Congenital Heart Diseases: Part I
Mª Josefa Fernández del Palacio, DVM, PhD
Hospital Clínico Veterinario. Universidad de Murcia
Congenital heart disease (CHD) is considered as an abnormality in cardiocirculatory structure or function that is present at birth, even if it is discovered much later. Cardiac malformations appear to result from an interaction between multifactorial genetic and environmental systems and in most instances, a causal factor cannot be identified. The true prevalence of CHD is difficult to determine accurately owing many animals die before a cardiac defect was diagnosed. Thus, there are both national and regional breed differences for certain cardiovascular malformations. In dogs the overall prevalence ranges from 0.46 to 0.85 per cent of hospital admissions. The prevalence of different types has changed over the past 20 years and aortic stenosis (or subaortic) is the most common defect in dogs in some countries, followed by patent ductus arteriosus (PDA), pulmonic stenosis (PS), ventricular septal defects (VSD), dysplasia of the atrioventricular valves (DAV), tetralogy of Fallot (TF), persistent left cranial vena cava (PCVC) and peritoneopericardial diaphragmatic hernia (PPDH). In cats the prevalence of congenital heart defects ranges from 0.2 to 1 per cent. AVD, VSD, endocardial fibroelastosis, PDA, TF, atrial septal defects, common atrioventricular canal, PS and PPDH are the most common defects reported in cats. Clinical evaluation of patients with CHD includes the history, physical examination, thoracic radiographs, electrocardiograms (ECG), routine hematologic tests and echocardiography. Definitive diagnosis requires Doppler echocardiography and less frequently cardiac catheterization and angiocardiography.
Obstruction of the left ventricular outflow (LVOT) tract is the most common congenital heart defect detected in large breed of dogs. Although supravalvular and valvular forms of aortic stenosis have been recognised, fibromuscular subaortic stenosis, causing a fixed form, is the most common form in the dog. Some dogs may present a dynamic form of LVOT. Fixed and dynamic obstruction has been observed coexisting with anomalies of the mitral valve complex. In cats aortic stenosis is uncommon, and also valvular and supravalvular forms have been reported. Earlier studies in Newfoundland dogs suggested that the SAS is not a true congenital defect but develops in the postnatal period. Three different grades of severity have been described: Grade 1: the lesion consisted of small, raised nodules of thickened endocardium of the interventricular septum below the aortic valve; Grade 2: lesions formed a narrow ridge of thickened fibrous endocardium that partially encircled the LVOT below the aortic valve; Grade 3: a fibrous band, ridge or collar completely encircled the LVOT just below the aortic valve. The most severe forms occur in dogs over six months of age, although sometimes pups of just a few weeks of age may be affected by severe forms of stenosis.
Breed predispositions: large breeds of dogs such as Newfoundlands (inherited condition), golden retrievers, rottweilers, boxers, German shepherd and Samoyeds are specially affected.
Pathophysiology: LVOT obstruction results in a ventricular pressure overload which requires higher systolic pressure to maintain adequate stroke volume. The ventricle hypertrophies, increasing the demand for myocardial blood flow. Aortic pressures are normal but left ventricular luminal pressures are higher than normal, so that the coronary driving pressure may be inadequate and sometimes coronary flow reversed direction in systole. Papillary muscles and subendocardial regions are the most commonly areas affected by the myocardial ischemia predisposing the myocardium to ventricular arrhythmias and syncope. Thus, left ventricular hypertrophy in association with ischemic lesions may determine a reduced ventricular compliance and diastolic dysfunction. Although cardiac output is normal at rest, the fixed obstruction makes difficult a normal increase of the stroke volume in response to the exercise requirements inducing a lower exercise tolerance and syncope. Also, the high intraluminal pressures in the left ventricle can trigger mechanoreceptors, which result in neurally-mediated reflex responses of bradycardia and hypotension and syncope. Severe forms of SAS can progress systolic dysfunction and congestive heart failure, so a reduction of the aortic velocity as well as pressure gradient.
