Congenital Heart Disease in Animals

Congenital Heart Disease

• Congenital heart disease (CHD) is a defect in the heart's structure present at birth.
• CHD can be inherited or develop spontaneously.
• Risk factors for CHD include genetic mutations, toxins, infections, environmental factors, drug-related factors, nutritional factors, and are often developmental anomalies.
• CHD is a common cause of heart disease in young animals.
• CHD prevalence in dogs and cats ranges from 0.5-0.9% for dogs and 0.05% for cats.
• CHD prevalence is <10% of all diagnosed heart disease in dogs and cats.
• CHD affects large animals, including horses (0.7-0.8%) and cattle (<0.2%).
• CHD prevalence is 1.5% for alpacas and 3.6% for referral hospitals.

Learning Objectives

• Identify clinical signs suggestive of CHD, including physical examination findings (breed/family history, history/clinical signs, physical findings).
• Differentiate between innocent and pathological heart murmurs in young animals; determining if a heart murmur is associated with structural heart disease.
• Compare and contrast fetal and adult circulatory systems.
• List major cardiovascular changes in the immediate postnatal period.
• List common CHD types in dogs, cats, horses, cattle, and camelids.
• Describe pathophysiology, clinical findings, and treatment strategies for specific CHDs (e.g., PDA, VSD, Tetralogy of Fallot).

Definition

• CHD is a defect in the cardiac structure present at birth.
• May be inherited; may develop spontaneously.
• Genetic mutations, toxins, infections, environmental factors, drug-related factors, nutritional factors can play a role, and are often developmental anomalies.

Prevalence

• Dogs and cats represent approximately 0.5-0.9% (dogs) and 0.05% (cats) of referral hospitals, with CHD accounting for less than 10% of all diagnosed heart disease.
• Large animals affected are approximately 0.7-0.8% of horses and <0.2% of cattle presented for necropsy.
• Prevalence is 1.5% in alpacas and 3.6% in referral hospitals.

When To Suspect CHD

• Breed/family history of CHD
• History/clinical signs: Failure to thrive, poor exercise tolerance, syncope, difficulty breathing, abdominal distension, and sudden death
• Suggestive physical examination findings: Heart murmur, arrhythmia, cyanotic mucous membranes, abnormal jugular venous or peripheral arterial pulses.

Heart Murmurs In Young Animals

• Innocent/Physiological: Softer or absent at rest, increases with excitement/exercise, soft murmur grade 1–2/6, single, no other associated abnormal heart sounds
• Pathological: Can be heard at all times, loud murmur grade 3/6 or louder, associated clinical signs or physical exam findings like cyanosis, jugular venous distention/pulsation, exercise intolerance, difficult breathing, ascites, murmur can be systolic, diastolic, or continuous; persists beyond 4–6 months of age.

Review: Normal Fetal Circulation

• Ductus venosus: Connects umbilical vein to caudal vena cava.
• Foramen ovale: Small opening in septum between right and left atria
• Ductus arteriosus: Connects pulmonary artery to descending aorta.

Review: Normal Postnatal Cardiovascular Changes

• Pulmonary circulation resistance decreases as lungs inflate.
• Systemic circulation resistance increases due to loss of placental circulation.
• Three fetal shunts (ductus venosus, foramen ovale, and ductus arteriosus) close postnatally due to changes in vascular pressure, oxygen tension, and loss of placental prostaglandins.

Things To Remember

• Blood flow is typically high pressure to low pressure.
• The heart responds to stress through concentric hypertrophy (wall thickening) responding to increased end-systolic wall stress or eccentric hypertrophy (chamber dilation) responding to increased end-diastolic wall stress.

Classification Of Congenital Heart Lesions

• Acyanotic CHD: Left-to-right shunts (PDA, VSD, ASD), obstruction to ventricular outflow (PS, SAS), or regurgitant valve lesions (mitral/tricuspid valve dysplasia).
• Cyanotic CHD: Right-to-left shunts (Tetralogy of Fallot), Eisenmenger's syndrome, reversed PDA, and reversed ventricular septal defect (VSD).

Most Common Defect By Species

• Specific congenital heart defects are more common in some species (e.g., PDA in dogs). Specific defects and their frequencies are listed for different species.

Pathophysiology of Left-To-Right Shunts

• Communication between pulmonary veins/left heart and systemic arterial circulation/systemic venous circulation/ Right heart/pulmonary arteries.
• Re-circulation of oxygen-rich blood leads to volume overload and eccentric hypertrophy of the affected heart structures.
• Most common L-to-R shunts: PDA, ventricular and atrial septal defects.

Patent Ductus Arteriosus (PDA)

• Failure of ductus arteriosus to close after birth.
• Normal closure occurs in minutes to hours for dogs/cats, and 3-4 days for horses.
• Anatomic closure occurs within 1 month.

Patent Ductus Arteriosus (PDA): Clinical Findings

• “Wind blowing through a tunnel”, “machinery” murmur, grade/intensity is typically IV–VI/VI, palpable thrill common, location is left heart base, and timing is continuous.

