The RVW thickness is normally less than 50 per cent of the left ventricular wall. A systolic murmur of pulmonary stenosis is recorded on the phonocardiogram (PCG). Parts of the tricuspid apparatus are visible within the right ventricular lumen.
Tetralogy of Fallot. It is possible in some cases to image both the hypertrophied right ventricle and the overriding aorta (Fig. 5). Since the aortic root is dextropositioned over the septal defect, the normal continuity between the ventricular septum and aortic root may be lost. Right ventricular hypertrophy is evidence of the pulmonary outflow obstruction, and decreased internal dimensions of the left atrium and ventricle suggest the reduced pulmonary flow caused by right-to-left shunting. Intracardiac shunting is readily identified through an injection of green dye into the cephalic or jugular vein. Contrast will appear first in the right ventricle and then in the left ventricle, concentrating between the anterior mitral valve and ventricular septum. 3 Aortic Stenosis. Congenital aortic stenosis in the dog usually consists of a subvalvular fibrous ring that may partially envelop the aortic valvules. Accordingly, it is possible to image the subvalvular obstruction by sweeping from the mitral to aortic valve positions. The lesion is identified as a slight narrowing in the outflow tract just below the aortic valve. An additional
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echocardiographic finding is poststenotic dilatation of the aorta, which can be appreciated by angulating the transducer dorsad and craniad from the aortic valve. Hypertrophy of the left ventricular wall and the ventricular septum are abnormalities that accompany aortic stenosis. Atrial Septal Defect. Atrial septal defect is rare as an isolated congenital lesion. The septal defect cannot be imaged, as the interatrial septum is not readily visualized by M-mode techniques. The consequences of the left-toright atrial shunt can be appreciated and include right ventricular dilatation, increased thickness of the right ventricular wall, and marked paradoxical septal motion. A cephalic vein injection of green dye may indicate some degree of right-to-left shunting, with contrast appearing in both the left atrium and left ventricle. Ventricular Septal Defect. Although the ventricular septum is readily visualized, it is unusual to image the "drop out" in echoes associated with a ventricular septal defect. Although two-dimensional techniques can readily identify the septal patency, the pinpoint image of the M-mode echogram limits this technique to determining the consequences of this lesion. Echographic findings vary but may illustrate shunt-induced enlargement of the left and right ventricles, left atrial enlargement, and slightly paradoxical septal motion. Echo-contrast studies can identifY the left-to-right cardiac shunt; however, the contrast material must be introduced directly into the left ventricle, as "conventional" microbubbles (green dye or saline) do not pass through the lungs. Inasmuch as this requires cardiac catheterization, there is no significant advantage to this technique versus routine angiocardiography in small animals. However, in large animals that are not readily imaged by radiography, this technique can substantiate the ventricular shunting (Fig. 6). Recently developed contrast materials capable of passing through the lungs may prove useful in the future and permit noninvasive delineation of this lesion. Patent Ductus Arteriosus. The echocardiographic findings in patent ductus arteriosus are those of left ventricular volume overload with increased left ventricular internal dimensions, normal ventricular function, and increased left atrial size. The amplitude of aortic root motion (see Fig. 3G) is generally exuberant, indicating a large stroke volume and increased left atrial filling. Immediately following surgical ligation of the ductus, the left ventricular dimension is smaller. Atrioventricular Valve Dysplasia. Tricuspid valve dysplasia, which results in marked increases in right ventricular internal dimension and paradoxical septal motion, is identified by recognition of abnormally thickened diastolic echoes returning from the tricuspid valve structures. Mitral valve dysplasia similarly may result in increased thickness of the valve, increased left atrial and ventricular internal dimensions, and increased fractional shortening of the left ventricle. Pericardial Diaphragmatic Hernia. The findings with congenital pericardial-peritoneal communications are similar to those with pericardia} effusion; however, the usual echo-free pericardia} space may be dense owing to the presence of the liver or another abdominal structure. The
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osu 621272 Figure 6. Contrast echocardiogram from a one-month-old female Holstein calf with a ventricular septal defect. Following injection of indocyanine green dye and the patient's blood, the image of the left ventricle (LV) becomes opacified. Bubbles (arrow) abruptly appear in the right ventricle (RV), indicating shunting of blood at the ventricular level. A transient arrhythmia (ectopic QRS ) was induced by introduction of a needle into the LV. RV = right ventricular lumen; IVS = interventricular septum; LVOT = left ventricular outflow tract; AMV = anterior leaflet of the mitral valve . (From Bonagura, J. D., and Pipers, F. S.: Diagnosis of cardiac lesions by contrast echocardiography. J. Am. Vet. Med. Assoc. , 182:396402, 1983; with permission. )
right ventricle is frequently displaced from the thoracic wall as a result of the hernia and its contents. Dilated Coronary Sinus. The coronary sinus is situated behind the left atrium in the atrioventricular groove . When there is abnormal persistence of the left cranial vena cava, the coronary sinus may be dilated, resulting in a large posterior echo-free space at the junction of the anterior mitral valve/aortic root and left atrial positions. Identification of this le sion may alter the approach to cardiac catheterization or cardiac surgery. Acquired Heart Disease
Pericardia[ Disease. The essential echocardiographic features of pericardia! effusion include separation of the left ventricular epicardium and parietal pericardium, displacement of the right ventricular free wall from the thoracic cage, identification of a sonolucent space between the epicardium and pericardium, and abnormal cardiac motion (Fig. 7). 2 •7 Technical considerations of recording are important since improper gain and reject settings prevent identification of the fluid-filled (echo-free) pericardia! space. The magnitude of the pericardia! effusion varies , with the greatest accumulation usually noted at the cardiac apex. As the transducer is angulated
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F igure 7. A , Schematic diagram of approximate pathway of the ultrasound beam in pericardia! effusion. The transducer is placed at the right side of the thorax. The anterior mitral valve (2) and ventricular (1) positions are shown. Pericardia! effusion results in an echofree space between the epicardium and parietal pericardium. RV = right ventricular lumen; IVS = interventricular septum; AMV = anterior leaflet of the mitral valve; PMV = posterior leaflet of the mitral valve. B, Echogram from a 5-year-old spayed female Collie with granulomatous pericarditis. Echo-free spaces representing pericardia) effusion (PE) anterior and posterior to the heart are recorded at the mitral valve position. At higher reject levels, only the most echo-dense structures are visualized. As demonstrated in the right panel, these echoes usually originate from the pericardia) structures. RVFW = right ventricular free wall; IVS = interventricular septum; PLV = posterior left ventricular wall. D epth (1 em) calibration dots are recorded every second. (From Bonagura, J. D. , and Pipers, F . S.: Echocardiographic features of pericardia! effusion in dogs. J. Am. Vet. Med. Assoc., 179:49-56, 1981; reproduced by permission.)
