Left-to-right shunts include patent ductus arteriosus, ventricular septal defects, and atrial septal defects.
Patent Ductus Arteriosus in Animals
Patent ductus arteriosus (PDA) is a common congenital defect of dogs and occurs less commonly in cats. Persistent postnatal flow through the ductus leads to excess blood flow (volume overload) to the pulmonary circulation and left heart chambers, most commonly leading to left-sided congestive heart failure (CHF) within the first 1–2 years of life.
PDA occurs most commonly in small-breed dogs, although breed predispositions also exist for some medium- and large-breed dogs. PDA has a strong sex (female) predilection.
Reverse (right-to-left) PDA is very uncommon and occurs when concurrent severe pulmonary hypertension leads to reversal of PDA flow and differential cyanosis. Other cyanotic heart diseases, including truncus arteriosus, ventricular septal defect with pulmonary atresia, double-outlet right ventricle, and D-transposition of the great arteries, are rare in dogs and cats.
Pathophysiology of Patent Ductus Arteriosus
Persistence of a normal fetal structure (ductus arteriosus) after birth allows for shunting of blood from the descending aorta to the pulmonary artery, leading to volume overload and, subsequently, left-sided heart failure. A continuous murmur heard at the left base and bounding pulses are classic changes on physical examination.
In fetal life, blood entering the right heart largely bypasses the uninflated lungs through either the foramen ovale or the ductus arteriosus. The ductus arteriosus effectively shunts blood from the pulmonary artery into the descending aorta (right-to-left shunt). At birth, several factors mediate closure of the ductus to separate systemic and pulmonary circulations. Inflation of the lungs and remodeling of the fetal pulmonary vasculature allow the pulmonary circulation to change from a high-pressure, high-resistance system to a low-pressure, low-resistance system. Closure of the ductus, which becomes the ligamentum arteriosum, occurs shortly after birth.
Persistence or patency of the ductus with an otherwise normal systemic and pulmonary circulatory system results in substantial shunting of blood from the descending aorta to the pulmonary artery (left to right). Because systolic and diastolic aortic pressures normally exceed pulmonary artery pressures, shunting is continuous throughout the cardiac cycle. The result is a continuous murmur and volume overload of the pulmonary arteries and veins, left atrium, and left ventricle.
Left atrial and left ventricular dilatation may result in cardiac arrhythmias.
Diastolic flow (also called diastolic runoff) through the ductus leads to a decrease in diastolic and mean systemic blood pressures. The widened difference between systolic and diastolic pressures creates an increased pulse pressure and bounding femoral pulses. See patent ductus arteriosus image.
Courtesy of Dr. Gheorghe Constantinescu.
Chronic volume overload and dilatation of the left cardiac chambers usually result in development of left-sided CHF in most untreated cases within the first 1–2 years of life. Animals with a small ductus and minimal shunting may reach adulthood, although most affected dogs have evidence of volume overload even at a young age. In some animals with a large PDA, increased pulmonary blood flow may induce pulmonary vasoconstriction and development of pulmonary hypertension.
In cases of severe pulmonary hypertension in which pulmonary pressures exceed systemic pressures, blood flow through the ductus can reverse and result in right-to-left shunting. This reverse PDA causes the disappearance of the classic machinery murmur and bounding pulses.
Delivery of oxygenated blood to the head and neck results in pink mucous membranes cranially, while delivery of hypoxemic blood caudally results in cyanotic mucous membranes caudally (vulva and prepuce). Differential cyanosis is a characteristic examination finding in animals with reverse PDA.
In addition, perfusion of the kidneys with deoxygenated blood causes excessive release of erythropoietin and subsequent polycythemia. The finding of polycythemia in a young animal with compatible clinical signs should prompt further diagnostic evaluation of the heart.
Clinical Findings of Patent Ductus Arteriosus
In animals with a left-to-right patent ductus arteriosus, a prominent, continuous, machinery-like murmur is present. The murmur is usually loudest at the left base of the heart and is often associated with a precordial thrill.
In some cases, the ductus remains open for several days after birth, so a continuous murmur may be detected during examination of the neonate.
Femoral pulses are typically bounding.
Most young animals do not demonstrate clinical signs. Those with a large shunt and older animals often have signs of left-sided CHF.
