Case Studies In Small Animal
Text from "Small Animal Cardiovascular Medicine"
Left ventricular outflow tract (LVOT) obstruction may be anatomically classified, in relation to the aortic valve, as valvular, supravalvular, or subvalvular. Further categorization may be based on the functional characteristics of the obstruction, as either fixed (static) or dynamic (labile). Canine subaortic stenosis (SAS) is the second most common heart defect in the dog (22%), while other forms of LVOT obstruction in dogs are rare. Congenital LVOT obstruction is rare in cats, yet both subvalvular, valvular and supravalvular have been recognized. In most cases of congenital SAS, the dimensions of the restrictive orifice are static or “fixed” by the anatomical characteristics of the lesion. The severity of the obstruction is not altered from beat to beat and does not change in degree as systole progresses. The dimensions of the lesion in dogs with “dynamic” obstruction, on the other hand, are labile. In this case, the severity of the obstruction is altered by changes in heart rate or inotropic state, and usually vary as systole progresses. In some cases, fixed and dynamic obstruction occur in conjunction.
The LVOT is formed during the development of the trunco-conal septum, the ventricular septum, and the anterior leaflet of the mitral valve. Abnormal development of any of these structures may result in subaortic stenosis. It is probably due to a primary faulty development in the region of transition from the conus to the truncus, a site that favors inflammation in later stages of development in humans. It has been suggested that in dogs the fibrocartilagenous ring of SAS is derived from persistent embryonal endocardial tissue which retains its proliferative capacity and has chondrogenic potential for some time after birth.
Subaortic stenosis has been demonstrated as a specific inherited trait in the Newfoundland retriever. The results of breeding experiments were not consistent with any simple genetic hypothesis, and indicate that SAS is inherited as a polygenic trait or as an autosomal dominant trait with modifiers. However, in these test matings many of the offspring were sacrificed or died before three weeks of age. The same investigators have reported that lesions of SAS were not found in dogs before three weeks of age and that only the mildest form of SAS was seen in dogs between 3 and 12 weeks of age. The youngest dog with a detectable lesion in this report was 24 days old at the time of death. These findings suggest that SAS in the Newfoundland retriever is not a true congenital defect, rather it develops in the postnatal period. If only dogs that survived beyond three weeks of age are considered in the genetic analysis, the results of all matings among Newfoundlands are consistent with the autosomal dominant hypothesis. Also, recent reexamination of the breeding studies in SAS in the Newfoundland and conotruncal defects in Keeshond dogs indicate that single major genes underlie these defects. A heritable basis for SAS is also strongly suspected in other commonly affected breeds.
ANATOMY AND PATHOLOGY
The left ventricular outflow tract is defined by several structures: the craniolateral portion of the left ventricular fee wall, the membranous and muscular portions of the basilar ventricular septum, and the anterior mitral valve leaflet and associated structures. Abnormalities in any of these areas may produce subaortic obstruction to left ventricular output. The classic description of SAS in the dog is that of a discrete fibrous ridge or collar that completely or partially encircles the left ventricular outflow tract just below the aortic valve. Pathologic studies in breeding colonies have shown, however, that a range in severity of the lesion exists, the mildest form of which is clinically silent. Pyle, et. al described three grades of SAS lesions based on postmortem studies in Newfoundland puppies. In the mildest form (grade 1) the lesions consisted of small, raised nodules of thickened endocardium on the intraventricular septum below the aortic valve. Grade 1 lesions were only identified in dogs between 3 and 12 weeks of age. Grade 2 lesions formed a narrow ridge of whitish, thickened endocardium that partially encircled the LVOT below the aortic valve, which in most cases originated at the base of the anterior mitral valve leaflet and traversed across the intraventricular septum for a variable distance. In the most severe form (grade 3), a fibrous band, ridge, or collar completely encircled the LVOT just below the aortic valve. The ridge was raised, sometimes 1-2 mm, above the endocardial surface and extended across the entire LVOT including the base of the anterior mitral valve leaflet. Grade 3 lesions were predominantly identified in dogs older than 6 months of age. In these dogs, the ventricular surfaces of the aortic valve leaflets were also thickened.