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toward the base, the sonolucent space diminishes in size owing to the firm attachments of the pericardium at the atrioventricular groove. The pericardium appears to be thickened in some cases, and pericardia! motion frequently is dampened (flat) with large effusions. Exudate or fibrin strands are occasionally identified as echo-dense structures within the effusion. Cardiac motion can be abnormal and is caused by actual swinging of the heart within the pericardia! effusion. It is difficult to assess valvular, septal, or ventricular wall excursions in such cases. Pericardia} effusion must be distinguished from pleural effusion. Inasmuch as pleural effusion is also noted behind the left atrium and thoracic radiography readily identifies most pleural effusions, this distinction is not difficult. Rarely, excessive pericardia! fat appears as a small echo-free space behind the epicardium. Constrictive pericarditis has been recognized in some animals and has been identified echocardiographically by the recordings of thickened epicardial echoes, abrupt flattening of ventricular diastolic motion, paradoxical septal motion, decreased ventricular dimensions, and flattened mitral valve E-F slope, indicating decreased transmitral flow and reduced ventricular filling. 2• 7 Myocardial Diseases. Myocardial diseases are common in veterinary practice. Ventricular dilatation and hypertrophy can be recognized based on echocardiographic assessment of left ventricular chamber size and wall thickness. Dilatation of the left ventricle, with minimal or absent wall hypertrophy, is recognized in association with idiopathic congestive cardiomyopathies of the dog and cat, chronic anemia, viral myocarditis, and doxorubicin (Adriamycin) toxicity. Chronic volume work, as occurs with valvular insufficiency or congenital shunts, also results in ventricular dilatation. Acquired causes of hypertrophy of the left ventricle, with normal or reduced internal ventricular dimensions, are (idiopathic) hypertrophic cardiomyopathies, hyperthyroidism, and systemic hypertension associated with renal disease. Congestive Cardiomyopathy. The echocardiogram is quite valuable in the diagnosis and assessment of patients with congestive cardiomyopathy and is indicated in the evaluation of animals with radiographic evidence of cardiomegaly. Salient echocardiographic features of congestive cardiomyopathy include increased ventricular internal dimensions, left atrial enlargement, normal or decreased ventricular and septal wall thickness and systolic thickening, and decreased indices ofleft ventricular function (Fig. 8). Mitral valve motion may be abnormal, with increased distance between the E point and ventricular septum (indicating ventricular dilatation), increased excursion of the posterior leaflet (the "double diamond sign"), and delayed closure of the mitral valve resulting in a visible hump ("B shoulder") rather than the normally straight line between the A and C points. This B shoulder usually indicates increased ventricular and atrial end diastolic pressures. Aortic root excursion frequently is diminished, suggesting reduced stroke volume and cardiac output. There may be regional wall motion abnorml!.lititl$ with disparity between the ventricular septum and the hypokinetic left ventricular free wall.
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Figure 8. Echogram from a six-year-old male Coonhound with congestive (dilated) cardiomyopathy. The left ventricle is grossly dilated with a left ventricular internal dimension (LV) of approximately 8.5 em. LV fractional shortening is markedly reduced and clearly less than the usual values of 30 to 40 per cent. The anterior mitral valve (arrow) is evident, and the distance between the mitral valve E point to the ventricular septum is increased, indicating ventricular dilatation. The left ventricular wall (W) and septal wall thickness are normal. Left atrial enlargement is evident in the right panel as an increased left atriumaortic root ratio. A phonocardiogram and electrocardiogram are recorded at the bottom of the record. RV = right ventricular lumen; paper speed = 50 mm per sec; depth (l em) calibration dots are recorded every second.
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These abnormalities are practically diagnostic of congestive cardiomyopathy, and such findings are quite useful in establishing the etiology of heart failure in breeds that are infrequently affected with this disorder. Other causes of reduced myocardial contractility, such as parvovirus myocarditis and therapy with the antineoplastic drug doxorubicin can lead to similar echocardiographic changes. Hypertrophic Cardiomyopathy. Echocardiography is a sensitive indicator of ventricular hypertrophy and thereby permits the clinician to distinguish between the major forms of cardiomyopathy in cats and dogs. Hypertrophic cardiomyopathy is characterized by increased thickness and systolic thicke ning of the left ventricular wall and the ventricular septum. The ventricular internal dimensions are normal to decreased, and the mitral valve appears to be "crowded" into the thickened ventricle (Fig. 9). Some patients appear to have dynamic left ventricular outflow tract obstruction characterized echocardiographically by marked systolic anterior motion of the mitral valve. Indices ofleft ventricular function are normal to increased. Secondary enlargement of the left atrium and a normal aortic root excursion are other anticipated findings. Valvular Heart Disease. Bacterial Endocarditis. The vege tative lesion of infective endocarditis can be recognized by M-mode echocardiography.
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Figure 9. Echogram from a male cat with hypertrophic cardiomyopathy. Significant abnormalities include normal to increased systolic thickening of the left ventricular wall (L) and ventricular septum (S), normal fractional shortening, normal to decreased ventricular dimension during systole (arrows), and systolic anterior motion (arrowhead) of the mitral valve (M), which is noted to strike the septum during midsystole. Right ventricular free wall motion is normal. Motion artifact is present in the electrocardiogram. Paper speed = 50 mm per sec; depth (1 em) calibration dots are recorded every second.
The typical valvular vegetation appears as a thickened, "shaggy," irregular echo-dense mass. The lesion is attached to the affected valve and exhibits phasic motion in conjunction with valvular excursions. Mitral valve vegetations result in a mild to severe, but nonuniform, thickening of the valve. Aortic valve vegetations are identified as an echo-dense mass within the aortic root (Fig. 10) or as a prolapsed mass density appearing in the left ventricular outflow tract during diastole. 4 • 6 • 19 If the valve leaflet is ruptured or flail, diastolic fluttering of the valve may be imaged. Inasmuch as vegetations must be at least 3 mm thick to be discerned, the echocardiogram in some animals with bacterial endocarditis may not be diagnostic. Furthermore, changes attributed to endocarditis must be distinguished from those of the degenerative (myxomatous) thickening of the atrioventricular valves that is so common in dogs . This requires careful integration of echocardiographic and clinical data. In general, the identification of a large valvular vegetation carries a guarded to poor prognosis, as most of these patients develop congestive heart failure or signs of systemic embolization. Mitral and Aortic Regurgitation. Degenerative valvular heart disease (endocardiosis) results in subtle alterations in the affected valve and obvious abnormalities in the intracavitary dimensions. 13 Chronic valvular disease in the dog is recognized by mild, "smooth" thickening of the mitral or tricuspid valves. These thickenings are most prominent during the E and A points of valve excursion. Increased ventricular internal dimensions are expected on the affected side of the heart. Left atrial enlargement is recognized as an increased left atrium-to-aortic root ratio. Even in the presence of leftsided congestive heart failure, indices of ventricular function can be normal
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Figure 10. Echogram from a seven-year-old female Boxer with aortic valvular endocarditis, aortic stenosis, and congestive heart failure. Images recorded through the aortic root (AO) demonstrate a large echo-dense mass (VEG) that appears during diastole and is barely visible against the posterior wall of the aortic root during systole. Compare this with Figures 2F and G. Left atrial enlargement is also noted. PW = posterior wall. Systolic (SYS) and diastolic (DIAS) murmurs are recorded on the phonocardiogram (Phono). Depth (1 em) calibration dots are recorded every second. (From Bonagura, J. D., and Pipers, F . S.: Diagnosis of cardiac lesions by contrast echocardiography. J. Am. Vet. Med. Assoc., 182:595- 599, 1983; with permission.)
