9 Congenital Heart Disease
The cause of CHD is largely unknown. In occasional cases, teratogenic factors such as maternal infectious disease might play a role. However, distinct breed predispositions for the development of some malformations are recognized and in these cases, a genetic basis for CHD is suspected or has been established.
CHD is most common in purebred dogs and many specific malformations have an obvious familial distribution. In some cases, breeding experiments have demonstrated that the malformation is genetically transmitted; on the basis of these studies, the mode of inheritance is known or suspected for some forms of CHD.
Interpretation of breed predisposition data must be undertaken with care. In some cases, apparent breed predispositions that do not take into account breed popularity have been reported in the literature. For example, German Shepherds have occasionally been reported to be at increased risk for patent ductus arteriosus (PDA) because they have been “overrepresented” in case series of dogs with PDA. However, the relatively high frequency with which PDA is diagnosed in German Shepherds, is partly due to the popularity of the breed. One way of overcoming the confounding influence of breed popularity is use of a statistic known as the odds ratio. Buchanan (see Bibliography), has reported odds ratios for heart disease in various breeds of dogs.
In a few cases, breeding experiments have demonstrated heritability of specific lesions. This is the case for subaortic stenosis in the Newfoundland, for PDA in Miniature Poodles, for pulmonic stenosis in Beagles, and for a spectrum of conotruncal defects in Keeshonden. In this last breed, it has recently been shown that conotruncal defects—meaning developmental abnormalities of ventricular outflow and the great vessels that include tetralogy of Fallot—are associated with a single genetic mutation.
It is noteworthy that every published report of a breeding experiment in purebred dogs with CHD supports heritability of the malformation. So, despite the fact that a breed predisposition is not itself evidence of genetic transmission, it is probably appropriate to assume that all familial CHD in dogs has a genetic basis. It is therefore reasonable to advise against breeding any dog affected by CHD. A breed predisposition for development of a specific congenital cardiac malformation, even a very strong one, is not a substitute for a specific diagnosis. In some ways then, breed predispositions are most helpful to dog breeders and those who counsel them.
With few exceptions, animals with CHD have cardiac murmurs, and these are typically heard on auscultation, usually in a pup presented for routine evaluation and immunization. Of the few clinically consequential malformations that do not usually result in a heart murmur, most cause other clinical signs such as cyanosis, exercise intolerance, respiratory distress, or failure to thrive.
As in all areas of clinical veterinary medicine, a carefully elicited medical history can provide diagnostically useful information. However, most animals with CHD are free of clinical signs when the defect is first identified. Unfortunately, the absence of clinical signs is no assurance that the malformation lacks clinical importance. Therefore further diagnostic evaluation is recommended for all animals with physical findings that suggest CHD.
Not many years ago, cardiac catheterization was the primary procedure for antemortem diagnosis of CHD. Although angiographic and manometric data obtained from catheterization studies remain a gold standard of sorts, echocardiography has become widespread and has largely replaced diagnostic cardiac catheterization. More specifically, Doppler echocardiography, which provides information regarding the velocity, direction, and character of blood flow, allows noninvasive diagnosis of almost all forms of CHD. Other noninvasive procedures such as diagnostic radiography and electrocardiography generally provide ancillary information. A definitive diagnosis of CHD generally requires a complete Doppler echocardiographic study performed by a trained individual.
Treatment of specific malformations is determined by the pathophysiology that results from the lesion. In general, CHD imposes an abnormal mechanical load—a pressure overload, volume overload, or both—on the malformed segments. Treatment is most appropriately mechanical and includes operative procedures that result in either palliation or definitive correction of the malformation. Some malformations such as PDA and some forms of pulmonic stenosis are amenable to surgical correction without cardiopulmonary bypass (CPB); unfortunately, however, CPB is required for definitive correction of most congenital cardiac lesions. Recently, interventional catheterization techniques have at least partly supplanted surgery in the management of CHD.