Animals with a reverse PDA usually have a history of lethargy, exercise intolerance, and hindlimb collapse that relates to severe pulmonary hypertension and venous admixture. Careful examination may reveal differential cyanosis. Cardiac auscultation may reveal a right-sided systolic murmur (tricuspid regurgitation), split S2, and a soft diastolic murmur (pulmonic insufficiency). A continuous murmur is not present, and femoral pulses are not bounding.
Diagnosis of Patent Ductus Arteriosus
Echocardiography
Agitated saline contrast (bubble) study if reverse PDA is suspected
Diagnosis is typically made by echocardiography, which is valuable in excluding concurrent congenital cardiac defects as well as documenting abnormal blood flow typical of PDA. Continuous turbulence in the main pulmonary artery is characteristic of a left-to-right shunting PDA. Left ventricular and left atrial dilatation are typically noted, and mild mitral regurgitation may be present secondary to annular dilation.
A bubble study—contrast echocardiography using agitated saline solution (0.9% NaCl)—helps to confirm the diagnosis of reverse PDA. After the injection of agitated saline solution into a peripheral vein, hyperechoic microbubbles can be observed within the abdominal aorta. Young animals with echocardiographic evidence of pulmonary hypertension (right ventricular dilation and hypertrophy, septal flattening, pulmonary artery dilation) should be evaluated for reverse PDA with a bubble study.
Electrocardiography may demonstrate left ventricular enlargement and possible arrhythmias. Electrocardiography frequently demonstrates tall R waves in lead II, indicative of left ventricular enlargement. A spectrum of cardiac arrhythmias may also be observed, including both atrial and ventricular premature complexes.
Thoracic radiographs can help support a tentative diagnosis of PDA. Left ventricular enlargement, left atrial enlargement, and an aortic ductus diverticulum (ductal bump) may be noted on radiographs, along with evidence of pulmonary overcirculation or possible presence of pulmonary edema. The severity of radiographic changes will likely depend on the size, shape, and duration of the ductus.
Treatment of Patent Ductus Arteriosus
Occlusion of the patent ductus arteriosus by device occlusion or surgical ligation
Treatment of CHF (pulmonary edema) if present
Because of the high risk of CHF early in life for animals with a PDA, timely closure is generally recommended. There are 2 major treatment options for PDA closure—interventional transvenous occlusion and surgical ligation:
Catheter-based transvenous occlusion is minimally invasive and involves placement of an occlusion device through a peripheral vessel (most commonly the femoral artery). The Amplatz canine ductal occluder (ACDO) is used most often and is highly successful with minimal complications (see ACDO image). Transcatheter coil closure and vascular plug placement have also been described. The major limitations to a transcatheter approach are ductal size and patient size.
Surgical ligation of the ductus is highly successful and is considered most often for very small dogs and cats.
If present, CHF should be medically managed before anesthesia and surgery are performed.
Courtesy of Dr. Sandra Tou.
Occlusion or ligation of the ductus is contraindicated with reverse PDA due to the risk of worsening pulmonary hypertension and right-sided heart failure. Treatment in these cases focuses on medical management of pulmonary hypertension and control of polycythemia through periodic phlebotomy or use of hydroxyurea. Closure can be considered if treatment of pulmonary hypertension leads to flow reversal (left-to-right flow).
Prompt diagnosis and treatment (either by interventional device occlusion or surgical ligation) before the development of clinical signs is associated with an excellent outcome.
Long-term prognosis for reverse PDA is guarded to poor.
Key Points
Patent ductus arteriosus (PDA) is a common congenital disease in dogs and also occurs in cats.
A loud, continuous, left basilar heart murmur and bounding femoral pulses are typically found on physical examination. Many affected cats have only a systolic murmur.
Left-sided heart failure (pulmonary edema) is the most common sequela of PDA, commonly occurring within the first year of life.
Treatment (interventional device occlusion or surgical ligation) before the onset of clinical signs is associated with an excellent prognosis.
Septal Defects in Animals
Ventricular Septal Defects
Ventricular septal defects (VSDs) are common congenital defects of cats and also occur in dogs and cows. This defect is heritable in miniature swine.
VSDs are most commonly located in the perimembranous portion of the septum, high in the ventricular septum immediately beneath the right aortic valve cusp on the left and just below the tricuspid valve commissure on the right. They vary in size and hemodynamic significance. Doubly committed juxta-arterial septal defects (located beneath the aortic valve on the left and beneath the pulmonic valve on the right) and muscular septal defects (located at any site along the muscular septum) are uncommon.
VSDs may be present with other congenital cardiac anomalies.
