Chapter 12 Congenital Heart Disease

Keith N. Strickland

INTRODUCTION

Incidence

• The incidence of congenital heart disease in dogs has been reported to be 6.8 to 8.0 per 1000 hospital admissions. On average, this equates to one case per 15 litters. The actual incidence is likely higher, as some defects result in neonatal death and are unreported.

• The most common congenital heart defects in dogs include patent ductus arteriosus (PDA), pulmonic stenosis (PS), aortic stenosis, ventricular septal defect (VSD), and tetralogy of Fallot (Box 12-1). Several breed predispositions have been identified (see Appendix 1: Canine Breed Predilections for Heart Disease).

• Less common defects in dogs include mitral valve dysplasia, atrial septal defects, tricuspid valve dysplasia, cor triatriatum dexter, and mitral stenosis. Rare defects include persistent truncus arteriosus, tricuspid stenosis, right ventricular hypoplasia, double outlet right ventricle, and transposition of the great vessels.

• Congenital heart disease is less common in cats than in dogs. The reported incidence is 0.2 to 1.0 per 1000 hospital admissions. No consistent breed or gender predilections have been adequately demonstrated.

• The most common congenital heart defects in cats include mitral valve dysplasia, tricuspid valve dysplasia, PDA, VSD, aortic stenosis, tetralogy of Fallot, persistent common atrioventricular canal, and endocardial fibroelastosis.

• Less common defects in cats include persistent truncus arteriosus, PS, atrial septal defect, tricuspid stenosis, and right ventricular hypoplasia.

Box 12-1 Classification of Congenital Defects According to Pathophysiology

CANINE	FELINE
Defects primarily causing volume overload	Defects primarily causing volume overload
Systemic to pulmonary (left-to-right) shunting
Common	Common
Patent ductus arteriosus	Ventricular septal defect
Ventricular septal defect	Patent ductus arteriosus
Uncommon	Atrial septal defect
Atrial septal defect	Endocardial cushion defect
Endocardial cushion defect	Uncommon
(Pseudo) truncus arteriosus	Truncus arteriosus
Valvular regurgitation	Valvular regurgitation
Common	Common
Mitral dysplasia	Mitral dysplasia
Tricuspid dysplasia	Tricuspid dysplasia
Uncommon
Pulmonic insufficiency
Aortic insufficiency
Defects primarily causing pressure overload	Defects primarily causing pressure overload
Common	Common
Pulmonic stenosis	Dynamic subaortic stenosis
Subaortic stenosis	Uncommon
Uncommon	Pulmonic stenosis
Valvular aortic stenosis	Pulmonary artery branch stenosis
Coarctation and interruption of the aorta	Fixed subaortic stenosis
Cor triatriatum dexter	Valvular aortic stenosis
	Cor triatriatum dexter
	Cor triatriatum sinister
Defects primarily causing cyanosis	Defects primarily causing cyanosis
Common	Common
Tetralogy of Fallot	Tetralogy of Fallot
Uncommon	Endocardial cushion defect
Pulmonary to systemic shunting (ventricular septal defect [VSD])	Uncommon
Pulmonary to systemic shunting (patent ductus arteriosus [PDA])	Pulmonary to systemic shunting (VSD)
Tricuspid atresia/right ventricular hypoplasia	Pulmonary to systemic shunting (PDA)
Double outlet right ventricle	Double outlet right ventricle
Transposition of the great vessels	Truncus arteriosus
Truncus arteriosus
Aorticopulmonary window
Miscellaneous cardiac and vascular defects	Miscellaneous cardiac and vascular defects
Common	Common
Peritoneopericardial diapharagmatic hernia	Peritoneopericardial diapharagmatic hernia
Persistent right aortic arch	Endocardial fibroelastosis
Persistent left cranial vena cava	Uncommon
Uncommon	Persistent right aortic arch
Endocardial fibroelastosis
Pericardial defects
Anomalous pulmonary venous return
Double aortic arch
Retroesophageal left subclavian artery
Situs inversus

Diagnosis

• Generally, a congenital heart defect is suspected when a heart murmur is detected in a young dog or cat (Table 12-1).

• A tentative diagnosis can be made based on results of a complete cardiovascular physical examination, routine ECG, and radiography (see Table 12-1).

• The specific diagnosis is confirmed in most cases by echocardiography, and rarely by cardiac catheterization. Information obtained echocardiographically is also helpful in determining the severity of the defect, especially if Doppler techniques are used (measurement of blood velocities within the heart).

Table 12-1 Auscultatory Findings in Congenital Heart Disease

Therapy

• Advances since the early 1990s allow for the treatment of certain stenotic lesions by balloon valvuloplasty, a technique employing a small balloon located at the end of a cardiac catheter, and transcatheter occlusion of PDA using a Gianturco coil or similar device. Surgical correction or palliation is possible for certain defects.

• Medical treatment is primarily for the control or prevention of complications, such as congestive heart failure, arrhythmias, and endocarditis, rather than for correction of the defect. Medical therapy is discussed briefly in this chapter.

INNOCENT (FUNCTIONAL) MURMUR

Not all young dogs and cats with heart murmurs have congenital heart disease. Innocent, or functional, murmurs are created by mild turbulence within the heart and great vessels and usually diminish in intensity or resolve by 4 to 5 months of age. The following characteristics of innocent murmurs help differentiate them from pathologic murmurs.

• Innocent murmurs are systolic in timing, usually occurring early in systole and of short duration (ejection-type). They are “soft” (i.e., grade III/VI or less in intensity) and often have a low-pitched, vibrating, or musical quality.