to supranormal. This is explained by the opportunity afforded the left ventricle to eject blood into the low-resistance left atrium. These findings are in marked contrast to patients with congestive cardiomyopathy, where indices of ventricular function are significantly reduced despite concurrent mitral regurgitation. Thus, the M-mode echocardiogram is useful in assessing the consequences and in determining the cause of mitral regurgitation. In congenital dysplasia, there may be marked thickening of the valve. With endocardiosis,
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mild valvular thickening is accompanied by normal left ventricular function; in endocarditis, the typical vegetative lesion may be recognized. Mitral regurgitation caused by congestive cardiomyopathy is allied with marked ventricular dilatation and hypokinesis. Rare causes of mitral regurgitation such as ruptured chordae tendineae also can be identified by noting abnormal valvular motion and chaotic fluttering of the flail valve. Potential causes of aortic insufficiency include congenital aortic valve disease, dilatation of the aortic root, bacterial endocarditis and, particularly in horses, degenerative valvular disease. Diagnostic criteria for these abnormalities have been previously described. It should also be recognized that diastolic fluttering of the aortic valve is nonspecific and can be observed with aortic regurgitation due to valvular degeneration or infective endocarditis. Diastolic fluttering of the mitral valve is diagnostic of aortic regurgitation from any cause and is present if the regurgitant jet strikes the anterior mitral valve leaflet (Fig. 11). The hemodynamic consequences of aortic insufficiency as assessed by echocardiography include increased left ventricular internal dimension, hyperkinesis of the left ventricle due to volume overload, and premature closure of the mitral valve due to increased ventricular diastolic pressure. 4 Other Lesions. The echocardiogram can document unsuspected or uncommon cardiac lesions and thus is an important aid in the diagnosis of such conditions. We have identified cases of mitral valve stenosis that have occurred secondary to marked endocardial inflammation. 13 Atrial thrombi, associated with cardiomyopathy, have been imaged as echo-dense structures within the sonolucent left atrial cavity. These usually are contiguous with the atrial wall. Occult dirofilariasis is suspected by the identification of right ventricular enlargement and of increased densities within the right ventricular cavity of a dog. The consequences of cardiac rhythm disturbances on cardiac motion and function also can be studied by echocardiography. Ventricular Function. Echocardiography is a useful adjunct in the assessment of myocardial performance. Left ventricular function is dependent on a number of factors, including myocardial contractility, regional wall function, and ventricular loading conditions. Although it is difficult to assess afterload (aortic impedance) with echocardiography, preloading can be estimated by determination of left ventricular end diastolic dimension. Ventricular fractional shortening (per cent Ll D), velocity of circumferential fiber shortening, ejection fraction, and systolic time intervals can be calculated from echocardiographically derived measurements (see Table 1 and Figs. 3 and 12). If the cardiac rhythm and loading conditions are constant, these indices are useful in the evaluation of myocardial contractility and assessment of directional changes in ventricular function. Unfortunately, these indices are of limited value when there is either significant mitral regurgitation or aortic stenosis because these conditions decrease and increase, respectively, the ventricular afterload. The calculation and basic interpretation of ejection phase indices of ventricular function and systolic time intervals are summarized in Table 1. Calculation of these indices requires the prior determination of left ventricular internal dimensions (at the chordal level), pre-ejection period (from
317
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the onset of the QRS until aortic valve opening), and left ventricular ejection time (from opening to closing of the aortic valve). 17 In general, a reduction in cardiac contractility will decrease the fractional shortening, velocity of circumferential fiber shortening, and ejection fraction, and will increase the ratio of pre-ejection period to ejection time derived from systolic time intervals. 8 • 12 Other estimates of ventricular function are efforts to evaluate the effects of contractility, stroke volume, or cardiac output on cardiac structures. These evaluations include per cent thickenings of the ventricular wall and septum, valvular separation during diastole (mitral valve) or systole
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Figure 12. Determination of systolic time intervals using the echocardiogram. The preejection period (pep) is measured from the onset of the QRS complex of the electrocardiogram (ECG) to the opening (O) of the aortic valve (AV). The left ventricular ejection time (et) is measured from the point of aortic valve opening to the point of aortic valve closure (C). This closure point can be ve rified since it occurs coincident with the initial high frequency vibrations of the second heart sound (2), as recorded on the phonocardiogram (PCG). Total electromechanical systole is the time from the onset of the QRS complex to the onset of the second heart sound. AO = aortic root; time lines are recorded every 10 msec.
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(aortic valve), amplitude of aortic root motion, and estimation of stroke volume by the cube formula as noted in Table 1. These have been described in humans but have not been adequately studied in animals. ACKNOWLEDGMENT
The author acknowledges the medical illustrations of Nancy schmidt.
J. Gold-
REFERENCES 1. Baylen, B. G., Garner, D. J., Laks, M. M., eta!.: Improved·echocardiographic evaluation of the closed-chest canine: Methods and anatomic observations. J. Clin. Ultrasound, 8:335-340, 1980. 2. Bonagura, J. D., and Pipers, F. S.: Echocardiographic features of pericardia! effusion in dogs. J. Am. Vet. Med. Assoc., 179:49-56, 1981. 3. Bonagura, J. D., and Pipers, F. S.: Diagnosis of cardiac lesions by contrast echocardiography. J. Am. Vet. Med. Assoc., 182:396-402, 1983. 4. Bonagura, J. D., and Pipers, F. S.: Echocardiographic features of aortic valve endocarditis. J. Am. Vet. Med. Assoc., 182:595-599, 1983. 5. Dennis, M. 0., Nealeigh, R. D., Pyle, R. L., eta!.: Echocardiographic assessment of normal and abnormal valvular function in Beagle dogs. Am. J. Vet. Res., 39:1591-1598, 1978. 6. Dillon, J. F., Feigenbaum, H., Konecke, L. L., eta!.: Echocardiographic manifestations of valvular vegetations. Am. Heart J., 86:698-704, 1973. 7. Feigenbaum, H.: Echocardiography. Edition 3. Philadelphia, Lea and Febiger, 1981. 8. Fortuin, N. J., and Pawsey, C. G. K.: The evaluation of left ventricular function by echocardiography. Am. J. Med., 63:1-9, 1977. 9. Gramiak, R., Shaw, P. M., and Kramer, D. H.: Ultrasound cardiography: Contrast studies in anatomy and function. Radiology, 92:939-948, 1969. 