Atrial Septal Defect

Atrial septal defect (ASD) is a congenital cardiac malformation characterized by an abnormal communication between the left and right atria, resulting from incomplete development of the interatrial septum. This defect allows shunting of blood between the systemic and pulmonary circulations, typically left-to-right due to the higher pressure in the left atrium. The magnitude and direction of the shunt depend on the size of the defect, the relative compliance of the ventricles, and pulmonary vascular resistance. ASDs are classified anatomically into several types: ostium primum (involving the atrioventricular septum), ostium secundum (in the region of the fossa ovalis), sinus venosus (near the superior or inferior vena cava), and coronary sinus defects. In veterinary medicine, ostium secundum defects are the most commonly reported. The clinical significance ranges from asymptomatic incidental findings to severe right heart volume overload, pulmonary hypertension, and congestive heart failure. Early recognition and appropriate management are crucial to prevent long-term complications.

The etiology of atrial septal defects in dogs and cats is primarily genetic, with a polygenic mode of inheritance suspected. Specific genetic mutations have not been fully characterized in veterinary species, but familial occurrence has been documented in certain breeds, suggesting a hereditary component. In humans, mutations in genes such as NKX2-5, GATA4, and TBX5 are associated with ASD, and similar pathways may be involved in animals. Teratogenic influences during fetal cardiac development, such as exposure to certain drugs, toxins, or maternal infections, may also contribute, although these are less commonly identified in veterinary patients. No specific viral, bacterial, or parasitic etiologies are recognized for congenital ASD. The defect arises from abnormal septation of the primitive atrium during embryogenesis, which can result from failure of the septum primum to fuse with the septum secundum, excessive resorption of the septum primum, or abnormal development of the endocardial cushions (in ostium primum defects).

Atrial septal defects are relatively uncommon congenital heart defects in dogs and cats, accounting for approximately 3-5% of all congenital cardiac anomalies. They are more frequently diagnosed in dogs than cats. Certain breeds appear to be overrepresented, including Boxers, Standard Poodles, Samoyeds, Doberman Pinschers, and Old English Sheepdogs. In cats, the defect is rare but has been reported in domestic shorthair and purebred cats. There is no strong sex predilection, although some studies suggest a slight female predominance. The age at diagnosis varies; many animals are asymptomatic and the defect is detected incidentally during routine examination or screening for other conditions. Clinical signs, if they develop, typically appear in young to middle-aged adults, but severe defects may cause signs in puppies or kittens. Geographic variation is not significant, but breed popularity influences the apparent prevalence in different regions.

The pathophysiology of ASD is centered on the abnormal communication between the atria, leading to a left-to-right shunt. In normal circulation, left atrial pressure is slightly higher than right atrial pressure (approximately 8-10 mmHg vs. 3-5 mmHg). The shunt volume depends on the size of the defect and the relative compliance of the right and left ventricles. In small defects, the shunt is minimal and hemodynamically insignificant. In larger defects, significant blood volume is shunted from the left atrium to the right atrium, causing volume overload of the right heart. This leads to right atrial and right ventricular dilation, increased pulmonary blood flow, and eventually pulmonary hypertension. Chronic volume overload can result in right-sided congestive heart failure, with signs such as ascites, hepatomegaly, and pleural effusion. If pulmonary hypertension becomes severe, the shunt may reverse to right-to-left (Eisenmenger syndrome), causing cyanosis and systemic hypoxemia. The development of pulmonary hypertension is influenced by the magnitude and duration of the shunt, as well as individual susceptibility. In ostium primum defects, there is often concurrent mitral valve malformation, leading to mitral regurgitation and additional left heart volume overload.

Predisposing factors for atrial septal defects include genetic predisposition, with certain breeds having a higher incidence. A family history of congenital heart disease increases the risk. Maternal factors during pregnancy, such as exposure to teratogenic drugs, toxins, or infections, may contribute, though these are rarely identified in veterinary cases. Concurrent congenital anomalies, such as mitral valve dysplasia or pulmonic stenosis, can be present and may influence the clinical presentation. Age is a factor in the development of clinical signs; younger animals with large defects may show signs earlier, while older animals may develop complications such as pulmonary hypertension or arrhythmias. Sex may play a minor role, with some studies suggesting a slight female predominance. Environmental factors are not well-defined, but poor maternal nutrition or health during gestation could theoretically affect fetal cardiac development.