The lesions of SAS are histologically characterized by large, uni- and multinucleated, rounded connective tissue cells that resemble chondrocytes. Adjacent connective tissue is rich in acid mucopolysaccharides, small collagen fibrils, and poorly developed elastic and reticular fibers. In advanced lesions, discrete bundles of collagen and cartilage may be found. Dogs with LVOT pressure gradients in excess of 35 mmHg, demonstrate remodeling of the intramural coronary arteries and arterioles characterized by luminal narrowing, intimal smooth muscle proliferation, medial hypertrophy, and medial smooth muscle disorganization. The coronary lesions are associated with focal areas of myocardial ischemia and fibrosis which are most prevalent in the papillary muscles and subendocardial regions of the left ventricular wall. Although the exact cause of the coronary lesions is not known, it is presumed that they may be precipitated by the increased left ventricular systolic pressure, increased systolic wall tension, a relative decrease in myocardial capillary density secondary to concentric hypertrophy, and abnormal coronary blood flow associated with the primary lesion.
Associated findings on gross examination illustrate the pathologic consequences of SAS. Concentric hypertrophy of the left ventricle may be present and its severity usually reflects the magnitude of left ventricular pressure increase in systole. Mild left atrial enlargement may be presented presumably due to a decrease in left ventricular compliance, myocardial failure, or concomitant mitral insufficiency and dilatation of the ascending aorta and aortic arch may reflect the turbulent flow in those areas. Mild malformations of the mitral valve that are not functionally important are also common in dogs with SAS.
In dogs with grade 1 lesions, clinical examination and cardiac catheterization failed do identify the lesion. Only occasional transient murmurs (1/6) were recognized in these puppies. Dogs with grade 2 lesions often demonstrated soft systolic murmurs (1-2/6) and mild systolic pressure gradients across the LVOT (less than 20 mmHg) while dogs with grade 3 lesions exhibited clear physical evidence of SAS and all had abnormal pressure gradients across the LVOT, ranging from 36-95 mmHg, and abnormal left ventricular angiograms. Thus, dogs with genetic coding for SAS may clearly remain unidentified using currently approved screening methods and genetic counseling is difficult in dogs of breeds known to be at increased risk for SAS with soft systolic murmurs of unknown origin.
Pathologic studies of SAS in dogs are complicated by evidence that structural lesions may not be present or may not be fully developed at birth. Such that, any obstruction may become progressively more severe during the developmental period. Further evidence suggests that any gradient, regardless of the age of the dog, may become progressively more severe over time. The mechanism by which SAS increases in severity is uncertain. Pyle, et. al. hypothesized that the fibrocartilagenous ring of SAS is derived from persistent embryonal endocardial tissue which retains its proliferative capacity and chondrogenic potential for some time after birth. It is unclear whether increases in body size and changes in the degree of left ventricular concentric hypertrophy may also contribute to the progression. Of clinical concern, it is also uncertain at what age an observed obstruction can be regarded as fully developed. This progressive element of SAS has implications relative to the identification of cardiac murmurs in puppies of breeds known to be at risk for SAS. It is probably inappropriate to “clear” dogs for SAS before they are full grown especially if the exhibit soft systolic murmurs of unknown origin. On the other hand, it is unlikely that any adult dog that does not demonstrate a cardiac murmur consistent with SAS will go on to develop SAS. However, if mild SAS has been documented or if a soft murmur consistent with SAS is identified, especially in young dogs, only a tentative prognosis should be given as the nature of the obstruction may ultimately be more severe.
