section name header

Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • A congenital defect (i.e. hole) in the interatrial septum that creates a communication between the right and left atria
  • Can be located in the atrial septum immediately adjacent to the ventricular septum (i.e. atrium primum defect), in the area of the foramen ovale (i.e. atrium secundum defect), or in the most basilar portion of the interatrial septum (i.e. sinus venosus-type defect)
  • ASD can occur in isolation or in conjunction with other cardiac anomalies in complex congenital cardiac disease
  • The atrial septum forms in the fetus from the septum primum and the septum secundum. The slit-like communication between these septa (i.e. the foramen ovale) allows passage of blood from right to the left atrium in the fetus
  • The foramen ovale is functionally closed in normal neonates within 24–48 h of birth, but anatomic closure may not be complete until 9 weeks

Pathophysiology!!navigator!!

  • A patent foramen ovale occurs when the foramen ovale fails to close
  • Failed formation of 1 of the 2 septa results in the other forms of ASD
  • Blood shunts from the higher pressure left atrium to the lower pressure right atrium in foals with ASD, creating a left atrial, right atrial, and right ventricular volume overload
  • Size of the ASD determines severity of the volume overload. In horses with a large ASD, the right and left atrial and right ventricular volume overload is severe
  • Over time, stretching of the tricuspid annulus occurs, and tricuspid regurgitation develops. As the tricuspid regurgitation becomes more severe, increases in right atrial pressure result in increased hepatic venous pressure and development of clinical signs of right-sided CHF

Systems Affected!!navigator!!

Cardiovascular

Genetics!!navigator!!

  • Not yet determined in horses
  • Although heritable in other species, it is rare in horses

Incidence/Prevalence!!navigator!!

These defects are uncommon as isolated congenital defects and more frequently occur in conjunction with complex congenital heart disease, particularly tricuspid and pulmonic atresia.

Signalment!!navigator!!

Most frequently diagnosed in neonates, foals, and young horses, but may be diagnosed at any age.

Signs!!navigator!!

General Comments

May be detected as an incidental finding, but usually is part of a more complex, congenital cardiac disorder.

Historical Findings

  • Exercise intolerance—medium-sized to large ASDs
  • CHF—large ASDs

Physical Examination Findings

  • No murmur may be present, or a coarse, band- or ejection-shaped, holosystolic murmur with point of maximal intensity in the pulmonic valve area may be detected
  • Premature beats or an irregularly irregular heart rhythm of AF may be present with larger ASDs

Causes!!navigator!!

  • Failed closure of the foramen ovale
  • Congenital malformation of the interatrial septum

Risk Factors!!navigator!!

  • Premature foal
  • Neonatal pulmonary hypertension
  • Neonatal respiratory distress syndrome

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Physiologic flow murmur—differentiate echocardiographically
  • Pulmonic stenosis (rare)—murmur usually louder; differentiate echocardiographically
  • Aortic stenosis (rare)—murmur usually louder; weak arterial pulses; differentiate echocardiographically
  • Tricuspid atresia—murmur usually louder; foal is unthrifty, tachycardic, and hypoxemic; differentiate echocardiographically
  • Pulmonic atresia—murmur usually louder; may have a continuous machinery murmur; foal is unthrifty, tachycardic, and hypoxemic; differentiate echocardiographically

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Other Laboratory Tests!!navigator!!

N/A

Imaging!!navigator!!

ECG

  • Atrial premature depolarizations or AF may be present in horses with right and left atrial enlargement
  • Persistent AF has been reported in some affected foals and horses

Echocardiography

  • Can determine location of the ASD
  • Atrial septal dropout is detected at the ASD location and should be confirmed by visualization in 2 mutually perpendicular planes
  • The left and right atria and right ventricle are enlarged, dilated, and have a rounded appearance
  • Paradoxical septal motion is detected with a severe right ventricular volume overload
  • Pulmonary artery dilatation is seen in horses with a large shunt
  • Interrogate the entire atrial septum with pulsed-wave or color-flow Doppler with suspected ASD
  • Contrast or color-flow Doppler reveals the shunt from the left to the right atrium through the ASD
  • A small amount of positive contrast may be seen in the left atrium in horses with normal pulmonary arterial pressures or with the Valsalva maneuver during contrast echocardiography
  • A jet of tricuspid regurgitation may be present in horses with a large ASD and marked right atrial and ventricular volume overload

Thoracic Radiography

Increased pulmonary vascularity and cardiac enlargement may be detected in horses with large shunts.

Other Diagnostic Procedures!!navigator!!

Cardiac Catheterization

  • Right-sided catheterization can be performed to directly measure right atrial, right ventricular, and pulmonary arterial pressures and to sample blood for oxygen content
  • Elevated right atrial, right ventricular, and pulmonary arterial pressures and increased oxygen saturation of right ventricular and pulmonary arterial blood have been seen in horses with larger ASDs

Continuous 24 h Holter Monitoring

Use in identifying intermittent atrial premature depolarizations.