Pathophysiology of Ventricular Septal Defects
Shunting of blood from the left ventricle into the right ventricle or right ventricular outflow tract occurs in the direction of the pressure gradient between the two ventricles. The magnitude of the shunt depends on the size of the defect, the ratio of pulmonary to systemic vascular resistance, and the relative compliance of the two ventricles. Blood shunted into the right ventricle is recirculated through the pulmonary vessels and left cardiac chambers, which causes dilatation of these structures. The right ventricle may dilate as well, especially in animals with large VSDs (which are rare). Small defects limit the volume of shunted blood and minimize hemodynamic effects, whereas large defects usually result in severe circulatory derangements and clinical signs.
Most VSDs are restrictive, in which the degree of shunting is small, thereby allowing the left and right ventricles to maintain normal pressures and a normal pressure gradient. Substantial shunting through the pulmonary arteries can induce vasoconstriction and pulmonary hypertension. As resistance rises, the shunt may reverse and result in right-to-left shunting of blood, cyanosis, and polycythemia. The reversal of shunt flow (right-to-left) through a septal defect as a consequence of pulmonary hypertension is referred to as Eisenmenger syndrome.
Clinical Findings of Ventricular Septal Defects
Clinical findings depend on the severity of the defect and the shunt direction. Defects can vary in location and size; most are small enough to restrict extensive shunting between ventricles, resulting in a high-pressure gradient and a loud-intensity (typically grade III/VI or higher), right-sided murmur.
A small, restrictive defect usually causes minimal or no signs. Larger defects may result in left-sided CHF.
Cattle are prone to developing signs of right-sided failure.
The development of Eisenmenger syndrome (right-to-left shunt flow through a septal defect as a consequence of pulmonary hypertension) is indicated by cyanosis, fatigue, and exercise intolerance.
Most animals with a restrictive VSD have a loud holosystolic murmur heard best over the right thorax that may be accompanied by a palpable thrill. This murmur is often absent or faint if shunting is minimal (due to equalization of ventricular pressures with large defects) or if shunting is right to left.
On occasion, aortic valvular insufficiency develops secondarily because the defect may disrupt aortic valve apposition. In these cases, a concurrent diastolic murmur may be present, and the combination systolic/diastolic murmur (“to-and-fro” murmur) may be mistaken for that of a PDA.
Diagnosis of Ventricular Septal Defects
Diagnosis of VSD can be made with echocardiography, although small defects may be missed (see VSD image). Doppler echocardiography can often confirm the presence and direction of shunting and assess for VSD complications.
Thoracic radiographs can demonstrate generalized cardiomegaly with overcirculation of the pulmonary vessels.
Courtesy of Dr. Sandra Tou.
Treatment of Ventricular Septal Defects
Most cases of restrictive VSD do not require treatment and have a good prognosis.
Treatment depends on use of the animal, severity of clinical signs, and direction of the shunt.
Animals with small VSDs do not typically require therapy and have a good long-term prognosis if substantial aortic insufficiency is not present. However, animals with a moderate to severe VSD and secondary volume overload to the left heart more commonly develop clinical signs, and treatment should be considered.
Surgical closure of the defect requires cardiopulmonary bypass and is often limited by expense and availability. Percutaneous transcatheter closure can be considered for muscular septal defects and involves placement of an occluder device but is not typically considered for perimembranous defects due to proximity to the aortic valve. Pulmonary artery banding to increase right ventricular outflow tract resistance, and thus decrease left-to-right shunting, can be considered.
Medically, the use of drugs to decrease systemic vascular resistance (eg, vasodilators) may help to decrease the degree of left-to-right shunting.
With right-to-left shunting, surgical closure of the defect is generally contraindicated.
Phlebotomy to relieve the effects of polycythemia or use of hydroxyurea to relieve clinical signs may be considered; however, the prognosis is poor to guarded.
Animals with a VSD should not be bred. The defect has been demonstrated to be heritable in at least one breed (English Springer Spaniels).
Key Points
Ventricular septal defect (VSD) is a common congenital disease in cats and also occurs in dogs. VSD occurs due to incomplete formation of the interventricular septum.
VSD may cause a loud systolic murmur (grade 3/6 or higher), heard best over the right thorax.
Long-term survival is common with restrictive VSD.
Atrial Septal Defects
Atrial septal defects (ASDs) are uncommon congenital defects of dogs and cats. The midseptal location (secundum-type) is most common.
