Brian A. Scansen,     Columbus, Ohio

Richard E. Cober,     Annapolis, Maryland

John D. Bonagura,     Columbus, Ohio

Prevalence

The prevalence of CHD in the dog and cat is difficult to quantify due to a lack of routine perinatal care, occurrence of conditions that are not apparent on routine physical examination, and hospital biases in published reports. The work of Detweiler and Patterson in the Philadelphia area suggested a CHD prevalence rate of 0.56% among 4831 dogs surveyed in the mid-twentieth century (Detweiler and Patterson, 1965), whereas Buchanan found a rate of 0.67% for all dogs brought to the University of Pennsylvania between 1987 and 1989 (Buchanan, 1992). In a study of 1679 puppies aged 6 to 18 weeks at a pet store, murmurs were observed in 11, which suggests an incidence of CHD of 0.66% (Ruble and Hird, 1993). However, this may be an underestimation because it fails to include those puppies that died from CHD before the age of 6 weeks or those animals whose disease was silent to auscultation at that age. Conversely, this study may have overestimated the incidence of CHD because further testing was not performed to determine if the murmur was reflective of true CHD or simply functional or physiologic in origin. Additionally, the pet store population is likely biased toward purebred dogs, which are known to carry a higher proportion of hereditary defects than mixed-breed dogs.

Although the true prevalence of CHD in dogs and cats is unknown, data are available that indicate the relative incidence of specific malformations in each species. In an attempt to compile these data, several studies of CHD prevalence were analyzed and the data were combined (Tables 174-1 and 174-2). Ten applicable studies of CHD prevalence in dogs were found in the veterinary literature, comprising a sum of 4694 defects (see Table 174-1). When the defects were totaled, patent ductus arteriosus (PDA) (25.7%), subaortic stenosis (SAS) (23.5%), and pulmonary valve stenosis (PS) (22.1%) were found to be the most common and of nearly equal incidence, together comprising almost three fourths of all instances of CHD in the dog. Defects in the ventricular septum (8.8%) and atrial septum (1.9%) accounted for an additional 10.7% of cases, whereas dysplasias of the mitral valve (4.3%) and tricuspid valve (4.6%) were present in nearly equal frequency. The constellation of defects known as tetralogy of Fallot (ToF) accounted for 2.3% of defects, whereas more complex lesions not easily classified into the aforementioned categories made up 3.4% of CHD. Not all studies provided data on vascular ring anomalies, which may present with noncardiac signs and therefore likely are underrepresented in these data.

Fewer data are available about CHD prevalence in the cat. Table 174-2 compiles CHD cases from five studies to provide cumulative data on 435 defects. Ventricular septal defect (VSD) appears to be the most common defect in the cat, comprising over 18% of cases. PDA is not considered a common defect in the cat, compared with the dog, but accounted for over 11% of defects tabulated in these studies. Mitral valve dysplasia (MVD) and tricuspid valve dysplasia (TVD) each accounted for an additional 10% of defects. Abnormalities of the atrioventricular septum (AVSDs; formerly known as endocardial cushion defects or atrioventricular canal defects) are not infrequent in the cat, comprising a tenth of the cases reported. However, not all studies clearly defined the location of tabulated atrial septal defects (ASDs) and VSDs, nor fully described the morphology of diagnoses of MVD and TVD—all of which may be forms of AVSD. For this reason, the proportion of at least partial AVSDs is likely higher than tabulated here. Earlier studies described a high proportion of endocardial fibroelastosis in kittens, particularly the Burmese and Siamese breeds, but this disease was not described in more recent studies. Compared with dogs, abnormalities of the outflow tracts (PS and SAS) appear much less common in cats.

Diagnostic Testing

Screening for congenital cardiac disease typically begins with a thorough physical examination performed at the first general health examination. One of the most important aspects of the cardiac physical examination and one of the initial diagnostic tests for CHD is a thorough auscultation of the heart and great vessels. Auscultation is imperative for screening for CHD because the most common forms of CHD, as well as many complex CHDs, typically present with a systolic murmur (SAS, PS, VSD, MVD, and TVD) or continuous murmur (PDA). Auscultation in puppies and kittens should be performed in a quiet room and all valve areas should be evaluated, including the mitral valve area where the apical impulse is felt most strongly, the semilunar valve area (aortic and pulmonary valves) at the base of the left side of the heart (the ventral second to fourth intercostal spaces), the area of the great vessels just dorsal to the base of the left side of the heart, and the tricuspid valve area on the right chest wall. The left and right thoracic inlet and sternal borders also should be auscultated so as not to miss atypically radiating murmurs or very focal defects. In many cats and very small dogs, distinguishing separate valve areas is not possible because of the small chest size, and most murmurs in cats tend to be loudest adjacent to the sternal borders. The use of a pediatric stethoscope head can aid in the auscultation of very small animals.