Clinical assessment: Dogs with mild SAS are usually asymptomatic. The most common sign noticed by the owner is the exertional syncope or sometimes sudden death. Congestive heart failure can develops in dogs with severe forms of SAS that do not succumb to sudden death in early adulthood. Physical examination reveals a systolic ejection murmur, which may be heard best over the left or right heart base and appears to be related to the severity of the obstruction. Peripheral pulses can be normal in mild SAS or weak in more severe forms. The ECG is normal in most dogs, although signs of left ventricular enlargement or ST segment changes with exercise or ventricular arrhythmias may be present. The most common significant thoracic radiograph findings in dogs with SAS are an increase in apico-basilar length of the heart and post-stenotic dilation in moderate to severe forms. Echocardiography can demonstrate most (but not all obstructions. Left ventricular hypertrophy and hyperechoic subendocardial areas of fibrosis are observed in severe cases. The diagnosis is confirmed by Doppler echocardiography. The measure of peak aortic velocity represents an indicator of the pressure gradient, useful for severity classification of the aortic stenosis, especially when considered with clinical signs, concurrent mitral lesions, arrhythmias and myocardial fibrosis. Based on transvalvular pressure gradient, the aortic stenosis may be classified as mild (<40 mmHg), moderate (40-80 mm Hg) and severe (> 80-100 mm Hg). The aortic velocity should be measured from the subcostal view for optimal alignment of blood flow for a Doppler study. In moderate to severe forms Doppler echocardiography may show abnormalities in diastolic mitral inflow. Because of the aortic velocity may be influenced by several factors, this parameter may not indicate a real parameter to differentiate normal patients from patients with mild lesions, not evident at the echocardiographic exam.
Clinical Management: Balloon catheter dilation may reduce the severity of the obstruction in severe cases, at least in the short term. Medical management of dogs with severe form of SAS is commonly based on beta-blockers in order to improve the ventricular diastolic inflow and the left ventricle performance and to reduce the severity of the LVOT obstruction. Beta-blockers also have a direct anti-arrhythmic effect reducing the incidence of syncope and sudden death. Prophylactic antibiotic therapy to prevent bacterial endocarditis should be considered when there is known potential for hematogenous exposure to infectious organisms.
PULMONIC STENOSIS (PS)
Although subvalvular and supravalvular forms of PS have been reported, the most common form of the right ventricular obstruction in dogs is the pulmonary valve dysplasia. It can be present as a single defect or associated with other heart defects. Two forms have been described in dogs: a) Type 1 pulmonary valve dysplasia, the valve leaflets appear slightly thickened, and the commissures fused with a central orifice. b) Type 2 pulmonary valve dysplasia characterized by moderate to severe thickening and hypoplasia of valve leaflets and obstruction of the orifice by redundant valve tissue. Thus, intermediate forms were also present in those dogs. In English Bulldog and Boxer, PS can be associated with an abnormal coronary artery which encircles the pulmonary trunk just below the valve causing subvalvular stenosis. A pulmonary artery atresia represents the extreme form of the right ventricular obstruction tract.
Breed predispositions: English Bulldog, Mastiff, Fox Terrier, Samoyed, miniature Schnauzer, Cocker Spaniel and West Highland White Terrier are specially predisposed. A hereditary form of pulmonary valve dysplasia has been found in the Beagle (polygenetic mode of transmission) and Boykin Spaniel.
Pathophysiology: Obstruction of the right ventricular outflow tract causes increased resistance to ejection and a proportional increase in ventricular systole pressure leading to right ventricular hypertrophy, leftward septal flattening and a systolic gradient across the pulmonary valve. High velocity and turbulent flow about the stenosis is associated with a left basilar ejection murmur, and poststenotic dilatation of the main pulmonary artery. Tricuspid valve malformation often develops in severe PS, a consequence of concurrent malformation or hypertrophy of the papillary muscles.