Cardiopulmonary bypass (CPB) is an extracardiac technique to circulate and oxygenate the blood. Because the heart is not included in the circuit—is bypassed—CPB allows a surgeon to perform open-heart procedures. The necessary instrumentation is relatively expensive and application of the technique requires considerable expertise. For these reasons, CPB is not widely practiced in veterinary surgery. A few institutions have programs that offer CPB in the surgical management of canine patients with congenital heart disease.
Cardiac catheterization refers to manipulating catheters within the cardiovascular system. Most cardiac catheterization procedures are performed under fluoroscopic guidance. Angiographic, manometric, and oximetric data can be collected with this technique. Beginning in the 1960s pediatric cardiologists devised ways to use intravascular catheters to treat cardiovascular disease, and many of these so-called interventional cardiac catheterization techniques have been modified for clinical use in dogs. Balloon catheter dilation of pulmonic stenosis is widely practiced, and various techniques for transcatheter occlusion of a PDA have been described. The applicability of interventional techniques is somewhat limited by the availability of fluoroscopy and veterinarians with expertise in these techniques, but despite this, transcatheter intervention for CHD has become routine.
Patent ductus arteriosus (PDA), subvalvular aortic stenosis (SAS), pulmonic stenosis (PS), and ventricular septal defect (VSD) are the most common congenital malformations in the dog. Other lesions such as atrial septal defect, atrioventricular valve dysplasia, and various forms of cyanotic heart disease are relatively uncommon.
The ductus arteriosus, or arterial duct, is a blood vessel that forms during embryonic development and provides a communication between the systemic and pulmonary circulations. It arises from the proximal descending aorta and courses cranioventrally, where it joins the dorsal aspect of the main pulmonary artery. In utero the duct provides a means for fetal blood to skirt the pulmonary capillary bed. Although the precise mechanisms are incompletely understood, in mammals, postnatal increases in oxygen tension result in a prostaglandin cascade that causes contraction of the muscular layer of the ductus arteriosus and closure of the duct. The process begins immediately after birth and in most species is complete within 3 to 4 days. In some animals, closure of the duct does not occur or is incomplete, and the vessel is then known as a patent ductus arteriosus (PDA).
In a newborn with a PDA, if pulmonary vascular resistance falls appropriately after birth, blood can cross the duct from the high-pressure/high-resistance systemic circulation to enter the low-resistance pulmonary circulation. Therefore some of the blood pumped by the left ventricle into the aorta exits the aorta through the PDA and enters the main pulmonary artery. From here, the shunted aortic blood together with the systemic venous return is conveyed to the lungs and then, through the pulmonary veins, to the left atrium and ventricle. The volume of blood that is shunted from the aorta through the PDA to augment the pulmonary venous return increases the volume of blood entering the left atrium and subsequently the left ventricle. So, despite the fact that the shunt direction is “left-to-right”, a PDA imposes a volume overload on the left atrium and left ventricle.
When pulmonary vascular resistance is normal, the direction in which blood is shunted through a PDA is from left to right. The increase in pulmonary venous return results in left ventricular dilation and hypertrophy; the degree of cardiac enlargement is roughly proportional to the proportion of blood that is shunted through the PDA. The volume load on the left heart can cause diastolic pressures within the left ventricle to rise, and this pressure elevation is reflected back upon the pulmonary venous circulation. If pulmonary venous pressures become sufficiently high, fluid is forced out of the pulmonary capillaries and pulmonary edema results. Therefore left-sided congestive heart failure is a potential consequence of a left-to-right PDA.
PDA is observed most commonly in small-breed dogs such as Miniature Poodles, Maltese, Bichon Frises, and Pomeranians. Females are affected more often than males, with a ratio of about 2.5 to 1. The lesion is usually detected early in life, often during the first routine veterinary visit. A typical signalment for a dog with a PDA would be a 3-month-old, female Pomeranian.
