PATENT DUCTUS ARTERIOSUS (PDA)

In the fetus, the ductus arteriosus connects pulmonary arterial blood to bypass the unexpanded lungs and enter the descending aorta for oxygenation in the placenta. At birth, the increase in oxygen tension leads to inhibition of local prostaglandins and causes functional closure of the ductus, followed by anatomic closure during the ensuing weeks of life. If the ductus fails to close blood shunts from the descending aorta to the pulmonary artery.

Breed predispositions: Bichesare predisposed to PDA, with numerous breeds over-represented, including Poodles, German shepherd, Collies, Pomeranians and Shetland, Maltese, English Springer spaniel, Keeshond and Yorkshire terrier dogs also are predisposed to the defect.

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 will shunt from the descending aorta to the pulmonary artery because the aortic pressure exceeds that of the pulmonary artery during all phases of the cardiac cycle. This results in increased pulmonary flow and increased venous return to the left atrium and left ventricle. Volumen overloading of the left side of the heart causes left atrial dilatation and left ventricular eccentric hypertrophy and mitral insufficiency. Left-sided congestive heart failure may develop from volume overload. When pulmonary vascular resistance increases, left-to-right shunting decreases and right-to-left shunting develops.

Clinical assessment: A clinical classification has been proposed by J. Buchanan according to the types of the PDA. The most common physical examination reveals a continuous murmur often accompanied by a thrill at the craniodorsal cardiac base. The point of maximal intensity of the murmur is over the main pulmonary artery, high on the left base, and radiates cranial to the manubrium and to the right base. Frequently, a systolic murmur is evident over the left apex if mitral incompetency is present. Hyperkinetic arterial pulses, and pink mucous membranes are typical. Characteristic ECG findings include wide P waves and increased voltage Q and R waves in craniocaudal leads II, III and aVF. Thoracic radiographs indicate pulmonary overcirculation and left atrial and left ventricular enlargement and may show dilation of the main pulmonary artery and descending aorta. Doppler echocardiography can be used to verify the diagnosis, to evaluate the severity and to rule out concurrent defects. Spectral Doppler reveals a velocity across the PDA of 5 m/s. If spectral Doppler velocity is lower than 5 m/s, pulmonary hypertension or systemic hypotension should be suspected.

Clinical management: Surgical ligation of the PDA is recommended in all cases of left-to-right shunting as soon as possible after diagnosis. Without ligation over 50% or more of the cases are expected to die within a year of diagnosis. Some dogs, presumably with small shunts may live for many years. An alternative to surgical correction of the ductus is coil embolization via transcatheter delivery of thin metal coils.

VENTRICULAR SEPTAL DEFECTS (VSD)

Depending on the location, VSD are classified as: a) membranous/perimembranous, the most common type in dogs, b) outflow (infundibular/supracristal), c) inflow (atrioventricular canal), and d) muscular (trabecular). VSD can appear as isolated lesion or in association con others heart defects (i.e., PS, pulmonary atresia, truncus arteriosus and double-outlet right ventricle). Some defects can predispose to prolapse of the aortic valve into the defect.

Pathophysiology: The physiologic consequences of a VSD are determined by the size of the defect and the relative resistance in the systemic and pulmonary vascular beds. If the defect is small ("restrictive" VSD), there is little or no functional disturbance, since pulmonary blood flow is increased only minimally. In contrast, if the defect is large ("non restrictive" VSD), the ventricular systolic pressures are equal and the magnitude of flow to the pulmonary and systemic circulation is determined by the resistances of the two beds. Initially, systemic vascular resistance excess pulmonary vascular resistance, so that left-to-right shunting predominates. As pulmonary flow increases, there is increased venous return to the left atrium and left ventricle, and the left ventricular diastolic pressure can increase. Left ventricular failure is likely when the left-to-right shunt is great. Over the time, the pulmonary vascular resistance usually increases, and the magnitude of left-to-right shunting declines. If the pulmonary vascular resistance equals or exceeds the systemic resistance the shunting of blood from left to right then ceases, and right-to-left shunting begins.

Breed predispositions: English bulldog, Keeshond (genetic basis documented), English springer spaniel and Beagle are predisposed.

Clinical assessment: Most animals that have survived through the first months without clinical signs, can tolerate the defect for many years without clinical signs. A harsh, holosystolic murmur, audible maximally over the ventral right thorax, often near to the sternum is present. Murmurs of the relative pulmonic stenosis or aortic regurgitation (prolapse of the aortic valve into the defect) may be audible with VSD.

Thoracic radiographic findings in dogs with VSD are variable, but usually left atrial and ventricular dilation, pulmonary overcirculation, a dilated main pulmonary artery, and variable degrees of right ventricular enlargement are present. The ECG may show evidence of left, right or biventricular enlargement, depending on hemodynamic consequences of the shunt. Echocardiography is useful to confirm the diagnosis of a VSD and to help assess the severity of the defect. Spectral Doppler shows a high velocity jet (Vmax > 4.5 m/sec) thorough a restrictive VSD and increased peak pulmonary artery velocity owing left-to right shunt.

Clinical management: Definitive treatment of symptomatic or severe lesions requires surgery using cardiopulmonary bypass and is rarely performed. Pulmonary artery banding can be used to create supravalvular pulmonary stenosis and decrease the magnitude of left-to-right shunting in dogs with large VSD. If surgical correction or palliation is not an option, medical management of congestive heart failure may be required. Prophylactic antibiotic therapy should be considered in patients with VSD when there is known potential for hematogenous exposure to infectious organisms.