Clinical assessment: Animals with mild PS are usually asymptomatic. Severe cases manifest signs related to low cardiac output, such as exercise intolerance and syncope and often right side cardiac failure. On thoracic auscultation a left basilar ejection murmur over the pulmonic valve area that irradiates dorsally is present. A holosystolic heart murmur is heard when tricuspid regurgitation is present. The ECG is normal in mild PS. Right axis deviation in the frontal plane and deep S waves on the left precordial chest leads are present in moderate to severe forms of PS. Thoracic radiographs show varying degrees of right ventricular enlargement, prominent main pulmonary artery and the lungs often appear underperfused. The echocardiography permits to visualize the features of the obstruction. When valvular fusion exists the semilunar cusps are fused toward the tips of the leaflet, resulting in doming of the valves. When hypoplastic type of PS is present the valve appears thickened and immobile and the annulus may be narrow and hypoplastic. The main hemodynamic effects of the PS can be visualized as concentric right ventricular hypertrophy, paradoxical septal motion and reduced size of the left atrium and left ventricle in moderate to severe PS. Dilatation of the main pulmonary artery can be see distally to the obstruction, close to the bifurcation. Spectral Doppler echocardiography of PS will record increased peak pulmonary artery velocity (> 1.6 m/sg) and often prominent pulmonary insufficiency. A Doppler gradient across the stenosis <50 mm Hg is considered as mild PS. Gradients between 50 and 100 mm Hg are moderate in severity and higher gradients are associated with severe PS.
Clinical management: Balloon valvuloplasty is the procedure of choice for significant PS (systolic Doppler pressure greater than 100 mmHg). Long-term successful reduction of the obstructive gradient in dogs with type 1 PS has been reported. In dogs with hypoplasia of the pulmonary annulus the results are more variable.
DYSPLASIA OF THE ATRIOVENTRICULAR VALVES (DAV)
A wide spectrum of lesions has been identified in dogs and cats with atrioventricular valve malformations, including anomalies in valve leaflets, chordae tendineae and/or papillary muscles. Dysplasia of the atrioventricular valves may appear as isolated lesion or in association with others defects (i.e., SAS, VSD, ASD, EP).
Breed predispositions: Great Danes, German Shepherds, bull terriers, golden retrievers, Newfoundlands, Dalmatians, mastiffs and cats are predisposed to mitral dysplasia. Tricuspid dysplasia has been reported in cats and Old English sheepdogs, German Shepherds, Weimaraners, and Labrador retrievers.
Pathophysiology: Functional sequelae to atrioventricular valve malformations include mainly mitral and tricuspid regurgitation and/or inflow obstruction. Valve regurgitation causes volumen overloading, atrial dilation and eccentric hypertrophy on the affected side and congestive heart failure. When valve stenosis is present, increased left or right atrial pressure results in pulmonary edema or ascites, respectively.
Clinical assessment: Clinical signs of heart failure may develops before one year of age although clinical signs may be absent several years. The typical physical examination finding is a holosystolic heart murmur heard best over the affected valve area. Thoracic radiographs demonstrate left or right-sided cardiomegaly. In patients with mitral dysplasia pulmonary venous congestion and pulmonary edema may appear if left congestive heart failure develops. Valvular stenosis may be suspected whenever the atrium is markedly dilated without enlargement of the ipsilateral ventricle. The ECG may reflect left or right-sided enlargement, atrial arrhythmias, and intraventricular conduction disturbances (tricuspid dysplasia). Definitive diagnosis requires echocardiography. Abnormal shape, location, motion or attachment of the valve apparatus may be observed by two-dimensional echocardiography. Doppler studies demonstrate a regurgitant jet and/or valvular stenosis.
Clinical management: Surgical replacement of the dysplastic valve has been successfully performed in some dogs. Also, balloon valvuloplasty has been reported in one dog with tricuspid stenosis. However, in most patients wit DAV medical management is instituted when congestive heart failure develops.
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M™ Josefa FernŠndez del Palacio, DVM, PhD