Independent of the nature of the obstruction, the principle hemodynamic consequence is an increased resistance to LV systolic outflow with a proportional elevation of LV systolic pressure, if flow remains constant. The pressure in the aorta is usually normal, but may be decreased if LV stroke volume is reduced. The magnitude of the resultant pressure gradient across the obstruction is directly related to both the quantity and rate of blood flow across the obstruction and indirectly related to the cross sectional area of the stenotic region. The pressure gradient is commonly used as an index of lesion severity. However, since it is affected by both lesion severity (resistance) and to flow, it is less accurate than determinations of lesion's cross-sectional area or resistance. Distal to the obstruction blood flow velocity increases and flow becomes turbulent. The force of this turbulent jet results in post stenotic dilatation of the ascending aorta.
The increase in LV systolic wall stress stimulates an increase in LV muscle mass (concentric hypertrophy) usually also proportional to the severity of the obstruction. The left ventricular hypertrophy normalizes ventricular systolic function and usually allows left ventricular stroke volume to remain within the normal range. However, due to a the increased resistance, peak LV ejection is delayed and may result in a late rising, diminished arterial pulse. A severely hypertrophied LV may also reduce the end-diastolic volume and may produce a relatively stiff ventricle, which in turn may reduce the ability of the ventricle to fill properly. This abnormal diastolic filling pattern may further reduce LV stroke volume, an increase in the a wave of the left atrial and pulmonary capillary wedge pressure pulses, and often leads to mild left atrial enlargement.
Mild degrees of aortic insufficiency are commonly observed in dogs with SAS and appear to be caused by thickening and impaired mobility of the valve cusps secondary to the trauma created by the high velocity jet. Aortic insufficiency may be further aggravated by involvement of the leaflets within the fibrous ring, dilation of the ascending aorta, or infective endocarditis. Damage to the aortic valve cusps by the turbulent jet predisposes to the development of infective endocarditis in both dogs and humans with discrete subaortic stenosis.
Exertional syncope and sudden death are the most common symptoms related to LVOT obstruction. The mechanism behind these signs remains speculative. The most widely accepted explanation of exertional syncope is acute reflex peripheral arterial and venous dilation, possibly mediated by the effects of sudden changes in LV systolic pressure on ventricular baroreceptor activity. However, some investigators contend that ventricular arrhythmias must be considered as a causative factor. On the other hand, some studies indicate that syncope is the primary event with malignant and/or fatal arrhythmias being secondary to the hemodynamic changes imposed by the collapse. Malignant ventricular arrhythmias are also probably responsible for sudden death whether they be associated with myocardial ischemia or induced by syncopal episodes.
Although left sided congestive heart failure may ultimately result from severe SAS, the overall incidence is quite low. The left ventricle responds to sudden production of severe obstruction by dilation and reduction of stroke volume. However, in SAS the obstruction is either present at birth or gradually develops over time. Left ventricular output is maintained by concentric hypertrophy and LV end-diastolic volume usually remains relatively normal until late in the course of the disease. The elevated left ventricular end-diastolic pressure resulting from decreased ventricular compliance is usually only mild and not generally enough to produce pulmonary edema. If congestive heart failure does develop in a patient with SAS, it is likely due to slowly developing myocardial failure and/or complicating factors such as moderate to severe mitral regurgitation or aortic insufficiency.
The M-mode echo demonstrates increased diastolic septal and LV wall thickness (LV concentric hypertrophy) and a normal LV shortening fraction. The narrowed subvalvular region can sometimes be identified while sweeping the M-mode transducer from the LV to the Ao level, but 2D evaluation is more accurate for this purpose. Other findings may include post-stenotic dilatation of the ascending aorta, secondary thickening of the aortic valve, and mid-systolic partial closure of the aortic valve. In some cases of SAS the mitral valve E-F slope decreases due to reduced compliance of the hypertrophied left ventricle. This should not be misinterpreted as mitral stenosis. Overall, M-mode examination alone is not very reliable for identification r confirmation of congenital SAS except in severe cases.