Clinical signs of atrial septal defect vary widely depending on the size of the defect and the presence of concurrent cardiac abnormalities. Many animals with small to moderate defects are asymptomatic and the defect is discovered incidentally. When signs occur, they typically include exercise intolerance, lethargy, and syncope. In cases of significant left-to-right shunting, signs of right-sided congestive heart failure may develop, including ascites, hepatomegaly, jugular venous distension, and pleural effusion. Coughing may occur due to pulmonary overcirculation or concurrent respiratory disease. In animals with severe pulmonary hypertension and right-to-left shunting, cyanosis, polycythemia, and signs of hypoxemia (e.g., weakness, collapse) may be observed. On physical examination, a systolic heart murmur is often auscultated, typically a soft to moderate intensity murmur best heard over the left heart base, due to increased flow across the pulmonary valve. A fixed split S2 may be present. In ostium primum defects, a holosystolic murmur of mitral regurgitation may be heard at the apex. Arrhythmias, particularly atrial fibrillation, may develop in animals with severe atrial enlargement.

Differential diagnoses for atrial septal defect include other congenital cardiac shunts and conditions that cause similar clinical signs or murmurs. These include: 1) Ventricular septal defect (VSD) – a murmur is typically loud, holosystolic, and best heard over the right sternal border; echocardiography reveals a defect in the interventricular septum. 2) Patent ductus arteriosus (PDA) – a continuous murmur is heard, and echocardiography shows a patent ductus with characteristic Doppler flow. 3) Pulmonic stenosis – a systolic ejection murmur is heard over the left heart base, and echocardiography shows thickening and fusion of the pulmonary valve leaflets with post-stenotic dilation. 4) Mitral regurgitation due to myxomatous mitral valve disease – a holosystolic murmur at the left apex, with echocardiographic evidence of mitral valve thickening and regurgitation. 5) Tricuspid regurgitation – a holosystolic murmur at the right apex, with echocardiographic evidence of tricuspid valve abnormalities. 6) Endocarditis – may cause murmurs and signs of systemic illness, with echocardiographic evidence of vegetative lesions. 7) Cardiomyopathy – dilated or hypertrophic cardiomyopathy can cause murmurs and signs of heart failure, but echocardiography shows characteristic myocardial changes. 8) Heartworm disease – can cause right heart enlargement and murmurs, but is diagnosed via antigen testing and echocardiography. 9) Pulmonary hypertension – can cause right heart failure and murmurs, but is often secondary to other conditions; echocardiography shows elevated pulmonary artery pressures. 10) Anemia or fever – can cause functional murmurs, but these are typically soft and resolve with treatment of the underlying cause.

The diagnostic algorithm for atrial septal defect begins with a thorough history and physical examination. If a heart murmur or clinical signs suggestive of cardiac disease are present, thoracic radiographs should be obtained. Radiographic findings may include cardiomegaly, particularly right atrial and right ventricular enlargement, and increased pulmonary vascularity. If radiographs are suggestive, echocardiography is the gold standard for diagnosis. A complete echocardiographic examination, including two-dimensional, M-mode, and Doppler studies, should be performed. Two-dimensional echocardiography can directly visualize the defect in the interatrial septum, and color-flow Doppler can demonstrate the shunt. Pulsed-wave Doppler can assess the velocity and direction of flow. In cases where echocardiography is inconclusive or for further characterization, transesophageal echocardiography (TEE) may be used. Cardiac catheterization is rarely needed but may be performed to measure pressures and calculate shunt fraction if surgical or interventional closure is planned. Electrocardiography (ECG) may show evidence of right atrial enlargement (P pulmonale) or right ventricular enlargement, and may detect arrhythmias. Blood tests, including complete blood count, serum biochemistry, and NT-proBNP, may be helpful to assess the severity of heart failure and rule out other causes.

