Overview of Congenital and Inherited Anomalies of the Cardiovascular System
Congenital anomalies of the cardiovascular system are defects that are present at birth and can occur as a result of genetic, environmental, infectious, toxicologic, pharmaceutical, nutritional, or other factors or a combination of factors. For several defects, an inherited basis is suspected based on breed predilections and breeding studies. Congenital heart defects are significant not only for the effects they produce but also for their potential to be transmitted to offspring through breeding and thus affect an entire breeding population. In addition to congenital heart defects, many other cardiovascular disorders have been shown, or are suspected, to have a genetic basis. Diseases such as hypertrophic cardiomyopathy, dilated cardiomyopathy, and degenerative valvular disease of small breeds of dogs may have a significant heritable component.
In dogs, the prevalence of congenital heart disease is estimated at <1%. In multiple large studies of congenital heart disease in dogs, the three most common defects are aortic stenosis, pulmonic stenosis, and patent ductus arteriosus (PDA). Less common defects include ventricular septal defect, atrial septal defect, mitral valve dysplasia, tricuspid valve dysplasia, tetralogy of Fallot, cor triatriatum, and persistent right aortic arch. However, because of regional differences, the most common congenital cardiac defects in dogs in the USA vary from those reported in the UK and may likely differ from those in Europe and other regions.
In cats, the prevalence of congenital heart disease has been estimated to be 0.2%–1% and includes atrioventricular (AV) septal defects (including ventricular septal defect, atrial septal defect, and endocardial cushion defects), AV valve dysplasia, endocardial fibroelastosis, PDA, aortic stenosis, and tetralogy of Fallot.
The most common defects in other species are as follows: cattle—ventricular septal defect, ectopic heart, and ventricular hypoplasia; sheep—ventricular septal defect; pigs—tricuspid valve dysplasia, atrial septal defect, and subaortic stenosis; horses—ventricular septal defect, PDA, tetralogy of Fallot, and tricuspid atresia. Arabian horses have a relatively higher incidence of congenital defects than other breeds; a variety of defects have been reported for this breed.
The early detection of a congenital heart defect is critical for several reasons. Certain defects are surgically correctable, and treatment should be performed before the onset of congestive heart failure (CHF) or irreversible cardiac damage; recently purchased animals may be returned to avoid economic loss; pets with congenital heart defects are likely to die prematurely, causing emotional distress; and animals purchased for performance have limited potential and will likely be unsatisfactory. Early detection also prevents incorporation of genetic defects into breeding lines.
The evaluation of most animals with a congenital cardiac defect usually consists of a physical examination, electrocardiography, radiography, and echocardiography. This allows for a definitive diagnosis and an assessment of the severity of the defect. The use of Doppler echocardiography has supplanted the use of invasive cardiac catheterization studies in the evaluation of most cardiac defects. Once the diagnosis has been made and severity determined, treatment options can be developed and a prognosis given.
The clinical significance of congenital heart disease depends on the particular defect and its severity. Mildly affected animals may exhibit no ill effects and live a normal life span. Defects causing significant circulatory derangement will likely cause neonatal death. Such defects, many incompatible with life, also cause fetal death and reduced litter size. Medical or surgical management is most likely to benefit animals with congenital cardiac defects of moderate or greater severity. Left-to-right shunting PDA is one notable exception in which surgical correction is indicated in nearly all affected animals.
Congenital heart defects produce signs of cardiac failure through a variety of pathophysiologic mechanisms. Defects such as pulmonic stenosis and subaortic stenosis cause ventricular outflow obstruction and may result in right- and left-side failure, respectively. Outflow obstruction also leads to concentric hypertrophy of the respective ventricle, increasing the risk of ischemia-induced arrhythmias and sudden death. PDA and septal defects are examples of abnormal communications between the systemic and pulmonary circulatory systems and typically result in left-to-right shunting of blood. The recirculation of blood through the pulmonary circulation and into the left-side chambers often precipitates signs of left-side CHF (eg, pulmonary edema, cough, fatigue). Larger defects typically result in a greater degree of volume overcirculation to the left-side chambers. PDA is a possible exception, with very large defects sometimes contributing to pulmonary hypertension and right-to-left shunting (see Right-to-Left Shunts (Cyanotic Heart Disease)), also called a reversed PDA. Animals with right-to-left shunting defects (tetralogy of Fallot, reversed PDA) may develop right heart failure but more often have clinical signs associated with polycythemia (see Erythrocytosis and Polycythemia), which develops subsequent to renal perfusion with deoxygenated blood. This results in an increase in erythropoietin production by the kidneys and consequent polycythemia.
It is imperative to appreciate that the presence of a heart murmur in a young animal is not pathognomonic for a congenital heart defect. Many young animals will have a low-grade systolic murmur (often grade II/VI or less) that is the result of mild turbulence and is not associated with a congenital heart defect. These murmurs usually disappear by 6 mo of age in dogs and cats. Innocent murmurs are heard in the absence of any other demonstrable evidence of cardiovascular disease. High-grade systolic murmurs (grade IV/VI or greater) and diastolic murmurs are indicative of cardiac disease and should prompt further investigation.