In cases of moderate to severe SAS, 2D echocardiography readily identifies the subvalvular obstruction as well as accompanying secondary changes. In mild cases, the obstruction may be difficult to distinguish except as minor irregularities in the LV outflow tract. The obstruction appears as a narrowing between the ventricular septum and the base of the anterior mitral valve leaflet, just proximal to the aortic valve on the right parasternal or left cranial long-axis views. It usually appears as a discrete membranous or fibromuscular ridge, rarely as a longer tunnel-type obstruction. The subvalvular fibrous narrowing may also be identified in the right short-axis view as a circular or oval ring in the outflow tract. The aortic valve often appears mildly thickened from the continuous trauma associated with the stenotic jet. Areas of ischemic fibrosis appear as hyperechoic areas in the LV papillary muscles or walls. In many cases, post-stenotic dilation of the ascending aorta is recognized, but this can be slight in dogs under 6 months old. It may be identified in the right long-axis view, but is usually identified best in the left cranial long-axis view. It is common to observe mildly thickened mitral valve leaflets with diminished diastolic motion, but these valves are usually functionally competent. The left atrium is normal or mildly dilated in most dogs with SAS and normal mitral valve function. Cats with aortic stenosis display similar echocardiographic findings.
An additional finding in some dogs with SAS is systolic anterior motion (SAM) of the mitral valve, resulting in a dynamic LV outflow obstruction between the anterior mitral valve leaflet and hypertrophied ventricular septum. Mitral valve SAM is recognized by a movement of the anterior mitral valve leaflet toward the ventricular septum in mid-systole, often approaching or contacting the septum. The underlying mechanism of this abnormal valve motion remains unclear. Although reported in dogs with concurrent congenital fixed SAS, we have recognized SAM and dynamic SAS more often in young dogs and cats with LV hypertrophy but without identifiable discrete SAS.
As with pulmonic stenosis, Doppler echocardiography is useful for determining the location of the stenosis and its severity, as well as accompanying complications. It is critical for the diagnosis of mild SAS in animals without clear abnormalities on the 2D examination. Color Doppler examination demonstrates a broad systolic jet in the proximal aorta and diastolic aortic valve regurgitation in the LV outflow tract. O'Grady found spectral Doppler evidence of mild aortic regurgitation in 86.8% of 53 dogs with SAS. Our experience, using 2D color Doppler imaging, is that mild aortic valve regurgitation is present in almost every dog with SAS, and that this may be one of the most sensitive indicators of SAS in dogs with the mildest defects. Continuous wave Doppler examination of the LVOT and proximal aorta can be performed from the left apical position, a subcostal position, and a thoracic inlet (suprasternal) position. Lehmkuhl & Bonagura recently studied 12 dogs with SAS and reported that highest peak velocities were obtained from the subcostal position in 10 (83%), suprasternal position in 1 (8%), and apical position in 1 (8%).94 The typical CW tracing shows an increased systolic velocity toward the aorta and a high velocity holodiastolic signal toward the left ventricle (Fig. 13-26D). The modified Bernoulli equation is used to calculate the peak systolic pressure gradient, and severity is categorized using the same pressure ranges discussed for dogs with PS.
Left ventricular and aortic pressure measurement demonstrates a pressure gradient across the obstruction and may also record and elevated end-diastolic LV pressure. The pressure gradient is invariably less than that recorded by Doppler echocardiography, often as much as 40-50 percent, due to the effects of general anesthesia. Such that, for purposes of prognosis and evaluating therapy gradients measured using Doppler echocardiography, in the awake patient, are more reliable. Recordings of LV end-diastolic pressure, left atrial pressure, and pulmonary capillary wedge pressure often demonstrate increased amplitude a waves and slightly increased mean diastolic pressures due to the decreased compliance of the hypertrophied left ventricle.
Left ventricular angiography outlines a relatively small LV cavity and usually illustrates LV concentric hypertrophy. In most cases, the actual obstruction and post stenotic dilation of the aorta are also readily identified. An aortic root angiogram should also be performed to assess the amount of aortic insufficiency associated with the obstruction.
©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.