Chapter 154 Congenital Heart Disease
ETIOLOGY
• Congenital heart disease (CHD) is a general term indicating malformation of the heart or great vessels. CHD is present at birth and represents the most common cause of cardiovascular disease in animals younger than 1 year of age.
• Diagnosis, staging, and management of CHD can be complicated, and the purpose of this chapter is to offer a framework for recognition and understanding of common cardiac malformations in dogs and cats.
• Common defects based on prevalence in referral centers include:
• In dogs: patent ductus arteriosus (PDA), subvalvular aortic stenosis (SAS), pulmonic stenosis (PS), atrial and ventricular septal defects (ASD, VSD), mitral and tricuspid valvular dysplasia (malformation), persistent right aortic arch, tetralogy of Fallot, and cor triatriatum dexter.
• In cats: atrial and ventricular septal defects, mitral and tricuspid valve dysplasia, patent ductus arteriosus, tetralogy of Fallot, and aortic stenosis.
• Congenital peritoneopericardial diaphragmatic hernia is a common defect of the diaphragm and pericardium (see Chapter 151).
• Even in cases of suspected autosomal dominant inheritance, the penetrance of the lesion may be incomplete, making clinical recognition difficult or impossible. Furthermore, there are no genetic tests available for detection of carriers. This situation makes genetic counseling difficult.
• Dogs or bitches with normal cardiac phenotype that produce CHD-affected puppies in more than one litter should be removed from breeding programs.
CLINICAL APPROACH
The vast majority of CHD cases are recognized initially by cardiac auscultation. A routine 6-lead electrocardiogram (ECG) and thoracic radiographs will also provide diagnostic information in cases of moderate to severe CHD; however, these studies are likely to be negative in mild disease. Definitive diagnosis requires advanced echocardiography with Doppler studies. The general approach to diagnosis based on initial auscultation findings is summarized in Figure 154-1 along with typical ECG and radiographic features. The following are some pointers regarding diagnosis and assessment of CHD in dogs and cats.
• As a general rule, soft ejection type murmurs in otherwise healthy puppies or kittens can be followed through the vaccine sequence. Increasing murmur intensity, other physical abnormalities (stunted growth, cyanosis), or any suggestion of clinical signs (exercise intolerance, dyspnea) should prompt immediate evaluation by a veterinarian with experience in congenital heart disease. A loud systolic murmur, a diastolic murmur, or a continuous murmur is not likely to be innocent and should be evaluated promptly.
• The signalment, physical examination findings, 6- or 9-lead ECG, and thoracic radiographs may provide a provisional diagnosis and help to guide any initial treatment plans.
• PDA, VSD, SAS, and mitral valve malformation are more likely to lead to left-sided cardiomegaly, a finding that may be evident on ECG or thoracic radiographs. With ASD, PS, and tricuspid valve malformation, right-sided cardiomegaly might be identified.
• Radiographs are particularly helpful for recognizing pulmonary vascular changes and congestive heart failure (CHF). However, such examinations are not definitive in terms of diagnosis, and do not substitute for expert evaluation using echocardiographic techniques.
• When a dog or cat is suspected of having CHD consider the following points:
• Animals presenting with one defect may have concurrent or complicating cardiac issues. For example, combinations of outflow tract obstruction and atrioventricular valvular dysplasia generate a less favorable prognosis. In chronic volume overload, as with uncorrected PDA, progressive myocardial failure or development of atrial fibrillation can impact prognosis negatively.
• Young animals with relatively severe lesions often appear totally normal to the client and may remain compensated for months to years. The presence of exertional symptoms is generally an indication of serious CHD. Delaying cardiac evaluation until the onset of clinical signs is a serious clinical error, as by that time therapeutic options may be limited or less effective.
• Early therapeutic intervention may slow or eliminate irreversible myocardial damage and prevent heart failure. For example, after successful closure of a PDA, most dogs will live a normal life without need for cardiac follow-up.
• Progressive pulmonary vascular disease develops in some patients with left-to-right shunts (PDA, VSD, ASD, aorticopulmonary window) or congenital mitral stenosis. Elevated vascular resistance can cause severe pulmonary hypertension (PH) in these patients.
• PH due to vascular injury and high resistance increases pulmonary artery, right ventricular (RV), and right atrial pressures; reduces left-to-right shunting; and limits venous return to the left atrium.
• This development usually improves symptoms (and facilitates growth), but the benefit is temporary. Severe pulmonary vascular disease eventually limits cardiac output, causes progressive exercise intolerance, and often leads to arterial desaturation (a reduced oxygen content due to low partial pressure of oxygen). Desaturation is related to either right-to-left shunting of blood across a defect (Eisenmenger’s physiology) or development of bronchoesophageal collateral vessels that mix low-oxygen venous return with pulmonary venous blood.
