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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Congenital cardiac abnormalities include any structural cardiac malformation present from birth, usually due to either a failure in embryologic development or a persistence of fetal circulation.

Pathophysiology!!navigator!!

  • VSD is the most common defect in foals and occurs when there is incomplete development of the ventricular septum during embryonic development
  • A high (paramembranous) defect in the ventricular septum, within the left ventricular outflow tract, is most common; a subpulmonic defect (less common) communicates below the pulmonary valve. Muscular VSDs occur in any portion of the muscular septum
  • ASDs—can involve various portions of the atrial septum due to failure of ingrowth from several areas, most often as part of complex congenital defects. Requires differentiation from PFO
  • PDA—ductus arteriosus shunts blood from the pulmonary artery to the descending aorta in the fetal circulation; it can remain patent as a single defect or, more commonly, as part of a complex congenital cardiac abnormality. The ductus arteriosus normally closes in response to changes in pressure gradients at birth, inhibition of prostaglandins, and increased local oxygen tension
  • PFO—the foramen ovale shunts blood from the right atrium to the left atrium to bypass the lungs in the fetal circulation. The foramen ovale normally closes when the lungs expand (pulmonary vascular resistance falls). This opening can remain patent if pulmonary hypertension is present. In normal foals functional closure occurs in the first 24–48 h of life
  • Truncus arteriosus—normally partitions into aorta and pulmonary artery in the fetus. If there is failure to partition, communication occurs across a large VSD
  • Tricuspid atresia—absence of the right atrioventricular orifice, preventing flow of blood from right atrium to right ventricle. Blood must be shunted across an ASD or PFO
  • Pulmonary atresia with VSD—the right ventricular outflow does not connect with the pulmonary artery. A large malalignment VSD is usually present, and the fetal truncus arteriosus is unequally partitioned, causing marked dilation of the aorta and severe atresia or hypoplasia of the pulmonary trunk
  • TOF includes VSD, right ventricular outflow obstruction, overriding aorta, and right ventricular hypertrophy. TOF is the most common cause of right-to-left shunt. Pentalogy of Fallot includes ASD in addition to the above abnormalities
  • Left-to-right shunting occurs with ASD, VSD, and PDA. Blood shunted from the left ventricle to the right heart will increase volume in the pulmonary artery, and will therefore increase venous return to the left atrium and left ventricle, eventually causing left ventricular hypertrophy and dilation. Larger shunts will result in left-sided heart failure and significant pulmonary hypertension. VSDs that allow pressures to equilibrate between left and right ventricles can lead to biventricular hypertrophy
  • Right-to-left shunting can occur in tricuspid valve atresia and ASD or pulmonary valve atresia and VSD, or in chronic pulmonary vascular disease

Systems Affected!!navigator!!

Genetics!!navigator!!

There is thought to be a genetic predisposition to congenital cardiac abnormalities, although the specific genetic factors are unknown at this time.

Incidence/Prevalence!!navigator!!

  • 0.7–0.8% of foals and fetuses that undergo postmortem examination have congenital cardiac abnormalities
  • 3.5% of congenital defects in foals were heart defects

Geographic Distribution!!navigator!!

N/A

Signalment!!navigator!!

  • Defects are present from birth; however, the time to recognition of the problem will vary depending on the severity of clinical signs. Cardiac anomalies are most often recognized in the first weeks to months after birth
  • Congenital cardiac defects appear to be more common in Arabians, Standardbreds, Welsh Mountain ponies, and possibly Morgan foals
  • VSD appears to have a genetic link in the Arabian breed. Standardbred and Quarter Horse breeds may also be more commonly affected with VSD
  • No sex predilection

Signs!!navigator!!

Commonly, left-to-right shunts secondary to congenital cardiac anomalies will result in left-sided or biventricular heart failure, leading to signs of exercise intolerance, respiratory distress, and jugular venous distention.

Historical Findings

  • Premature foals may be at higher risk of left-to-right shunts at the foramen ovale
  • Exercise intolerance
  • Stunted growth

Physical Examination Findings

  • Heart murmur—usually grade 3/6 or higher, but absence of a murmur does not rule out congenital cardiac anomaly
    • VSD—holosystolic, auscultated on both sides, but point of maximal intensity usually on right; louder murmur generally correlates with smaller defect
    • PDA—continuous machinery murmur, usually loudest on left; heard in most foals for the first 15 min of life; can be heard in normal foals for up to the first 3 days of life
    • TOF—loud, left-sided systolic murmur; may have a palpable thrill
  • Lethargy, weakness
  • Cyanotic mucous membranes at rest or with exercise (most common with right-to-left shunting)
  • Tachypnea/dyspnea; may have harsh lung sounds
  • Jugular venous distention
  • Bounding pulses

Causes!!navigator!!

  • Teratogenic exposure, viral infection, or hypoxic damage early in pregnancy are suspected causes of congenital cardiac abnormalities
  • Breed is likely a risk factor
  • Prematurity may also be associated with failure to revert from fetal circulation

Risk Factors!!navigator!!

See Causes.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Pneumonia—thoracic radiographs and 2D echocardiography should elucidate the cause of respiratory compromise
  • Physiologic cardiac murmur—usually left-sided, systolic; no echocardiography abnormalities, and foal should not show clinical signs of cardiac disease
  • Anemia—lethargy and weakness

CBC/Biochemistry/Urinalysis!!navigator!!