10. Kerber, R. E., Koschos, J. M., and Lauer, R. M.: TI.e use of an ultrasonic contrast method in the diagnosis of valvular regurgitation and intracardiac shunts. Am. J. Cardiol., 34:722-727, 1974. 11. Mashiro, I., Nelson, R. R., Cohn, J. N., et a!.: Ventricular dimensions measured noninvasively by echocardiography in the awake dog. J. Appl. Physiol., 41:953-959, 1976. 12. Pipers, F. S., Andrysco, R. M., and Hamlin, R. L.: A totally noninvasive method for obtaining systolic time intervals in the dog. Am. J. Vet. Res., 39:1822-1826, 1978. 13. Pipers, F. S., Bonagura, J. D., Hamlin, R. L., eta!.: Echocardiographic abnormalities of the mitral valve associated with left-side heart disease in the dog. J. Am. Vet. Med. Assoc., 179:580-586, 1981. 14. Pipers, F. S., and Hamlin, R. L.: Echocardiography in the horse. J. Am. Vet. Med. Assoc., 170:815-819, 1977. 15. Pipers, F. S., Muir, W. W., and Hamlin, R. L.: Echocardiography in swine. Am. J. Vet. Res., 39:707-710, 1978. 16. Pipers, F. S., Reef, V., and Hamlin, R. L.: Echocardiography in the domestic cat. Am. J. Vet. Res., 40:882-886, 1979. 17. Sahn, D. J., DeMaria, A., Kisslo, J., et al.-The Committee on M-mode Standardization of the American Society of Echocardiography: Recommendations regarding quantitation in M-mode echocardiography: Results of a survey of echocardiographic measurements. Circulation, 58:1072-1083, 1978. 18. Seward, J. B., Tejik, A. J., Spangler, J. G., eta!.: Echocardiographic contrast studies: Initial experience. Mayo Clin. Proc., 50:163-192, 1975. 19. Wray, T. M.: The variable echocardiographic features in aortic valve endocarditis. Circulation, 52:658-663, 1975. 1935 Coffey Road Columbus, Ohio 43210
M-Mode Echocardiography Basic Principles
john D. Bonagura, D.V.M., M.S.*
The clinical application of ultrasound has expanded the diagnostic capabilities of the veterinarian and has provided a noninvasive method of cardiac imaging. Echocardiography employs the piezoelectric crystal or ultrasound transducer to generate high-frequency sound waves (1 to 7 MHz). Ultrasound, when transmitted through the thorax, can penetrate the heart and subsequently be collected and processed to delineate cardiac structures. Such imaging is possible because ultrasound behaves according to the laws of optics and will reflect (echo) from the interface of cardiac tissues of different acoustic densities. Accordingly, the hand-held transducer can be directed to transmit ultrasound, collect the reflected sound waves, and dispense the resultant signal to the echocardiograph for spatial arrangement and display. If the transducer is maintained in a constant position during the cardiac cycle, the phasic motion of cardiac structures can be recorded. The resultant record is termed a motion, or M-mode, echocardiogram. The echocardiogram is recorded on paper, with the horizontal axis and electrocardiogram serving as a time reference. The cardiac structures are discerned by observing their typical motion and the relative vertical distance of each structure from the transducer artifact. Since the Y axis of the M-mode echogram is calibrated in millimeters, the thickness, size, and excursions of the cardiac chambers and valves can also be determined. Echocardiography is useful in the evaluation of patients with congenital or acquired heart diseases and can be employed to estimate left ventricular function (Table 1). Among the cardiac lesions readily identified by ultrasonography are pericardial effusion, valvular vegetations, atrial and ventricular dilatation and hypertrophy, and abnormal cardiac motion. Since the Mmode echogram affords only an "icepick" view of the heart, certain lesions cannot be imaged. Among these are atrioventricular valvular insufficiency, *Diplomate, American College of Veterinary Internal Medicine (Internal Medicine and Cardiology); Assistant Professor, Department of Veterinary Clinical Sciences, The Ohio State University Veterinary Teaching Hospital, Columbus, Ohio
Veterinary Clinics of North America: Small Animal Practice-Yo!. 13, No. 2, May 1983
299
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Table 1. PARAMETER
Echocardiographic Indices of Left Ventricular Function* MEASUREMENTS REQUIRED
CALCULATION
Left ventricular diastolic dimension (LVDD), left ventricular systolic dimension (LVSD)
Fractional shortening (%) LVDD- LVSD
Velocity of circumferential fiber shortening (VcF) (circumferences/sec)
LVDD, LVSD, left ventricular ejection time (ET)
VcF (circumferences/sec) LVDD- LVSD
Ejection fraction (EF)
End diastolic volume (EDV), LVDD, end systolic volume (ESV), LVSD
EDV = LVDD 3 ESV = LVSD 3 EDV- ESV EF (%) EDV
Fractional shortening (FS, or % .lD)
INTERPRETATION
Decreased contractility decreases FS
LVDD Decreased contractility decreases VcF
LVDD x ET
Pre-ejection period (PEP) Left ventricular ejection time (LVET)
Phases of ventricular systole, preejection period, ET
PEP LVET
Decreased contractility decreases EF
Decreased contractility increases ratio of PEP to LVET
*Ejection phase indices and systolic time intervals are difficult to interpret in the presence of significant mitral valve insufficiency.
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Figure l. The echocardiographic examination. The transducer is held gently over the heart against the ventral right hemithorax. The tranducer is angulated in order to record the standard echocardiographic images. The patient is awake and restrained in left lateral recumbency. An electrocardiogram is recorded for purposes of timing.
ventricular or atrial septal defects , and regional dysfunction of the ventricles. Some of these limitations are obviated by the recognition of associated cardiac changes (such as dilatation) or the use of intravascular contrast agents that outline blood flow. The development of newer imaging techniques , such as real-time cardiac sector scanning and Doppler echocardiography, has expanded the diagnostic capabilities of cardiac ultrasound. Yet, in spite of the aforementioned limitations, the properly interpreted Mmode echocardiogram remains a powerful diagnostic test in the evaluation of patients with heart disease. Technical Considerations The equipment needed for M-mode echocardiography includes the echocardiograph, an electrocardiograph, the ultrasonic transducer, a coupling gel, and a method of recording the echocardiographic display (such as a strip chart recorder). Ancillary equipment is needed for simultaneous phonocardiography, apexcardiography, and hemodynamic and pressure determinations. Selection of the transducer depends principally on the size of the patient, as a shorter wavelength offers less penetration but greater resolution and a more detailed image. For cats and small dogs, a 5- or 7MHz unfocused transducer is used. A 3.5-MHz medium-focused transducer is appropriate for imaging dogs weighing 15 kg or more. Examination of horses and cattle requires a transducer that emits a wavelength of 2. 25 or 1.6 MHz and an echocardiograph that has been adjusted to image to a depth of up to 35 em. The small animal patient is generally positioned in left lateral recumbency (Fig. 1). Some animals are examined more effectively in a 30 to 45° left lateral oblique position, which can be maintained with foam wedges of the type used for radiography. Occasionally, the standing position is necessary to obtain a satisfactory echogram.