• A typical consequence of shunt reversal is progressive secondary polycythemia, a complication that can lead to a markedly elevated hematocrit and hyperviscosity syndrome.
• Advanced pulmonary vascular disease changes the examination findings to that of marked RV hypertrophy, pulmonary artery dilation, and reduced size of the left heart chambers. In the case of shunt lesions, the right-to-left shunt serves as a needed pressure-relief valve for the right side, but predisposes to arterial desaturation and polycythemia. This situation is generally irreversible and explains why a “reversed” PDA cannot be corrected surgically without fatal consequences.
DIAGNOSIS
Despite the technical advances in cardiology, a thorough history and physical examination is an essential part of the diagnostic process. In the evaluation of CHD, cardiac auscultation is especially important (Fig. 154-1), and experienced examiners can often establish a tentative diagnosis based on auscultation and breed predisposition (Table 154-1).
Table 154-1 BREED AND SEX PREDILECTIONS FOR CERTAIN CONGENITAL CARDIAC DEFECTS*
| Defect | Predilection |
|---|---|
| Patent ductus arteriosus (PDA) | Poodle, Bichon friese, collie, Pomeranian, German shepherd, Shetland sheepdog, and many other breeds (female:male, 2.2:1) |
| Pulmonic stenosis (PS) | Beagle, bulldog, fox terrier, miniature schnauzer, Chihuahua, Samoyed, Labrador retriever |
| Subaortic stenosis (SAS) | Newfoundland, boxer, German shepherd, German shorthaired pointer, golden retriever, rottweiler, bull terrier |
| Ventricular septal defect (VSD) | English bulldog, springer spaniel |
| Atrial septal defect (ASD) | Samoyed, boxer, Doberman pinscher |
| Mitral dysplasia | Great Dane, German shepherd, bull terrier (male > female) |
| Tricuspid dysplasia | Great Dane, German shepherd, Weimaraner, Labrador retriever (male > female) |
| Tetralogy of Fallot | Keeshond, English bulldog |
* Note: SAS, PDA, and PS are the most common defects in dogs. ASD/VSD and atrioventricular valve dysplasias are the most common defects in cats.
Signalment and History
Age, breed, and sex should be considered when evaluating animals for suspected CHD. As many specific breed predilections are identified—especially in dogs—a predisposition can be useful in formulating the differential diagnoses. Table 154-1 is a guide to breed for the most commonly recognized disorders, but is not a comprehensive listing.
• Most animals with CHD are asymptomatic when first examined, often in the face of hemodynamically severe defects. Thus do not conclude that the defect is mild in a patient with an unremarkable history.
• Stunted growth often indicates a patient with CHF or a right-to-left shunt (cyanotic heart disease). Signs of CHF may include coughing, exercise intolerance, orthopnea, tachypnea, respiratory distress, or abdominal distension.
Physical Examination
Precordial Palpation
• A precordial heave is an apical impulse that is stronger than normal and often indicates hypertrophy or enlargement of the underlying ventricle. RV hypertrophy is suggested by a right-sided impulse of greater intensity than that on the left. Caudal displacement of the left apical impulse beyond the 6th intercostal space (ICS) may indicate cardiac dilatation.
Cardiac Murmurs
• Murmur characteristics may be suggestive of a particular cardiac defect (see Fig. 154-1 and Chapter 142). Most murmurs associated with CHD are systolic. A continuous murmur is due to PDA or, rarely, from an aorticopulmonary window. A diastolic murmur is most likely due to aortic regurgitation, a condition that may coexist with VSD, aortic valvular malformation, or SAS complicated by aortic valvular endocarditis. Atrioventricular valvular stenosis (or supravalvular stenotic ring) is a rare cause of diastolic heart murmurs.
• Distinguishing an innocent (functional) murmur from one caused by CHD, especially in the case of mild aortic stenosis or trivial mitral malformation, may be difficult if not impossible.
• Murmurs caused by clinically-significant CHD are typically louder and of longer duration than innocent murmurs.
• Innocent murmurs are often intermittent, may be musical, and usually diminish or resolve by 16 to 24 weeks of age. Murmurs that persist are more likely to suggest cardiac malformation.
• With the exception of ventricular outlet obstruction (SAS or PS), murmur intensity or duration does not correlate reliably with the severity of the cardiac lesion. For example, dogs with severe tricuspid valve dysplasia may have soft murmurs, whereas a small and restrictive VSD can create a very loud murmur.
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