TETRALOGY OF FALLOT (TF)

TF is the most common cyanotic congenital heart defect characterised by a large ventricular septal defect, an aorta that overrides the left and right ventricles, obstruction of the right ventricular outflow tract and right ventricular hypertrophy. These anomalies are produced by a failure of the embryologic conotruncal septum to align properly.

Breed predispositions: Keeshond, bulldogs, beagles and Samoyeds are specially predisposed.

Pathophysiology: As a result of the right ventricular outflow tract obstruction and elevated right ventricular systolic pressure, desaturated blood is shunted through the ventricular septal defect to mix with blood coming from the left ventricle. Chronic hypoxemia may results in polycythemia by increasing renin production and release of erythropoietin.

Clinical assessment: Presenting signs include failure to grow, cyanosis, shortness of breath, exercise intolerance, weakness, syncope, and seizures. An ejection murmur of pulmonic stenosis is usually heard over the left cardiac base (or over the right cranial thorax) unless pulmonary atresia and/or polycythemia with hyperviscosity is present. Most dogs are cyanotic but acyanotic cases are not uncommon. Exercise or excitement may induce cyanosis by accentuating right-to-left shunting. Thoracic radiography usually shows a normal-size heart with rounding of the right ventricular border. Pulmonary undercirculation and an inconspicuous left auricle are due to decreased venous return. The ECG usually exhibits a right axis deviation. Echocardiography can be used to establish the diagnosis, as well as to assess the presence of associated abnormalities, the level and severity of the obstruction of the right ventricular outflow tract, and location and size of the ventricular septal defect. Cardiac catheterization and angiocardiography permit to confirm the diagnosis and obtain additional anatomical and hemodynamic data.

Clinical management: Definitive correction is preferred method of treatment for TF, but this procedure is rarely done in veterinary medicine because cardiopulmonary bypass is required. Surgical creation of a systemic-to-pulmonary shunt can be palliative in patients with clinical signs related to severe hypoxemia. Medical management includes exercise restriction, phlebotomy (if polycythemia exists to prevent signs related to hyperviscosity. The use of B-blockers has been reported to cause symptomatic improvement owing they reduce right-to-left shunting by modulating the heart rate and inhibiting exertion.

EISENMERGER'S SYNDROME (ES)

The ES is characterized by elevated pulmonary vascular resistance and right-to-left shunting of blood through a systemic-to-pulmonary circulation connection such as PDA, VSD, ASD, and aorticopulmonary septal defect.

Pathophysiology: With substantial left-to-right shunting, the exposure of the pulmonary vasculature to increased blood flow as well as increased pressure often results in pulmonary vascular obstructive disease, leading to increased pulmonary vascular resistance. As the pulmonary vascular resistance approaches or exceeds systemic resistance, the shunt is reversed and cyanosis appears. The right ventricle becomes hypertrophied because it must eject against a high pulmonary vascular resistance. Perfusion of the kidneys with hypoxemic blood leads to secondary polycythemia.

Clinical assessment: The altered physiology and shunt reversal usually become established shortly, before 6 months of age, but symptoms may not appear until adult. Affected patients become hypoxemic and cyanotic, especially with exertion, and eventually develop polycythemia and hyperviscosity (usually when the patients are 18 to 24 months of age). The most common auscultatory finding is an accentuated and split-second heart sound overt the left heart base. The ECG shows a right axis deviation. On thoracic radiographs the right heart may appear enlarged, the lungs appear underperfused and the main pulmonary artery and proximal lobar arteries are dilated. Echocardiography demonstrates right ventricular hypertrophy, the underlying cardiac defect and pulmonary hypertension. A peak velocity of pulmonary insufficiency of > 2.2 m/s and a peak velocity of tricuspid regurgitation of > 2.8 m/s without PS are indicative of pulmonary hypertension. Cardiac catheterization permits to assess the severity of pulmonary disease and to quantify the magnitude of intracardiac shunting.

Clinical management: management of patients with ES consists of limitation of exercise, avoidance of stress, and maintenance of the PCV between 58 and 65% per cent by periodic phlebotomy. Therapy with hydroxyurea has been reported to decrease the red blood cell volume and to improve clinical signs in dogs with right-to-left shunting PDA.

ATRIAL SEPTAL DEFECTS (ASD)

ASD are uncommon congenital heart defects in small animals. They can appear as an isolated defect or they may be part of a complex malformation. Four types of ASD have been described: 1) ostium secundum, involving the region of the fossa ovalis; 2) ostium primum or partial atrioventricular canal defect, located in the lowermost portion of the interatrial septum and associated with endocardial cushion defects; 3) sinus venosus involving the superior portion of the interatrial septum; 4) coronary sinus, located in the posteroinferior angle of the atrial septum.

Small ASD are not usually hemodynamically significant. Because the pressures between the two atria are similar, the shunt flow across an ASD does not generate murmur, but a murmur of relative pulmonic stenosis may be detected. Only right heart failure develops in severe cases.

ATRIOVENTRICULAR CANAL DEFECTS (AVC)

Complete AVC also termed complete endocardial cushion defect consists of ostium primum ASD, VSD in the inlet portion of the ventricular septum and a cleft in the anterior mitral valve leaflet and the septal leaflet of tricuspid valve. When the ventricular septum is intact, the defect is termed partial AVC or ostium primum ASD. AVC defects have been most commonly reported in cats than dogs. In patients with complete AVC all four chambers communicate with each other owing atrioventricular septal defects and insufficiency or atrioventricular valves. Congestive heart failure with or without pulmonary hypertension usually develops early.

REFERENCES

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