Murmur location and intensity are useful in differentiating between types of CHD as well as in distinguishing an innocent or functional murmur from that caused by CHD. Left (mitral regurgitation) and right (tricuspid regurgitation) apical systolic murmurs commonly are due to defects of mitral valve dysplasia and tricuspid valve dysplasia, respectively. The murmur of a VSD typically is systolic at the right sternal border. Left basilar systolic murmurs can be more problematic, however, because they can indicate either an innocent murmur or a CHD such as SAS, PS, or ASD, especially when soft in intensity. Murmur intensity coupled with location can be helpful in discerning different types of CHD and differentiating CHD from an innocent or functional murmur. High-intensity murmurs (grade 3 out of 6 or louder) usually indicate the presence of significant CHD regardless of location and are an indication for more advanced diagnostic testing. However, less intense murmurs (grade 3 or lower) either may originate from a functional cause or may indicate CHD. In general, a soft left basilar systolic murmur heard in a young puppy or kitten that decreases in intensity over the first months of life or disappears by 6 months of age is more likely to be innocent. However, if the murmur persists at the same intensity or becomes louder as the animal ages, it is more likely to reflect CHD, and advanced diagnostic testing such as echocardiography should be recommended. A continuous murmur, a diastolic murmur, other cardiac signs (arrhythmia, cough, syncope), and a very loud murmur all likely indicate CHD, and further testing should be advised.

Other aspects of a cardiac physical examination are evaluations of the mucous membranes, jugular veins, and pulse quality. Mucous membranes should be assessed for evidence of cyanosis or prolonged capillary refill time, which may be indicative of a right-to-left intracardiac shunt or decreased cardiac output and reduced peripheral perfusion even in the absence of an obvious heart murmur. Variations in pulse quality also may be suggestive of CHD. Hypokinetic pulses commonly are detected in moderate to severe subaortic stenosis, whereas a hyperkinetic pulse quality is suggestive of a wide pulse pressure caused by PDA or severe aortic insufficiency. Jugular pulsation with or without distention can indicate tricuspid regurgitation or elevated right-sided pressures due to TVD, PS, or pulmonary hypertension from severe left-to-right shunting defects.

Although the physical examination is an important aspect of screening for the presence of CHD, it is difficult to make a definitive diagnosis from signalment and physical examination alone. Ancillary diagnostic tests, like thoracic radiography and electrocardiography (ECG), may be useful in narrowing the differential diagnostic list and determining disease severity but can also add cost to the initial evaluation without providing definitive diagnostic information.

Thoracic radiographs should be evaluated in a systematic manner and should include examination of orthogonal views, the cardiac silhouette, pulmonary vasculature, pulmonary parenchyma, and vena cava. Thoracic radiography is useful in diagnosing congestive heart failure (CHF) as well as in detecting cardiac changes due to moderate to severe CHD. However, radiographs have limited diagnostic capability in mild CHD because there may be minimal to no radiographic change. The radiographic diagnosis of left-sided CHF includes left-sided cardiomegaly, specifically left atrial enlargement; pulmonary venous distention; and interstitial to alveolar infiltrates in the perihilar to caudodorsal lung fields in dogs. Cats with CHF also have left atrial enlargement and venous congestion but may have a variety of pulmonary parenchymal changes such as a ventrally distributed interstitial pattern, patchy interstitial to alveolar infiltrates, and pleural effusion. Right-sided CHF has the radiographic signs of enlargement of the right side of the heart, caudal vena caval dilation, hepatomegaly, and varying degrees of either loss of abdominal detail from ascites or signs of pleural effusion.

Although the specific radiographic diagnosis of CHD is beyond the scope of this chapter, in general left atrial and ventricular enlargement are common secondary to mitral regurgitation (MVD) or left-to-right shunting defects (PDA, VSD). Conversely, left atrial enlargement without ventricular enlargement more likely is related to mitral valve stenosis. Left-to-right shunting defects, in addition to left-sided cardiomegaly, also have signs of overcirculation (dilatation) of both the pulmonary arteries and the pulmonary veins. Right atrial and ventricular enlargement may occur with tricuspid regurgitation (TVD), PS, ASD, or ToF. Rarely, left-to-right shunting defects may result in pulmonary vascular changes and secondary pulmonary hypertension, which also may cause radiographic evidence of enlargement of the right side of the heart. Isolated right atrial enlargement without right ventricular enlargement can be caused by tricuspid valve stenosis and TVD. SAS can show left ventricular enlargement in conjunction with dilation of the aortic root and ascending aorta, whereas PS results in dilation of the main pulmonary artery segment in addition to enlargement of the right side of the heart.

