Introduction

• Cause: 6.2-14% of canine congenital heart defects; hereditary (polygenic) in Keeshond Keeshond.
• Approximately half have a concurrent defect: including Tetralogy of Fallot Heart: tetralogy of Fallot, atrial septal defect Atrial septal defect ; patent ductus arteriosus Patent ductus arteriosus ; aortic stenosis Heart: aortic stenosis ; pulmonic stenosis Pulmonic stenosis.
• May decrease in size, even close, by 1-2 years old but this is rare.
• Signs: stunted growth, congestive heart failure.
• Diagnosis: ultrasonography.
• Treatment: symptomatic.
• Prognosis: depends on severity good → poor.
  Print off the owner factsheet Congenital heart disease - ventricular septal defect (VSD) to give to your client.

Pathogenesis

Etiology

• Embryological - may be inherited - failure of closure of interventricular septum.
• Most commonly defect is in membranous portion of ventricular septum (upper region); under septal leaflet of tricuspid valve; and in left ventricular outflow tract below aortic valve. Rarely defect is in lower, muscular region of septum.

Predisposing factors

• Heredity; ?breed lines.

Specific

• Polygenic in Keeshond Keeshond.
• Autosomal dominant with incomplete penetrance or polygenic in English Springer Spaniel.
• Autosomal recessive in Beagle.

Pathophysiology

• Embryological failure of interventricular septum, leaving conduit between left ventricle and right ventricle → left ventricular pressure exceeds normal right ventricular pressure in systole → systolic left-to-right shunting occurs across the defect.
• Large defects in lower part of septum (less common) → volume overload of right ventricle, pulmonary circulation, left atrium and left ventricle.
• Defects higher in septum (more common) → discharge into the pulmonary artery → overloaded pulmonary circulation, left atrium and left ventricle.
• Majority of shunting occurs in systole when blood is leaving right heart out pulmonary artery → increased blood volume out pulmonary artery → volume overloaded pulmonary circulation → increased pulmonary venous return to left atrium (dilates) and left ventricle (dilation and eccentric hypertrophy), volume overloaded left heart → eventually left-sided congestive heart failure Heart: congestive heart failure.
• Magnitude of flow through VSD depends on size of defect and systemic and pulmonary vascular resistance.
• Small defects, where the small orifice is 'restricting' left-to-right shunting, are known as restrictive VSDs. Restrictive VSD prevents transmission of left ventricular pressures to right ventricle, so pressure gradient between right and left ventricle remains high.
• Higher pressure gradient between right and left ventricle with smaller defects mean smaller defects have louder murmurs; degree of murmur not related to severity of disease.
• Large defects are associated with loss of the pressure gradient between the left and right ventricles and shunt from left to right may even be laminar (so low grade murmur). Large defects may have diastolic bidirectional shunting, even with systolic left-to-right shunting. Therefore, can have continuous murmur.
• The aortic valve may prolapse into the VSD, resulting in aortic regurgitation during diastole which can contribute to left ventricular overload.

• Small VSDs carry a good prognosis, and may close completely due to scar tissue forming across the defect, or a portion of the septal leaflet of the tricuspid valve may seal the defect.
• Right ventricular dilation can occur with large VSDs that allow significant flow left to right during diastole. Right ventricular volulme overload may also occur with more apically located VSD.
• If the left-to-right shunt is severe, pulmonary hypertension Pulmonary Arterial Hypertension (PHT) can uncommonly develop (4% of cases). This can cause an increase in right heart pressures leading to right ventricular hypertrophy and right-to-left shunting. This may manifest as cyanosis, dyspnea, and polycythemia. This is called 'Eisenmenger's Syndrome'.

Timecourse

• From birth.
• Progression or development of clinical signs depends on size of defect and presence of concurrent defects.
• If large defect, clinical signs may be seen <2 years of age.

Diagnosis

Treatment

Outcomes