Patent Ductus Arteriosus (PDA): Prognosis and Recommendations

• If untreated, 65% of dogs die of heart failure within one year.
• Surgical closure is highly curative.
• Screen relatives of affected animals; do not breed affected animals; if a continuous murmur is heard, don't monitor, refer to a cardiologist.

Ventricular Septal Defect (VSD)

• Defect in the interventricular septum (IVS) allowing direct communication of ventricles, is common in large animals and cats.
• Majority are located in the basilar/upper portion of the IVS ("perimembranous").

Ventricular Septal Defect (VSD): Clinical Findings

• Grade/intensity: Variable, smaller VSD = louder murmur
• Location: Right heart (apex or base, depending on location of VSD)
• Timing: Systolic

Ventricular Septal Defect (VSD): Prognosis and Recommendations

• Clinical picture/prognosis depends on the size/volume of the shunt..—most are small and well-tolerated; moderate/large shunts may require treatment for CHF; small defects-no intervention.
• Surgical closure of larger defects or minimally invasive occlusion are possible in some cases but not widely available.

Lesions causing obstruction (stenosis) of ventricular outflow

• High-pressure distal to the obstruction leads to concentric hypertrophy of the ventricles.
• Low-pressure distal to the obstruction, will cause high-velocity, turbulent flow causing an audible murmur and post-stenotic dilatation.
• Narrowing of the ventricular outflow tract increases resistance to blood flow.

Potential Consequences of Outflow Tract Stenosis

• Congestive heart failure is one of the more frequent heart issues found more commonly in RV, rather than LV.
• Sudden cardiac death
• Lethal ventricular arrhythmias due to fibrosis/ischemia
• “Critical” stenosis: Flow too low to satisfy body’s demands (syncope, weakness, lethargy).
• The risk of complications increases with lesion severity. More details on pressure gradient ranges and their correlations to clinical severity are included.

Obstruction (Stenosis) of Ventricular Outflow: Clinical Features

• Murmur: Variable intensity/grade, located at the left heart base; timing is systolic.
• Femoral pulses usually normal for RV obstruction; can be reduced in LV obstruction if stenosis is severe enough.

Obstruction (Stenosis) of Ventricular Outflow: Species-Specific Clinical Features

• Most animals showing CHD will be asymptomatic for an extended period; the owner may notice exertional fatigue, syncope, or other signs of CHF in those with severe stenosis. Specific clinical features for common types of outflow obstruction are included.

Obstruction (Stenosis) of Ventricular Outflow: Clinical Management

• B1-adrenergic blockers are frequently utilized for moderate-severe stenosis to reduce myocardial oxygen demand.
• Antiarrhythmic medications are used as well.
• Percutaneous balloon valvuloplasty for pulmonic stenosis, although not as frequently utilized for subaortic stenosis.
• Breeding considerations vary based on severity, as significant defects greatly reduce the chances of survival and proper heart function.

Pathology of Right-to-Left Shunts

• Abnormal communication between systemic venous circulation/right heart/pulmonary arteries and pulmonary veins/left heart/systemic arterial circulation.
• Oxygenated blood bypasses the lungs, so the systemic circulation doesn't receive enough oxygen.
• Cyanosis is a visible sign when PaO2 < 40mm Hg, which may result in polycythemia.
• Common examples include Tetralogy of Fallot, Eisenmenger's syndrome (including reversed PDA and reversed VSD). More specific details on the pathology of each are included.

Eisenmenger's Syndrome

• Reversal of normal left-to-right heart shunt due to increased right-sided pressures as a result of pulmonary hypertension.
• Shunt reverses when pulmonary vascular resistance exceeds systemic vascular resistance.
• Common issue occurring in instances of PDA, VSD, or ASD.

Tetralogy of Fallot

• Most common cyanotic heart disease in animals.
• Upper interventricular septum malformation leads to four abnormalities.

Tetralogy of Fallot: Clinical Features

• Asymptomatic or signs of severe exercise intolerance syncope and seizures.
• Cyanosis is present in 90% of affected animals at rest; the cyanosis worsens during exercise.
• A systolic murmur is frequently present left basilar location.
• Pulses are usually normal.

Tetralogy of Fallot: Prognosis and Recommendations

• Medical therapy: Intermittent phlebotomy and exercise restriction.
• Surgical options: Palliative systemic-to-pulmonary shunts (e.g., modified Blalock-Taussig shunt, subclavian artery-to-pulmonary artery conduit) or cardiac bypass. Specific surgical options and success rates are included.

Take Home Points

• CHD severity ranges from trivial (well-tolerated) to life-threatening.
• CHD is the most common type of heart disease in young animals, although signs may not be noticed until adulthood.
• Diagnosis can be made through echocardiography.
• Prognosis is heavily dependent on the type and severity, as well as availability of treatment options.
• Prompt recognition from a veterinarian. Referral to a cardiologist for the most appropriate diagnosis is often required.