ECG may be beneficial in detecting cardiac changes related to CHD, although it is most useful for characterizing dysrhythmias heard on auscultation. Diseases affecting the right side of the heart such as PS, TVD, and ToF can show P pulmonale (tall P waves), indicating right atrial enlargement, or a right-axis deviation of the QRS complex, indicating right ventricular enlargement or right bundle branch block. Diseases affecting the left side of the heart can show P mitrale (wide P waves), indicating left atrial enlargement, or a tall or wide QRS complex, indicating left ventricular enlargement. CHDs that cause ventricular enlargement, such as PS and SAS, may provide a substrate for ventricular arrhythmias secondary to myocardial ischemia. However, ECG is a test that is more specific than sensitive because animals with significant CHD may have a normal ECG. ECG also is unlikely to be of use for definitive diagnosis of a CHD, although it is very useful for definitive diagnosis of dysrhythmias associated with CHD.

In recent years the use of cardiac biomarkers to screen animals for the presence of cardiovascular disease has gained in popularity. There have been limited studies looking at the usefulness of these tests in animals with CHD. The biomarkers receiving the most attention in veterinary cardiology are cardiac troponin I (cTnI) and B-type natriuretic peptide (BNP), especially the N-terminal (amino end) of the prohormone (NT-proBNP). The biomarker cTnI is a myocardial protein released in the setting of myocyte damage or death; for this reason, the level of cTnI is profoundly elevated in the setting of myocardial necrosis, and high-sensitivity assays also have found mild to modest elevations in conditions leading to cardiac remodeling. B-type natriuretic peptide is a hormone released by the myocardium in response to stretch, hypertrophy, or hypoxia; the N-terminal of the prohormone is more stable in serum and is therefore the preferred assay target.

There are few studies evaluating the use of cTnI as a biomarker in animals with CHD. A mild elevation (median, 0.20 ng/ml; range, 0.20 to 1.29) was shown in 30% of dogs with PS before balloon valvuloplasty (Saunders et al, 2009b). However, a study by a different group found normal cTnI values (≤0.05 ng/ml) in dogs with PDA and PS before intervention (Shih et al, 2009). Based on these limited data, cTnI level does not appear to be a reliable screening test for CHD in animals, although further studies are needed.

More data are available to suggest that NT-proBNP levels are elevated in dogs with CHD. A study by Saunders and colleagues (2009a) found median NT-proBNP values of 724 pmol/L (range, 50 to 3000) in dogs with PDA, 746 pmol/L (range, 278 to 3000) in dogs with PS, and 833 pmol/L (range, 388 to 2194) in dogs with ASD compared with 333 pmol/L (range, 161 to 826) in normal dogs. Also in 2009, Farace and colleagues found that NT-proBNP values in dogs with SAS were elevated and correlated with the severity of the stenosis: normal dogs had a mean NT-proBNP level of 361 pmol/L (range, 307 to 415), dogs with mild SAS had a level of 344 pmol/L (range, 239 to 448), dogs with moderate SAS showed a level of 1011 pmol/L (range, 86 to 1936), and those with severe SAS had a level of 2529 pmol/L (range, 1841 to 3217). Although NT-proBNP level does appear to have some usefulness in determining the presence or absence of CHD, it cannot distinguish the type of defect present. More comprehensive studies are needed to better define the value of NT-proBNP in screening young animals for the presence of CHD.

History, physical examination, thoracic radiography, ECG, and NT-proBNP level all are useful diagnostic tools for determining the presence of CHD but are less capable of providing a definitive diagnosis, assessing disease severity, or ruling out the presence of multiple or complex congenital lesions. In veterinary medicine, Doppler echocardiography is the noninvasive gold standard for definitive diagnosis of CHD. All animals in which CHD is suspected should undergo a detailed echocardiographic examination performed by a veterinarian trained in the diagnosis of CHD. Echocardiography allows identification of specific types of CHD and detection of concomitant defects, as well as assessment of disease severity and risk of complications, determination of the direction of cardiac shunting, estimation of intracardiac pressures, and assessment of the likelihood of CHF. Echocardiography is necessary for determining the need for therapy as well as for planning interventional or surgical therapies.