Laboratory findings in atrial septal defect are often unremarkable in asymptomatic animals. In animals with congestive heart failure, serum biochemistry may show elevated liver enzymes (ALT, AST, ALP) due to hepatic congestion, and elevated BUN and creatinine due to decreased renal perfusion. NT-proBNP (N-terminal pro-brain natriuretic peptide) may be elevated in animals with cardiac disease and can help differentiate cardiac from non-cardiac causes of respiratory signs. In animals with right-to-left shunting and cyanosis, polycythemia may be present on complete blood count, with increased hematocrit and hemoglobin. Arterial blood gas analysis may reveal hypoxemia. In animals with pulmonary hypertension, there may be evidence of secondary polycythemia. Urinalysis is typically normal. Specific biomarkers such as troponin I may be elevated if there is myocardial damage, but this is not specific for ASD.

Thoracic radiography in animals with atrial septal defect may show generalized cardiomegaly, with prominence of the right atrium and right ventricle. The main pulmonary artery segment may be enlarged, and pulmonary vascularity may be increased due to increased pulmonary blood flow. In animals with congestive heart failure, signs of right-sided failure may be present, including pleural effusion, ascites, and hepatomegaly. Echocardiography is the definitive imaging modality. Two-dimensional echocardiography from the right parasternal long-axis and short-axis views can visualize the defect in the interatrial septum. The defect appears as an echo dropout in the septum. Color-flow Doppler demonstrates left-to-right shunting across the defect. Pulsed-wave Doppler can measure the flow velocity. In ostium primum defects, the defect is located in the lower portion of the septum, and there may be associated mitral valve abnormalities. Sinus venosus defects are located near the entrance of the vena cava. Transesophageal echocardiography (TEE) provides better visualization of the atrial septum and is useful in cases where transthoracic imaging is suboptimal. Cardiac catheterization with angiography can confirm the defect and measure pressures, but is rarely needed. Advanced imaging such as CT or MRI may be used for research or complex cases, but is not routinely indicated.

Cytology and histopathology are not typically used in the diagnosis of atrial septal defect. However, if a biopsy of the atrial septum is obtained during surgical repair, histopathology would show the anatomical defect with a lack of normal septal tissue. In cases of congestive heart failure, histopathology of the lungs may show pulmonary vascular changes, including medial hypertrophy of pulmonary arteries, intimal fibrosis, and plexiform lesions in severe pulmonary hypertension. Liver biopsy may show centrilobular congestion and fibrosis in chronic right-sided heart failure. These findings are not specific to ASD but reflect the hemodynamic consequences.

Treatment of atrial septal defect depends on the size of the defect, the presence of clinical signs, and the development of complications. Small defects with no hemodynamic significance may not require treatment, but regular monitoring is recommended. For moderate to large defects with evidence of right heart volume overload or clinical signs, closure of the defect is indicated. Closure can be achieved surgically via open-heart surgery with cardiopulmonary bypass, or minimally invasively using transcatheter occlusion devices. Surgical closure is the standard of care but requires specialized equipment and expertise. Transcatheter closure using an Amplatzer septal occluder is an alternative in selected cases. Medical management is aimed at controlling congestive heart failure if present. This includes the use of diuretics such as furosemide (2-4 mg/kg IV, IM, SC, or PO q8-12h), ACE inhibitors such as enalapril (0.5 mg/kg PO q12-24h) or benazepril (0.25-0.5 mg/kg PO q24h), and pimobendan (0.25-0.3 mg/kg PO q12h) in cases of right-sided heart failure. In animals with pulmonary hypertension, sildenafil (1-2 mg/kg PO q8-12h) may be used. In cases of right-to-left shunting and polycythemia, phlebotomy may be necessary to reduce hematocrit, and oxygen therapy may be indicated. Antiarrhythmic drugs may be needed for atrial fibrillation, such as diltiazem (0.5-1.5 mg/kg PO q8h) or digoxin (0.005-0.01 mg/kg PO q12h).

The prognosis for atrial septal defect is generally good for small defects that do not cause hemodynamic compromise. Many animals with small ASDs live normal lives without treatment. For moderate to large defects, the prognosis is more guarded, especially if left untreated. The development of pulmonary hypertension and right-to-left shunting significantly worsens the prognosis. With successful closure of the defect, the prognosis is excellent, and most animals return to normal activity. However, if the defect is associated with other congenital anomalies, such as mitral valve dysplasia, the prognosis depends on the severity of the concurrent lesion. The long-term prognosis is also influenced by the development of arrhythmias, particularly atrial fibrillation, which can be managed but may affect survival. Overall, with appropriate management, many animals with ASD can have a good quality of life for several years.