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The transducer is placed at the lower right sternal border between the second to fourth intercostal spaces and is coupled to the thorax with an ultrasonic gel (see Fig. 1). Gentle pressure is applied as the transducer is angulated caudomedially in order to obtain an image. Suitable adjustments are made in the depth of field, gain, time-gain compensation, and reject controls in order to delineate the image. The technical aspects of ultrasound imaging have been described elsewhere. 7 By changing the transducer angle or location, the standard positions of Feigenbaum7 can be recorded (Fig. 2). With little exception, echocardiograms from animals are qualitatively similar to those from humans. 1• 5 • 11· 14- 16
Normal Findings The identification of cardiac structures requires both a preconceived notion of the expected echocardiographic image and a knowledge of the changes that occur in disease. The echobeam initially traverses the right ventricle; therefore, this structure is closest to the transducer artifact. The right ventricular dimension is normally quite small, since the ultrasonic beam usually passes through the tip of this crescent-shaped chamber (Fig. 3B). The ventricular septum is the next structure encountered, followed by the left ventric~lar lum~n and posteriorleftventricularwalL The ventricular walls are particularly thiek at the level of the papillary m:uscles (Fig. 3A). Ventricular dimensions are measured just below the mitral valve (Fig. 3B). If the transducer is angled dorsally, the mitral apparatus appears. The normal mitral valve is imaged as two structures: the anterior (cranial) and posterior (caudal) leaflets and associated chordae tendineae of the valve. The anterior mitral valve leaflet has a wider excursion and inscribes a typical "M" shape. The posterior mitral valve leaflet inscribes a "W," or the mirror image of the anterior mitral valve leaflet. Normal mitral valve motion has been designated as follows: C, point of mitral valve systolic closure; D, end of systolic mitral closure; E, maximum early diastolic separation of the anterior and posterior mitral valve leaflets; F, semiclosure of the valve; and A, mitral opening as the result of atrial activation (Fig. 3C to E). The mitral valve normally may appear very thick during systole when the annulus and valve move toward the transducer and the echobeam strikes the apparatus in a tangential manner. The tricuspid apparatus has similar motion and can be imaged by angulating the transducer slightly craniad (Fig. 3D); however, the lateral and septal leaflets are more readily observed when there is right ventricular dilatation. If the transducer is angulated dorsally and slightly cranially, the aorticleft atrial position is imaged (Fig. 3F and G). On a smooth sweep from the mitral position, the anterior aspect of the aortic root blends with the ventricular septum, and the posterior part of the root is continuous with the anterior mitral valve (see Fig. 2). The aortic root is recognized as two parallel lines that move toward the transducer in systole. The left atrium is caudal to the aorta and is contiguous 'with the posterior wall of the aortic root. Aortic motion results from both the systolic ejection of blood and from the filling of the left atrium. One or two aortic valvules are usually visible. These structures open toward the wall of the aorta during systole and close in the center of the aortic root during diastole. The left atrial size is
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Figure 2. A and B, Schematic illustration of the standard echocardiographic positions as described by Feigenbaum. 7 Position one traverses the right ventricle, ventricular septum, and left ventricle at the level of the papillary muscles. As the transducer is angulated dorsally, the chordae tendineae and cusps of the posterior and anterior mitral valve are imaged (positions 2 and 3). In the most dorsal position, the aortic root and the left atrium are recorded. The approximate motion mode (M-mode) echocardiograms obtained as the transducer is directed through these different positions are illustrated below. (Modified from Feigenbaum, H.: Echocardiography. Edition 3. Philadelphia, Lea & Febiger, 1981.)
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Figure 3 (Continued). D, Echogram obtained at the mitral valve level. Both AMV and PMV are evident and inscribe the typical M and W configurations. Parts of the tricuspid valve (TV) apparatus are imaged in the right ventricle. E, Normal AMV motion. The systolic (C to D) and diastolic excursions of the AMV are evident. Note the normal recording of multiple mitral echoes during systole . The C point occurs coincident with the first heart sound at the onset of mitral closure. F, E chogram recorded at the aortic root-left atrial position. The aortic root is evident as two parallel lines that move toward the transducer during systole. The aortic valve (AV) is imaged and closes at the time of the aortic incisura and the second heart sound (arrows). During systole, the valvulae move towards the walls of the aorta (arrowhead) and achieve maximal separation during rapid ejection of blood into the aortic root (dotted line). G, Measurements obtained from the aortic root position. The left atrium (LA) is noted to fill during ventricular systole. The amplitude (a) of aortic motion correlates to ventricular stroke volume and atrial filling. The normal ratio of the LA to aortic root dimension (AO) is approximately 0.8 to 1.2 in the dog.
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determined at its greatest dimension during ventricular systole (Fig. 3G). The posterior wall motion of the left atrium is attenuated and can thus be distinguished from the left ventricular wall. The ratio of the left atrial dimension to the aortic root dimension differs between species, with a normal ratio of approximately 0. 8 to l. 2 in the dog and cat and approximately 0.4 to 0.8 in the horse and cow. Intracardiac dimensions and ventricular wall thickness vary with the size of the animal and are best indexed to body weight or body surface area. Valve motion and the per cent of ventricular shortening normalized to the initial cavity dimension (fractional shortening) are relatively constant between species, though slightly higher in the cat. Other normal findings include concordant motion of the ventricular septum and left ventricular wall (toward each other during systole), and a ratio of septal thickness to left ventricular wall thickness of approximately 1.0 in normal small animals. Special echocardiographic contrast studies can be performed using the patient's blood, saline solution, carbon dioxide, or indocyanine green dye. 3 · 9 • 10· 18 When injected into the heart or vascular system, these agents induce microcavitation, forming echo-dense "targets" that are readily imaged in the heart (see Fig. 6). These targets tend to follow the path of blood flow, and thus permit the detection of intracardiac shunts and mitral regurgitation. I have experienced the greatest success with an equal mixture of patient blood and green dye, using a dose of 3 to 6 ml of green dye for most small animal patients.
ECHOCARDIOGRAPHIC EVALUATION OF HEART DISEASE Congenital Heart Disease M-mode echocardiography is helpful in the diagnosis of congenital heart disease. Although it is difficult to directly identifY some congenital cardiac lesions, the echogram still provides information about the responses of the heart. For example, right ventricular or left ventricular hypertrophy can be identified with pulmonic or aortic stenosis. Paradoxical (toward the transducer) systolic septal motion and increased right ventricular dimensions are noted in right ventricular volume overloading due to atrial septal defect, tricuspid valve dysplasia, and severe pulmonic valve stenosis with secondary tricuspid regurgitation. Left ventricular dilatation, as occurs with patent ductus arteriosus, aortic regurgitation, mitral dysplasia, and ventricular septal defect, can be determined. Finally, echo-contrast studies can identify intracardiac shunting. Pulmonic Valve Stenosis. The pulmonic valve is difficult to image and the dysplastic valve may not be apparent. However, the response to this pressure overload is usually evident with right ventricular hypertrophy, increased right ventricular internal dimension, and paradoxical septal motion imaged in most cases (Fig. 4). The left atrium and left ventricle may be smaller than normal owing to reduced right ventricular output. The tricuspid valve may be prominent with enlargement of the right ventricular cavity.
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Figure 4. Echocardiogram obtained from a four-month-old Great Dane with pulmonic stenosis and mild tricuspid valve dysplasia. Salient features include hypertrophy of the right ventricular wall (RVW), increased right ventricular internal dimension (RV), which is approximately equal to the left ventricular dimension (LV), and paradoxical movement of the ventricular septum (S) during systole. The RVW thickness is normally less than 50 per cent of the left ventricular wall. A systolic murmur of pulmonary stenosis is recorded on the phonocardiogram (PCG). Parts of the tricuspid apparatus are visible within the right ventricular lumen.