Pathologic Findings!!navigator!!

  • Defect in the atrial septum
  • Jet lesions along the defect margins and on the adjacent right atrial endocardium
  • Left atrial, right atrial, and right ventricular enlargement and thinning of the left atrial, right atrial, and right ventricular free wall in horses with a significant shunt
  • Pulmonary artery dilatation in horses with a large shunt or that have developed pulmonary hypertension
  • With CHF, ventral and peripheral edema, pleural effusion, pericardial effusion, chronic hepatic congestion, and, occasionally, ascites may be detected

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Management by intermittent monitoring in horses with small ASDs
  • Palliative care in horses with large ASDs and those with complex congenital cardiac defects

Appropriate Health Care!!navigator!!

  • Most affected horses require no treatment and can be monitored on an outpatient basis
  • Monitor horses with large shunts on an annual basis
  • Affected horses with CHF can be treated for CHF with positive inotropic drugs, vasodilators, and diuretics. Consider humane destruction if CHF develops, however, because only short-term, symptomatic improvement can be expected

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

  • Affected horses are safe to continue in full athletic work until significant tricuspid regurgitation or AF develops
  • Horses with small defects can be in unrestricted activity and may be able to compete reasonably successfully in upper level athletic competition
  • Monitor horses with hemodynamically significant defects echocardiographically on an annual basis to ensure they are safe to ride and compete. These horses can be used for lower level athletic competition but are unlikely to compete at the upper levels of athletic performance
  • Affected horses that develop AF need a complete cardiovascular examination to determine if they are safe to use for lower level athletic performance
  • Horses with significant pulmonary artery dilatation no longer are safe to ride

Diet!!navigator!!

N/A

Client Education!!navigator!!

  • Regularly monitor cardiac rhythm; any irregularities of the rhythm, other than second-degree atrioventricular block, should prompt ECG
  • Carefully monitor for exercise intolerance, respiratory distress, prolonged recovery after exercise, increased resting respiratory or heart rate, cough, generalized venous distention, jugular pulses, or ventral edema; if detected, obtain a cardiac reexamination

Surgical Considerations!!navigator!!

  • Closure of the ASD would be possible with a transvenous umbrella catheter if the diameter of the umbrella was large enough to close the defect
  • Surgical closure is not financially feasible or practical for obtaining equine athletes at this time

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

N/A

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Frequently monitor cardiac rate, rhythm, and respiratory rate and effort.

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

Large ASD—AF; CHF.

Expected Course and Prognosis!!navigator!!

  • Horses with small defects should have a normal performance life and life expectancy
  • Horses with moderate defects also have a normal life expectancy. These horses usually perform successfully only at lower levels of athletic competition, and they may develop AF
  • Horses with large defects have a guarded prognosis, because they may have a shortened life expectancy and performance life, even at the lower levels of athletic competition
  • Affected horses with CHF usually have a guarded to grave prognosis for life. Most such horses being treated for CHF should respond to the supportive therapy and transiently improve; however, once CHF develops, euthanasia is recommended

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Complex congenital cardiac disease, particularly tricuspid and pulmonic atresia, is likely
  • Tricuspid regurgitation can develop in horses with significant left atrial, right atrial, and right ventricular volume overload secondary to stretching of the tricuspid annulus
  • Pulmonic regurgitation can develop in horses with isolated defects
  • Pulmonic valve leaflets may no longer coapt with stretching of the pulmonary artery from the volume overload

Age-Related Factors!!navigator!!

Young horses are more likely to be diagnosed.

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Breeding affected horses is discouraged. The condition is rare, however, and the heritable nature of this defect in horses is not known.

Synonyms!!navigator!!

N/A

Abbreviations!!navigator!!

  • AF = atrial fibrillation
  • ASD = atrial septal defect
  • CHF = congestive heart failure

Suggested Reading ⬆ ⬇

Physick-Sheard PW, Maxie MG, Palmer NC, Gaul C. Atrial septal defect of the persistent ostium primum type with hypoplastic right ventricle in a Welsh Pony foal. Can J Comp Med 1985;49:429–433.

Reef VB. Cardiovascular disease in the equine neonate. Vet Clin North Am Equine Pract 1985;1:117–129.

Reef VB. Echocardiographic findings in horses with congenital cardiac disease. Compend Contin Educ Pract Vet 1991;13:109–117.

Reef VB. Cardiovascular ultrasonography. In: Reef VB, ed. Equine Diagnostic Ultrasound. Philadelphia, PA: WB Saunders, 1998:215–272.

Reppas GP, Canfield PJ, Hartley WJ, et al. Multiple congenital cardiac anomalies and idiopathic thoracic aortitis in a horse. Vet Rec 1996;138:14–16.

Taylor FG, Wooton PR, Hillyer MH, et al. Atrial septal defect and atrial fibrillation in a foal. Vet Rec 1991;128:80–81.

Author(s) ⬆

Author: Virginia B. Reef

Consulting Editors: Celia M. Marr and Virginia B. Reef