A communication between the atria may be the result of a patent foramen ovale or a true ASD. During fetal life, the foramen ovale, a flapped oval opening of the interatrial septum, allows shunting of blood from the right atrium to the left atrium to bypass the uninflated lungs. This flapped oval opening develops between two septa that later fuse to make up the interatrial septum: the septum primum and septum secundum. At birth, the drop in right atrial pressure and rise in left atrial pressure cause the foramen ovale to close and shunting to cease. Increased right atrial pressure may prevent closure of the foramen ovale and lead to persistent shunting through a patent foramen ovale. This does not represent a true ASD because the septa have formed normally.
A true ASD is an opening or hole within the interatrial septum, with shunting occurring in the direction of the pressure gradient between the two atria. Septum secundum defects occur high in the interatrial septum, near the foramen ovale, and are the most common type. Septum primum defects are located in the most apical portion of the interatrial septum, near the atrioventricular (AV) junction, and are often a component of AV septal (AV canal) defects. Sinus venosus defects are most often located at the junction of the right atrium and cranial vena cava. Coronary sinus defects occur when the wall between the coronary sinus and left atrium fails to form properly.
Pathophysiology of Atrial Septal Defects
In most cases of ASD, blood shunts from the left atrium to the right atrium, causing a volume overload of the right-sided chambers. The magnitude of shunting depends on the size of the defect, the ratio of pulmonary to systemic vascular resistance, and the relative compliance of the two ventricles. Excessive blood flow through the right-sided chambers results in their dilation and hypertrophy. Pulmonary vasoconstriction and development of pulmonary hypertension may occur as a consequence of excessive pulmonary blood flow and may precipitate right-sided CHF.
In conditions associated with high right atrial pressure (eg, pulmonic stenosis), reverse shunting may occur (from right to left) across a patent foramen ovale or ASD, potentially resulting in cyanosis and polycythemia.
Clinical Findings of Atrial Septal Defects
With an ASD, signs of right-sided heart failure (eg, ascites, jugular venous distention) may be present. A soft ejection-type systolic murmur is usually present over the pulmonic valve area, reflecting increased blood flow through the pulmonic valve. Blood flow through the defect itself does not produce a murmur due to the low velocity of shunt flow. Prolonged ejection time of the right ventricle may result in a split second heart sound (S2).
Diagnosis of Atrial Septal Defects
Diagnosis is achieved by echocardiography. Echocardiography demonstrates varying degrees of right atrial and right ventricular dilatation, as well as identification of the defect as a loss of echogenicity at the interatrial septum (see ASD image). The normal loss of echogenicity of the fossa ovale (artifactual dropout) should not be interpreted as an ASD. Doppler evaluation confirms shunting through the defect and increased ejection velocities across the pulmonic valve. Echocardiography is also important to assess the need for and feasibility of treatment.
Electrocardiography may reveal evidence of right ventricular or right atrial enlargement (right axis shift, deep S waves, tall P waves in lead II). Right bundle-branch block and arrhythmias can also be noted.
Radiography shows variable degrees of right ventricular enlargement and prominence of the pulmonary vessels indicating pulmonary overcirculation.
Courtesy of Dr. Sandra Tou.
Treatment of Atrial Septal Defects
Open-heart surgery
Transcatheter occlusion
Animals with small or medium-sized septum secundum defects can tolerate the defects well, and many of these defects are noted as an incidental finding in older animals.
Small defects may be hemodynamically insignificant and require no treatment. Large defects may result in substantial interatrial shunting, leading to volume overload of the right heart chambers and pulmonary vasculature.
Larger defects, such as noted with septum primum defects, are more likely to cause right-sided CHF. The prognosis is guarded to poor in these cases, although many dogs show no clinical signs for years.
For dogs with large defects and substantial volume overload, surgical correction or interventional occlusion may be possible. Surgical closure of the defect requires cardiopulmonary bypass and is often limited by expense and availability. Transcatheter closure using a septal occluder has been reported for secundum defects.
Key Points
Atrial septal defect (ASD) is an uncommon congenital heart defect caused by malformation of the interatrial septum, typically resulting in left-to-right shunting across the defect. The most common location is the septum secundum (secundum ASD).
ASD is commonly diagnosed incidentally during echocardiography for concurrent heart disease. Large ASDs with substantial shunting may lead to right heart overload, right-sided heart failure, and acquired pulmonary hypertension.
Some types of ASD can be closed either surgically or by interventional occlusion.