Tetralogy of Fallot. It is possible in some cases to image both the hypertrophied right ventricle and the overriding aorta (Fig. 5). Since the aortic root is dextropositioned over the septal defect, the normal continuity between the ventricular septum and aortic root may be lost. Right ventricular hypertrophy is evidence of the pulmonary outflow obstruction, and decreased internal dimensions of the left atrium and ventricle suggest the reduced pulmonary flow caused by right-to-left shunting. Intracardiac shunting is readily identified through an injection of green dye into the cephalic or jugular vein. Contrast will appear first in the right ventricle and then in the left ventricle, concentrating between the anterior mitral valve and ventricular septum. 3 Aortic Stenosis. Congenital aortic stenosis in the dog usually consists of a subvalvular fibrous ring that may partially envelop the aortic valvules. Accordingly, it is possible to image the subvalvular obstruction by sweeping from the mitral to aortic valve positions. The lesion is identified as a slight narrowing in the outflow tract just below the aortic valve. An additional
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echocardiographic finding is poststenotic dilatation of the aorta, which can be appreciated by angulating the transducer dorsad and craniad from the aortic valve. Hypertrophy of the left ventricular wall and the ventricular septum are abnormalities that accompany aortic stenosis. Atrial Septal Defect. Atrial septal defect is rare as an isolated congenital lesion. The septal defect cannot be imaged, as the interatrial septum is not readily visualized by M-mode techniques. The consequences of the left-toright atrial shunt can be appreciated and include right ventricular dilatation, increased thickness of the right ventricular wall, and marked paradoxical septal motion. A cephalic vein injection of green dye may indicate some degree of right-to-left shunting, with contrast appearing in both the left atrium and left ventricle. Ventricular Septal Defect. Although the ventricular septum is readily visualized, it is unusual to image the "drop out" in echoes associated with a ventricular septal defect. Although two-dimensional techniques can readily identify the septal patency, the pinpoint image of the M-mode echogram limits this technique to determining the consequences of this lesion. Echographic findings vary but may illustrate shunt-induced enlargement of the left and right ventricles, left atrial enlargement, and slightly paradoxical septal motion. Echo-contrast studies can identifY the left-to-right cardiac shunt; however, the contrast material must be introduced directly into the left ventricle, as "conventional" microbubbles (green dye or saline) do not pass through the lungs. Inasmuch as this requires cardiac catheterization, there is no significant advantage to this technique versus routine angiocardiography in small animals. However, in large animals that are not readily imaged by radiography, this technique can substantiate the ventricular shunting (Fig. 6). Recently developed contrast materials capable of passing through the lungs may prove useful in the future and permit noninvasive delineation of this lesion. Patent Ductus Arteriosus. The echocardiographic findings in patent ductus arteriosus are those of left ventricular volume overload with increased left ventricular internal dimensions, normal ventricular function, and increased left atrial size. The amplitude of aortic root motion (see Fig. 3G) is generally exuberant, indicating a large stroke volume and increased left atrial filling. Immediately following surgical ligation of the ductus, the left ventricular dimension is smaller. Atrioventricular Valve Dysplasia. Tricuspid valve dysplasia, which results in marked increases in right ventricular internal dimension and paradoxical septal motion, is identified by recognition of abnormally thickened diastolic echoes returning from the tricuspid valve structures. Mitral valve dysplasia similarly may result in increased thickness of the valve, increased left atrial and ventricular internal dimensions, and increased fractional shortening of the left ventricle. Pericardial Diaphragmatic Hernia. The findings with congenital pericardial-peritoneal communications are similar to those with pericardia} effusion; however, the usual echo-free pericardia} space may be dense owing to the presence of the liver or another abdominal structure. The
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osu 621272 Figure 6. Contrast echocardiogram from a one-month-old female Holstein calf with a ventricular septal defect. Following injection of indocyanine green dye and the patient's blood, the image of the left ventricle (LV) becomes opacified. Bubbles (arrow) abruptly appear in the right ventricle (RV), indicating shunting of blood at the ventricular level. A transient arrhythmia (ectopic QRS ) was induced by introduction of a needle into the LV. RV = right ventricular lumen; IVS = interventricular septum; LVOT = left ventricular outflow tract; AMV = anterior leaflet of the mitral valve . (From Bonagura, J. D., and Pipers, F. S.: Diagnosis of cardiac lesions by contrast echocardiography. J. Am. Vet. Med. Assoc. , 182:396402, 1983; with permission. )
right ventricle is frequently displaced from the thoracic wall as a result of the hernia and its contents. Dilated Coronary Sinus. The coronary sinus is situated behind the left atrium in the atrioventricular groove . When there is abnormal persistence of the left cranial vena cava, the coronary sinus may be dilated, resulting in a large posterior echo-free space at the junction of the anterior mitral valve/aortic root and left atrial positions. Identification of this le sion may alter the approach to cardiac catheterization or cardiac surgery. Acquired Heart Disease
Pericardia[ Disease. The essential echocardiographic features of pericardia! effusion include separation of the left ventricular epicardium and parietal pericardium, displacement of the right ventricular free wall from the thoracic cage, identification of a sonolucent space between the epicardium and pericardium, and abnormal cardiac motion (Fig. 7). 2 •7 Technical considerations of recording are important since improper gain and reject settings prevent identification of the fluid-filled (echo-free) pericardia! space. The magnitude of the pericardia! effusion varies , with the greatest accumulation usually noted at the cardiac apex. As the transducer is angulated
311
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F igure 7. A , Schematic diagram of approximate pathway of the ultrasound beam in pericardia! effusion. The transducer is placed at the right side of the thorax. The anterior mitral valve (2) and ventricular (1) positions are shown. Pericardia! effusion results in an echofree space between the epicardium and parietal pericardium. RV = right ventricular lumen; IVS = interventricular septum; AMV = anterior leaflet of the mitral valve; PMV = posterior leaflet of the mitral valve. B, Echogram from a 5-year-old spayed female Collie with granulomatous pericarditis. Echo-free spaces representing pericardia) effusion (PE) anterior and posterior to the heart are recorded at the mitral valve position. At higher reject levels, only the most echo-dense structures are visualized. As demonstrated in the right panel, these echoes usually originate from the pericardia) structures. RVFW = right ventricular free wall; IVS = interventricular septum; PLV = posterior left ventricular wall. D epth (1 em) calibration dots are recorded every second. (From Bonagura, J. D. , and Pipers, F . S.: Echocardiographic features of pericardia! effusion in dogs. J. Am. Vet. Med. Assoc., 179:49-56, 1981; reproduced by permission.)
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toward the base, the sonolucent space diminishes in size owing to the firm attachments of the pericardium at the atrioventricular groove. The pericardium appears to be thickened in some cases, and pericardia! motion frequently is dampened (flat) with large effusions. Exudate or fibrin strands are occasionally identified as echo-dense structures within the effusion. Cardiac motion can be abnormal and is caused by actual swinging of the heart within the pericardia! effusion. It is difficult to assess valvular, septal, or ventricular wall excursions in such cases. Pericardia} effusion must be distinguished from pleural effusion. Inasmuch as pleural effusion is also noted behind the left atrium and thoracic radiography readily identifies most pleural effusions, this distinction is not difficult. Rarely, excessive pericardia! fat appears as a small echo-free space behind the epicardium. Constrictive pericarditis has been recognized in some animals and has been identified echocardiographically by the recordings of thickened epicardial echoes, abrupt flattening of ventricular diastolic motion, paradoxical septal motion, decreased ventricular dimensions, and flattened mitral valve E-F slope, indicating decreased transmitral flow and reduced ventricular filling. 2• 7 Myocardial Diseases. Myocardial diseases are common in veterinary practice. Ventricular dilatation and hypertrophy can be recognized based on echocardiographic assessment of left ventricular chamber size and wall thickness. Dilatation of the left ventricle, with minimal or absent wall hypertrophy, is recognized in association with idiopathic congestive cardiomyopathies of the dog and cat, chronic anemia, viral myocarditis, and doxorubicin (Adriamycin) toxicity. Chronic volume work, as occurs with valvular insufficiency or congenital shunts, also results in ventricular dilatation. Acquired causes of hypertrophy of the left ventricle, with normal or reduced internal ventricular dimensions, are (idiopathic) hypertrophic cardiomyopathies, hyperthyroidism, and systemic hypertension associated with renal disease. Congestive Cardiomyopathy. The echocardiogram is quite valuable in the diagnosis and assessment of patients with congestive cardiomyopathy and is indicated in the evaluation of animals with radiographic evidence of cardiomegaly. Salient echocardiographic features of congestive cardiomyopathy include increased ventricular internal dimensions, left atrial enlargement, normal or decreased ventricular and septal wall thickness and systolic thickening, and decreased indices ofleft ventricular function (Fig. 8). Mitral valve motion may be abnormal, with increased distance between the E point and ventricular septum (indicating ventricular dilatation), increased excursion of the posterior leaflet (the "double diamond sign"), and delayed closure of the mitral valve resulting in a visible hump ("B shoulder") rather than the normally straight line between the A and C points. This B shoulder usually indicates increased ventricular and atrial end diastolic pressures. Aortic root excursion frequently is diminished, suggesting reduced stroke volume and cardiac output. There may be regional wall motion abnorml!.lititl$ with disparity between the ventricular septum and the hypokinetic left ventricular free wall.