A continuous murmur heard over the left craniodorsal aspect of the heart base is the auscultatory hallmark of a PDA. The murmur is often but not always loud and is usually relatively coarse; it peaks in intensity at or about the second heart sound and then fades during diastole. This murmur is sometimes referred to as a machinery murmur. Continuous murmurs must be distinguished from to-and-fro or “bellows” murmurs, which consist of separate systolic and diastolic murmurs. In dogs, to-and-fro murmurs most commonly result from endocarditis of the aortic valve or ventricular septal defects that are complicated by aortic valve regurgitation.
Careful auscultation of the dog with a PDA may also reveal a systolic murmur over the left cardiac apex. This murmur is due to functional mitral valve regurgitation; that is, the cause is mitral valve regurgitation that results from distortion of the mitral apparatus associated with ventricular dilation, and not a structural abnormality of the mitral leaflets. In dogs with a large PDA, a third heart sound that reflects high transmitral flow rates and probably reduced ventricular compliance may result in a gallop rhythm.
In a dog with a PDA, the femoral arterial pulse is brisk or bounding, sometimes described as a “water-hammer” pulse. The amplitude or strength of the femoral arterial pulse reflects the difference between systolic and diastolic pressures, which is called the pulse pressure. With a PDA, the low-resistance pulmonary circulation provides a sink for diastolic “run-off”. As a result, diastolic pressures tend to be abnormally low and the pulse pressure is wide.
In animals with moderate or large PDAs (left-to-right shunts), dilation and hypertrophy of the left ventricle is evident echocardiographically (Fig. 9-1). Echocardiographic examination also demonstrates left atrial enlargement. Myocardial dysfunction, reflected in a large end-systolic ventricular dimension and often a low fractional shortening index, is sometimes evident. Direct visualization of the PDA is usually possible when the study is performed by an experienced echocardiographer. The PDA extends from the descending aorta to the bifurcation of the main pulmonary artery. It is best demonstrated by a cranial left parasternal image of the pulmonary artery bifurcation.
Figure 9-1 M-mode echocardiographic image obtained from an 8-week-old male Shetland Sheepdog. M-mode echocardiography provides a one-dimensional view of the heart; the ordinate measures distance from the transducer and the abscissa, time. The two-dimensional image from which this M-mode was derived is shown in the inset. The left ventricle is moderately dilated and systolic performance is normal.
Doppler flow studies are used to confirm that blood from a PDA flows into the main pulmonary artery. In animals with a PDA, this will be evident on color-flow Doppler studies as a diastolic color mosaic that originates near the pulmonary artery bifurcation and extends retrograde toward the pulmonic valve. In many cases, the jet extends beyond the pulmonic valve and results in mild pulmonic regurgitation. On spectral Doppler studies, continuously disturbed flow will be evident within the pulmonary artery (Fig. 9-2).
Figure 9-2 Continuous-wave Doppler echocardiographic study performed with the ultrasound beam directed through the patent duct of 16-month-old female Maltese dog. There is a high-velocity jet directed toward the transducer and into the main pulmonary artery. The signal is continuous, but the velocity peak occurs at about the T-wave of the electrocardiogram; this event corresponds to the second heart sound and the Doppler profile provides a graphic depiction of the dog’s continuous murmur.
Additional findings detected by Doppler studies in a dog with a PDA include mitral valve regurgitation. When a substantial shunt is present, aortic flow velocities are often higher than normal. This does not necessarily reflect aortic obstruction but rather a larger left ventricular stroke volume.
PDA is essentially a physical diagnosis. When a continuous murmur is present in a dog with a typical signalment, the cause is almost always a PDA. However, Doppler echocardiographic examination is recommended as a noninvasive means of confirming the diagnosis. The echocardiographic evaluation of systolic function may also provide prognostic information that can influence therapy (see Fig. 9-3). Additionally, echocardiography provides a noninvasive means of detecting concurrent cardiac defects that might have an impact on prognosis and management.