Polycythemia may be seen in response to chronic hypoxemia.

Other Laboratory Tests!!navigator!!

Blood gas analysis—arterial hypoxemia, usually minimally responsive to oxygen supplementation, especially with right-to-left shunts. Arterial CO2 is normal or reduced.

Imaging!!navigator!!

  • Echocardiography—2D views can often confirm the defect, although Doppler studies are often needed to find VSDs in atypical locations and to estimate the pressure difference across the ventricles. Contrast echocardiography using agitated normal saline solution administered through the jugular vein (“bubble study”) allows identification of right-to-left shunting
  • Radiography—cardiomegaly, pulmonary edema, and overcirculation of the pulmonary vasculature may be detected

Other Diagnostic Procedures!!navigator!!

ECG should be used to detect arrhythmias. Cardiac catheterization can be performed to measure pulmonary artery and pulmonary capillary wedge pressures

Pathologic Findings!!navigator!!

Cardiac malformations are detected on gross examination at necropsy.

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

There are no current surgical recommendations for horses with cardiac defects. Although PDA and VSD have been successfully treated surgically in dogs, there are no surgical procedures currently described for use in foals. Medical management may be used for horses with CHF or acute decompensation.

Nursing Care!!navigator!!

Supportive care with optimization of perfusion, oxygenation, and appropriate exercise restriction constitute care.

Activity!!navigator!!

Foals with significant shunts or complex cardiac defects are usually exercise intolerant and can become cyanotic when exercised.

Diet!!navigator!!

N/A

Client Education!!navigator!!

Breeding of affected animals should be discouraged.

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Digoxin (0.011 mg/kg PO once daily and 0.0022 mg/kg IV every 12 h) to improve myocardial function and control tachycardia. Digoxin levels should be monitored to avoid toxic levels
  • Furosemide (1–2 mg/kg IV or IM PRN) to treat pulmonary edema. Monitor serum electrolyte concentrations and acid–base status with prolonged therapy

Contraindications!!navigator!!

Digoxin and furosemide should not be used in dehydrated patients. Acid–base or electrolyte abnormalities may increase the toxicity of digoxin in individual patients. Digoxin should not be used in horses with existing ventricular arrhythmias. Furosemide may cause electrolyte and acid–base abnormalities with prolonged therapy.

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

Free serum concentrations of digoxin may be increased if quinidine is given concurrently.

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Horses with small VSDs can have successful athletic careers but should be monitored for development of CHF and atrial fibrillation (exercise intolerance, coughing, dyspnea, lethargy)
  • Repeat echocardiography (recommended yearly) is useful for evaluating development of ventricular enlargement or hypertrophy as well as valvular insufficiency
  • Perform exercise ECG in horses with moderate to large VSDs, as part of a prepurchase examination, or when presenting with exercise intolerance

Prevention/Avoidance!!navigator!!

Horses with congenital cardiac anomalies should not be used for breeding, as the defects may be heritable.

Possible Complications!!navigator!!

CHF

Expected Course and Prognosis!!navigator!!

  • The prognosis with a VSD is dependent on the size of the defect, size of the cardiac chambers, maximal shunt velocity, and presence of significant aortic or mitral regurgitation, pulmonary hypertension, or CHF. Horses with membranous VSDs that measure <2.5 cm at the largest diameter or a VSD-to-aortic root diameter <0.4 (and that have a higher peak velocity of shunt flow >4.5 m/s) tend to have a good athletic prognosis. Horses with moderate defects may only be normal at rest
  • Horses with large VSDs or with multiple congenital cardiac defects may be small and stunted and are likely to develop signs of CHF

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Mares with significant shunting of blood due to congenital cardiac defects may develop signs of CHF during late-term pregnancy due to increased demand for cardiac output.

Abbreviations!!navigator!!

  • 2D = two-dimensional
  • ASD = atrial septal defect
  • CHF = congestive heart failure
  • PDA = patent ductus arteriosus
  • PFO = patent foramen ovale
  • TOF = tetralogy of Fallot
  • VSD = ventricular septal defect

Suggested Reading ⬆ ⬇

Bonagura JD, Reef VB. Disorders of the cardiovascular system. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 2e. St. Louis, MO: WB Saunders, 2004:355–459.

Chope K. Cardiac disorders. In: Paradis MR, ed. Equine Neonatal Medicine: A Case-Based Approach. Philadelphia, PA: Saunders, 2006:247–258.

Marr CM, Bowen M. Cardiology of the Horse, 2e. Edinburgh, UK: Saunders Elsevier, 2010.

Reef VB. Evaluation of ventricular septal defects in horses using two-dimensional and doppler echocardiography. Equine Vet J Suppl 1995;19:86–95.

Reef VB, Bonagura R, Buhl MKJ, et al. Recommendations for management of equine athletes with cardiovascular abnormalities. J Vet Intern Med 2014;28:749–761.

Author(s) ⬆

Author: Laura K. Dunbar

Consulting Editor: Margaret C. Mudge

Acknowledgment: The author acknowledges the prior contribution of Margaret C. Mudge.