313
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Figure 8. Echogram from a six-year-old male Coonhound with congestive (dilated) cardiomyopathy. The left ventricle is grossly dilated with a left ventricular internal dimension (LV) of approximately 8.5 em. LV fractional shortening is markedly reduced and clearly less than the usual values of 30 to 40 per cent. The anterior mitral valve (arrow) is evident, and the distance between the mitral valve E point to the ventricular septum is increased, indicating ventricular dilatation. The left ventricular wall (W) and septal wall thickness are normal. Left atrial enlargement is evident in the right panel as an increased left atriumaortic root ratio. A phonocardiogram and electrocardiogram are recorded at the bottom of the record. RV = right ventricular lumen; paper speed = 50 mm per sec; depth (l em) calibration dots are recorded every second.
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These abnormalities are practically diagnostic of congestive cardiomyopathy, and such findings are quite useful in establishing the etiology of heart failure in breeds that are infrequently affected with this disorder. Other causes of reduced myocardial contractility, such as parvovirus myocarditis and therapy with the antineoplastic drug doxorubicin can lead to similar echocardiographic changes. Hypertrophic Cardiomyopathy. Echocardiography is a sensitive indicator of ventricular hypertrophy and thereby permits the clinician to distinguish between the major forms of cardiomyopathy in cats and dogs. Hypertrophic cardiomyopathy is characterized by increased thickness and systolic thicke ning of the left ventricular wall and the ventricular septum. The ventricular internal dimensions are normal to decreased, and the mitral valve appears to be "crowded" into the thickened ventricle (Fig. 9). Some patients appear to have dynamic left ventricular outflow tract obstruction characterized echocardiographically by marked systolic anterior motion of the mitral valve. Indices ofleft ventricular function are normal to increased. Secondary enlargement of the left atrium and a normal aortic root excursion are other anticipated findings. Valvular Heart Disease. Bacterial Endocarditis. The vege tative lesion of infective endocarditis can be recognized by M-mode echocardiography.
314
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Figure 9. Echogram from a male cat with hypertrophic cardiomyopathy. Significant abnormalities include normal to increased systolic thickening of the left ventricular wall (L) and ventricular septum (S), normal fractional shortening, normal to decreased ventricular dimension during systole (arrows), and systolic anterior motion (arrowhead) of the mitral valve (M), which is noted to strike the septum during midsystole. Right ventricular free wall motion is normal. Motion artifact is present in the electrocardiogram. Paper speed = 50 mm per sec; depth (1 em) calibration dots are recorded every second.
The typical valvular vegetation appears as a thickened, "shaggy," irregular echo-dense mass. The lesion is attached to the affected valve and exhibits phasic motion in conjunction with valvular excursions. Mitral valve vegetations result in a mild to severe, but nonuniform, thickening of the valve. Aortic valve vegetations are identified as an echo-dense mass within the aortic root (Fig. 10) or as a prolapsed mass density appearing in the left ventricular outflow tract during diastole. 4 • 6 • 19 If the valve leaflet is ruptured or flail, diastolic fluttering of the valve may be imaged. Inasmuch as vegetations must be at least 3 mm thick to be discerned, the echocardiogram in some animals with bacterial endocarditis may not be diagnostic. Furthermore, changes attributed to endocarditis must be distinguished from those of the degenerative (myxomatous) thickening of the atrioventricular valves that is so common in dogs . This requires careful integration of echocardiographic and clinical data. In general, the identification of a large valvular vegetation carries a guarded to poor prognosis, as most of these patients develop congestive heart failure or signs of systemic embolization. Mitral and Aortic Regurgitation. Degenerative valvular heart disease (endocardiosis) results in subtle alterations in the affected valve and obvious abnormalities in the intracavitary dimensions. 13 Chronic valvular disease in the dog is recognized by mild, "smooth" thickening of the mitral or tricuspid valves. These thickenings are most prominent during the E and A points of valve excursion. Increased ventricular internal dimensions are expected on the affected side of the heart. Left atrial enlargement is recognized as an increased left atrium-to-aortic root ratio. Even in the presence of leftsided congestive heart failure, indices of ventricular function can be normal
315
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Figure 10. Echogram from a seven-year-old female Boxer with aortic valvular endocarditis, aortic stenosis, and congestive heart failure. Images recorded through the aortic root (AO) demonstrate a large echo-dense mass (VEG) that appears during diastole and is barely visible against the posterior wall of the aortic root during systole. Compare this with Figures 2F and G. Left atrial enlargement is also noted. PW = posterior wall. Systolic (SYS) and diastolic (DIAS) murmurs are recorded on the phonocardiogram (Phono). Depth (1 em) calibration dots are recorded every second. (From Bonagura, J. D., and Pipers, F . S.: Diagnosis of cardiac lesions by contrast echocardiography. J. Am. Vet. Med. Assoc., 182:595- 599, 1983; with permission.)