• Innocent murmurs are usually loudest along the left sternal border and are poorly transmitted. Their intensity may vary with changes in position, with the phase of respiration, with exercise, and from day to day.

• The most important characteristic of the innocent murmur is that it is heard in the absence of any other demonstrable evidence of cardiovascular disease (e.g., lack of clinical signs or radiographic abnormalities).

• Loud systolic murmurs (grade IV/VI or greater), precordial thrills, and diastolic murmurs are indicative of cardiac disease and should prompt further diagnostics.

SPECIFIC DEFECTS

Patent Ductus Arteriosus

• In fetal circulation, the ductus arteriosus serves to shunt maternally oxygenated blood into the aorta, thereby bypassing the nonfunctional lungs. Shortly after birth, several factors contribute to effect closure of the ductus. Pulmonary vascular resistance drops dramatically, vasodilatory prostaglandin levels decrease, and oxygen tension increases, resulting in a marked increase in pulmonary blood flow and vasoconstriction of the ductus. Following closure by vasoconstriction, the ductus is permanently closed by fibrous contracture, which produces the ligamentum arteriosum. Failure of the ductus to close is termed patent ductus arteriosus (PDA) or persistent ductus arteriosus.

Pathophysiology

• The consequences of a PDA depend primarily on the diameter of the ductus and the pulmonary vascular resistance.

• When pulmonary vascular resistance is normal, blood continually shunts from the aorta (high resistance) into the pulmonary circulation (low resistance). Shunting in this fashion (systemic to pulmonary) is referred to as left-to-right and represents the most common pattern in PDA (Figure 12-1).

• When pulmonary vascular resistance increases and exceeds systemic vascular resistance, blood will shunt from the pulmonary artery into the aorta (so-called right-to-left or reversed PDA).

• Failure of ductal closure is due to an abnormal amount of elastic fibers compared to contractile smooth muscle fibers (so-called extension of the noncontractile wall structure of the aorta into the ductus arteriosus). Varying amounts of normal smooth muscle causes varying degrees of ductal closure; this creates a structure that ranges from a funnel-shaped ductus that narrows on the pulmonary artery side of the ductus (Figure 12-2) to a tubelike structure without narrowing.

• Blood flow through the ductus is also dependent upon ductal diameter. With a small ductus, the volume of blood shunted is restricted and there may be no hemodynamic effects. In cases with a large ductus and significant shunting, one of two major consequences usually occurs. In most cases, volume overloading of the left atrium and left ventricle results in eventual left-sided failure. In the largest of shunts, the excessive pulmonary blood flow induces dramatic increases in pulmonary vascular resistance, pulmonary hypertension and shunt reversal (Eisenmenger’s physiology)

Figure 12-1 Pathophysiology and genesis of clinical findings in PDA.

(Modified from Fox PR, Sisson D, Moise N, eds: Textbook of canine and feline cardiology: principles and clinical practice, ed 2, Philadelphia, 1999, WB Saunders.)

Figure 12-2 Postmortem specimen from a young German shepherd of the transverse and proximal descending aorta on top, the pulmonary artery branches ventrally, and a left-to-right shunting patent ductus arteriosus between them. The aortic end of the PDA is wide open. The pulmonary artery end is partially constricted. This results in a funnel shape to the PDA.

(From Kittleson MD, Kienle RD, eds: Small animal cardiovascular medicine, St Louis, 1998, Mosby. Courtesy Dr. Mark Rishniw.)

Diagnosis

Etiology and Breed Disposition

• PDA occurs in both the dog and cat, with higher frequency in the dog. There is also a higher frequency in the female.

• Breeds predisposed or at increased risk include Bichon Frisé, Chihuahua, Cocker Spaniel, Collie, English Springer Spaniel, German Shepherd, Keeshond, Labrador Retriever, Maltese, Newfoundland, Poodle, Pomeranian, Shetland Sheepdog, and Yorkshire Terrier.

• The defect has been shown to be inherited in the miniature poodle, transmitted as a polygenic trait.

History and Clinical Signs

• Clinical signs are related to the degree of shunting and may range from none to severe congestive heart failure.

• Depending on the degree of heart failure, owners may report signs such as cough, labored breathing, exercise intolerance, and collapse. Reports of seizures and cyanosis are suggestive of a right-to-left shunting PDA.

• Cats seldom display signs of cardiac failure until decompensation is advanced and life threatening.

Physical Examination

Diagnostic Testing

Thoracic Radiographs

• The radiographic signs of PDA (Figures 12-3 and 12-4) vary considerably with the volume of blood being shunted, the age of the animal, and the degree of cardiac decompensation.

• Dorsoventral radiographs often reveal three prominences along the left cardiac silhouette.

• The aneurysmal bulge of the aortic arch

• The enlarged pulmonary outflow tract

• The enlarged left atrial appendage

• Mild to moderate left ventricular and left atrial enlargement are usually present.

• Overcirculation of the lung field can often be visualized.

• Severe cardiomegaly, pulmonary congestion, and pulmonary edema will be present when there is congestive heart failure.

• Right ventricular enlargement and prominent tortuous pulmonary arteries are seen in animals with severe pulmonary hypertension (R to L PDA).

Figure 12-3 Patent ductus arteriosus (lateral projection) in a 3-month-old poodle. There is generalized heart enlargement, engorgement of the right cranial lobar artery (white arrows), and engorgement of the right cranial lobar vein (black arrows). Vascular engorgement, perivascular congestion, and alveolar edema are seen in the caudal lung lobes.