Follow-up for animals with atrial septal defect depends on the severity and treatment. For animals with small defects that are not treated, re-evaluation every 6-12 months is recommended, including physical examination, thoracic radiographs, and echocardiography to monitor for changes in defect size or the development of pulmonary hypertension. For animals that have undergone surgical or transcatheter closure, follow-up is typically more frequent initially, with re-evaluation at 1, 3, 6, and 12 months post-procedure, then annually. Echocardiography is used to assess the position of the occluder device, residual shunting, and right heart remodeling. For animals on medical therapy for heart failure, regular monitoring of clinical signs, body weight, renal function, and electrolytes is necessary. Serial NT-proBNP measurements may be helpful to assess response to therapy. In animals with pulmonary hypertension, periodic echocardiographic assessment of pulmonary artery pressure is recommended.

Pearls: 1) A soft systolic murmur at the left heart base with a fixed split S2 is classic for ASD, but the murmur may be absent in small defects. 2) Echocardiography is the gold standard for diagnosis; always perform a thorough examination of the interatrial septum from multiple views. 3) In ostium primum defects, look for concurrent mitral valve abnormalities. 4) In animals with right-to-left shunting, avoid the use of ACE inhibitors and diuretics as they may worsen the shunt. 5) Transcatheter closure is a viable option for selected cases and avoids the risks of open-heart surgery. Pitfalls: 1) Failing to recognize that a murmur may be absent in small defects, leading to delayed diagnosis. 2) Misinterpreting the murmur as pulmonic stenosis, leading to unnecessary diagnostic testing. 3) Overlooking the presence of pulmonary hypertension, which can change the management and prognosis. 4) In animals with right-to-left shunting, administering drugs that reduce systemic vascular resistance can increase the right-to-left shunt and worsen cyanosis. 5) Not considering ASD as a cause of right-sided heart failure in young animals.

For congestive heart failure secondary to ASD: Furosemide: 2-4 mg/kg IV, IM, SC, or PO q8-12h; adjust dose based on clinical response and renal function. Enalapril: 0.5 mg/kg PO q12-24h; start at lower dose and titrate. Benazepril: 0.25-0.5 mg/kg PO q24h. Pimobendan: 0.25-0.3 mg/kg PO q12h; may be used in right-sided heart failure. Spironolactone: 1-2 mg/kg PO q12h; may be added as an adjunct diuretic. For pulmonary hypertension: Sildenafil: 1-2 mg/kg PO q8-12h; start at lower dose and titrate. For atrial fibrillation: Diltiazem: 0.5-1.5 mg/kg PO q8h; or Digoxin: 0.005-0.01 mg/kg PO q12h (monitor serum levels). For polycythemia secondary to right-to-left shunting: Phlebotomy to maintain hematocrit below 65%, and consider hydroxyurea (50 mg/kg PO q48h) if frequent phlebotomies are needed. Always adjust dosages for renal or hepatic impairment and monitor for drug interactions.

Evidence-based literature on atrial septal defect in veterinary medicine is limited, but several studies have reported on the clinical features, diagnosis, and treatment. A retrospective study by Chetboul et al. (2006) described the echocardiographic findings and outcome in dogs with ASD, noting that ostium secundum defects were most common and that surgical closure was associated with a good prognosis. Another study by Saunders et al. (2015) evaluated transcatheter closure of ASD in dogs using the Amplatzer device, reporting successful closure with minimal complications. Consensus guidelines from the ACVIM on the diagnosis and treatment of canine congenital heart disease (Boon et al., 2016) recommend echocardiography as the primary diagnostic tool and suggest that closure is indicated for defects with significant shunting. In cats, ASD is rare, but a case series by MacDonald et al. (2009) reported similar findings. The use of NT-proBNP as a biomarker for heart failure in dogs with congenital heart disease has been supported by studies such as that by Oyama et al. (2008). Overall, the literature supports the use of echocardiography for diagnosis and the consideration of surgical or interventional closure for clinically significant defects.