to supranormal. This is explained by the opportunity afforded the left ventricle to eject blood into the low-resistance left atrium. These findings are in marked contrast to patients with congestive cardiomyopathy, where indices of ventricular function are significantly reduced despite concurrent mitral regurgitation. Thus, the M-mode echocardiogram is useful in assessing the consequences and in determining the cause of mitral regurgitation. In congenital dysplasia, there may be marked thickening of the valve. With endocardiosis,
316
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BONAGURA
mild valvular thickening is accompanied by normal left ventricular function; in endocarditis, the typical vegetative lesion may be recognized. Mitral regurgitation caused by congestive cardiomyopathy is allied with marked ventricular dilatation and hypokinesis. Rare causes of mitral regurgitation such as ruptured chordae tendineae also can be identified by noting abnormal valvular motion and chaotic fluttering of the flail valve. Potential causes of aortic insufficiency include congenital aortic valve disease, dilatation of the aortic root, bacterial endocarditis and, particularly in horses, degenerative valvular disease. Diagnostic criteria for these abnormalities have been previously described. It should also be recognized that diastolic fluttering of the aortic valve is nonspecific and can be observed with aortic regurgitation due to valvular degeneration or infective endocarditis. Diastolic fluttering of the mitral valve is diagnostic of aortic regurgitation from any cause and is present if the regurgitant jet strikes the anterior mitral valve leaflet (Fig. 11). The hemodynamic consequences of aortic insufficiency as assessed by echocardiography include increased left ventricular internal dimension, hyperkinesis of the left ventricle due to volume overload, and premature closure of the mitral valve due to increased ventricular diastolic pressure. 4 Other Lesions. The echocardiogram can document unsuspected or uncommon cardiac lesions and thus is an important aid in the diagnosis of such conditions. We have identified cases of mitral valve stenosis that have occurred secondary to marked endocardial inflammation. 13 Atrial thrombi, associated with cardiomyopathy, have been imaged as echo-dense structures within the sonolucent left atrial cavity. These usually are contiguous with the atrial wall. Occult dirofilariasis is suspected by the identification of right ventricular enlargement and of increased densities within the right ventricular cavity of a dog. The consequences of cardiac rhythm disturbances on cardiac motion and function also can be studied by echocardiography. Ventricular Function. Echocardiography is a useful adjunct in the assessment of myocardial performance. Left ventricular function is dependent on a number of factors, including myocardial contractility, regional wall function, and ventricular loading conditions. Although it is difficult to assess afterload (aortic impedance) with echocardiography, preloading can be estimated by determination of left ventricular end diastolic dimension. Ventricular fractional shortening (per cent Ll D), velocity of circumferential fiber shortening, ejection fraction, and systolic time intervals can be calculated from echocardiographically derived measurements (see Table 1 and Figs. 3 and 12). If the cardiac rhythm and loading conditions are constant, these indices are useful in the evaluation of myocardial contractility and assessment of directional changes in ventricular function. Unfortunately, these indices are of limited value when there is either significant mitral regurgitation or aortic stenosis because these conditions decrease and increase, respectively, the ventricular afterload. The calculation and basic interpretation of ejection phase indices of ventricular function and systolic time intervals are summarized in Table 1. Calculation of these indices requires the prior determination of left ventricular internal dimensions (at the chordal level), pre-ejection period (from
317
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""'... Figure 11. Diastolic fluttering of the anterior mitral valve (MV) in a six-month-old female terrier-cross with aortic regurgitation. These vibrations (arrows) are indicative of aortic valve insufficiency with the regurgitant jet directed against the mitral valve. The cause in this case was a congenital abnormality of the aortic valve sinus of Valsalva. The E and A points of mitral excursion are indicated. LVW = left ventricular wall; P = posterior pericardium; paper speed = 50 mm per sec. (From Pipers, F. S., Bonagura, J. D., Hamlin, R. L., et al.: Echocardiographic abnormalities of the mitral valve associated with left-sided heart disease in the dog. J. Am. Vet. Med. Assoc., 179:580-586, 1981; with permission.
the onset of the QRS until aortic valve opening), and left ventricular ejection time (from opening to closing of the aortic valve). 17 In general, a reduction in cardiac contractility will decrease the fractional shortening, velocity of circumferential fiber shortening, and ejection fraction, and will increase the ratio of pre-ejection period to ejection time derived from systolic time intervals. 8 • 12 Other estimates of ventricular function are efforts to evaluate the effects of contractility, stroke volume, or cardiac output on cardiac structures. These evaluations include per cent thickenings of the ventricular wall and septum, valvular separation during diastole (mitral valve) or systole
318
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Figure 12. Determination of systolic time intervals using the echocardiogram. The preejection period (pep) is measured from the onset of the QRS complex of the electrocardiogram (ECG) to the opening (O) of the aortic valve (AV). The left ventricular ejection time (et) is measured from the point of aortic valve opening to the point of aortic valve closure (C). This closure point can be ve rified since it occurs coincident with the initial high frequency vibrations of the second heart sound (2), as recorded on the phonocardiogram (PCG). Total electromechanical systole is the time from the onset of the QRS complex to the onset of the second heart sound. AO = aortic root; time lines are recorded every 10 msec.
319
M-MonE EcHOCARDIOGRAPHY
(aortic valve), amplitude of aortic root motion, and estimation of stroke volume by the cube formula as noted in Table 1. These have been described in humans but have not been adequately studied in animals. ACKNOWLEDGMENT
The author acknowledges the medical illustrations of Nancy schmidt.
J. Gold-
REFERENCES 1. Baylen, B. G., Garner, D. J., Laks, M. M., eta!.: Improved·echocardiographic evaluation of the closed-chest canine: Methods and anatomic observations. J. Clin. Ultrasound, 8:335-340, 1980. 2. Bonagura, J. D., and Pipers, F. S.: Echocardiographic features of pericardia! effusion in dogs. J. Am. Vet. Med. Assoc., 179:49-56, 1981. 3. Bonagura, J. D., and Pipers, F. S.: Diagnosis of cardiac lesions by contrast echocardiography. J. Am. Vet. Med. Assoc., 182:396-402, 1983. 4. Bonagura, J. D., and Pipers, F. S.: Echocardiographic features of aortic valve endocarditis. J. Am. Vet. Med. Assoc., 182:595-599, 1983. 5. Dennis, M. 0., Nealeigh, R. D., Pyle, R. L., eta!.: Echocardiographic assessment of normal and abnormal valvular function in Beagle dogs. Am. J. Vet. Res., 39:1591-1598, 1978. 6. Dillon, J. F., Feigenbaum, H., Konecke, L. L., eta!.: Echocardiographic manifestations of valvular vegetations. Am. Heart J., 86:698-704, 1973. 7. Feigenbaum, H.: Echocardiography. Edition 3. Philadelphia, Lea and Febiger, 1981. 8. Fortuin, N. J., and Pawsey, C. G. K.: The evaluation of left ventricular function by echocardiography. Am. J. Med., 63:1-9, 1977. 9. Gramiak, R., Shaw, P. M., and Kramer, D. H.: Ultrasound cardiography: Contrast studies in anatomy and function. Radiology, 92:939-948, 1969. 10. Kerber, R. E., Koschos, J. M., and Lauer, R. M.: TI.e use of an ultrasonic contrast method in the diagnosis of valvular regurgitation and intracardiac shunts. Am. J. Cardiol., 34:722-727, 1974. 11. Mashiro, I., Nelson, R. R., Cohn, J. N., et a!.: Ventricular dimensions measured noninvasively by echocardiography in the awake dog. J. Appl. Physiol., 41:953-959, 1976. 12. Pipers, F. S., Andrysco, R. M., and Hamlin, R. L.: A totally noninvasive method for obtaining systolic time intervals in the dog. Am. J. Vet. Res., 39:1822-1826, 1978. 13. Pipers, F. S., Bonagura, J. D., Hamlin, R. L., eta!.: Echocardiographic abnormalities of the mitral valve associated with left-side heart disease in the dog. J. Am. Vet. Med. Assoc., 179:580-586, 1981. 14. Pipers, F. S., and Hamlin, R. L.: Echocardiography in the horse. J. Am. Vet. Med. Assoc., 170:815-819, 1977. 15. Pipers, F. S., Muir, W. W., and Hamlin, R. L.: Echocardiography in swine. Am. J. Vet. Res., 39:707-710, 1978. 16. Pipers, F. S., Reef, V., and Hamlin, R. L.: Echocardiography in the domestic cat. Am. J. Vet. Res., 40:882-886, 1979. 17. Sahn, D. J., DeMaria, A., Kisslo, J., et al.-The Committee on M-mode Standardization of the American Society of Echocardiography: Recommendations regarding quantitation in M-mode echocardiography: Results of a survey of echocardiographic measurements. Circulation, 58:1072-1083, 1978. 18. Seward, J. B., Tejik, A. J., Spangler, J. G., eta!.: Echocardiographic contrast studies: Initial experience. Mayo Clin. Proc., 50:163-192, 1975. 19. Wray, T. M.: The variable echocardiographic features in aortic valve endocarditis. Circulation, 52:658-663, 1975. 1935 Coffey Road Columbus, Ohio 43210