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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • A congenital defect (i.e. hole) in the interventricular septum, resulting in communication between the right and left ventricles
  • Can be located in any portion of the interventricular septum—membranous (most common)

Pathophysiology!!navigator!!

  • Blood shunts from the higher pressure left ventricle to the lower pressure right ventricle, then returns to the left atrium and ventricle via the lungs, creating primarily a left atrial and ventricular volume overload and, to a lesser degree, a right ventricular volume overload
  • The size and location of the VSD determine the severity of the volume overload and the degree of right ventricular involvement
  • With a large, membranous VSD, the left atrial and ventricular volume overload is severe. Over time, stretching of the mitral annulus occurs, and MR develops. As the MR becomes more severe, increases in left atrial pressure cause increased pulmonary venous pressure, increased pulmonary capillary pressure, pulmonary edema, pulmonary hypertension, and clinical signs of left-sided CHF. As pulmonary hypertension becomes more severe, clinical signs of right-sided CHF appear
  • With a large, muscular VSD, the left atrial and ventricular volume overload and right ventricular volume overload are severe, and clinical signs of right-sided heart failure may predominate
  • When a large VSD is located immediately below the aortic valve, the aortic valve may become incompetent, which further exacerbates left ventricular volume overload

Systems Affected!!navigator!!

Cardiovascular

Genetics!!navigator!!

Not yet determined in horses, but likely heritable.

Incidence/Prevalence!!navigator!!

Welsh Mountain ponies, Arabians, and Standardbreds at higher risk.

Signalment!!navigator!!

  • Murmurs present at birth
  • Diagnosed most frequently in foals and young horses but may be found at any age

Signs!!navigator!!

General Comments

Often detected as an incidental finding in mature horses if the VSD is small.

Historical Findings

  • Medium to large VSDs—poor performance
  • Large VSDs—CHF

Physical Examination Findings

  • Grade 3–6/6, coarse, band-shaped, pansystolic murmur with PMI in the tricuspid valve area; membranous defect has loudest murmur here
  • Grade 3–6/6, coarse, band or crescendo–decrescendo, ejection shaped, holosystolic or pansystolic murmur with PMI in the pulmonic valve area; outflow defect has loudest murmur here. With membranous VSD, pulmonic murmur is usually 1 grade quieter than the right-sided murmur owing to relative pulmonic stenosis
  • Other less common findings—accentuated third heart sound, grade 1–6/6 holodiastolic decrescendo murmur with PMI in the aortic valve area, and AF

Causes!!navigator!!

Congenital malformation of the interventricular septum.

Risk Factors!!navigator!!

N/A

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Tricuspid regurgitation—no pulmonic murmur; differentiate echocardiographically
  • VSD with pulmonic stenosis or bicuspid pulmonic valve—loudest murmur usually in the pulmonic valve area; differentiate echocardiographically
  • Tetralogy of Fallot—foals are often stunted and may be tachycardic and hypoxemic; loudest murmur usually in the pulmonic valve area; differentiate echocardiographically

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Imaging!!navigator!!

ECG

Atrial premature complexes or AF may be present with left atrial enlargement. Ventricular premature complexes also occur in some cases.

Echocardiography

  • The most common location is the membranous portion of the interventricular septum, immediately beneath the septal leaflet of the tricuspid valve and right or noncoronary leaflet of the aortic valve. VSDs that are <2.5 cm in 2 mutually perpendicular planes or with a ratio of the VSD to the aortic root of <1:3 are generally restrictive with little hemodynamic impact
  • Outflow VSD is less common and more difficult to detect echocardiographically because of its location ventral to the aortic and pulmonic valves
  • Muscular VSD in other portions of the interventricular septum is less common; however, the entire ventricular septum should be examined
  • More than 1 defect may be present
  • Left atrium and ventricle—enlarged, dilated, and rounded in appearance
  • Left ventricular free wall and interventricular septum—thinner than normal; pattern of left ventricular volume overload if the ventricle is coping well with the left-to-right shunt
  • Normal or decreased fractional shortening with left ventricular enlargement is consistent with myocardial dysfunction
  • Pulmonary artery dilatation with a large shunt fraction
  • Pulsed-wave or color-flow Doppler reveals left-to-right shunt through the VSD
  • Hemodynamic significance determined by the peak velocity of the shunt through the VSD—a peak velocity >4 m/s is consistent with a restrictive defect; a peak shunt velocity ≤3 m/s indicates a very hemodynamically significant shunt and a large VSD
  • MR may be present with a large VSD and marked left atrial and ventricular volume overload

Thoracic Radiography

  • An enlarged cardiac silhouette, tracheal elevation, and increased pulmonary vascularity may be detected with a large VSD
  • Pulmonary edema may be present in affected horses with CHF

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 ventricular pressure should be detected with an increased oxygen saturation of blood obtained from the right ventricle and pulmonary artery if the shunt is left to right

Continuous 24 h Holter Monitoring

Useful in the diagnosis of suspected arrhythmias.

Pathologic Findings!!navigator!!

  • Most frequently found in the membranous septum underneath the septal leaflet of the tricuspid valve and the right or noncoronary leaflet of the aortic valve but also can be present in any portion of the interventricular septum
  • Associated jet lesions along the margins of the defect and on the adjacent right ventricular endocardium
  • Left atrial and ventricular enlargement and thinning of the left atrial and ventricular myocardium and interventricular septum with a large shunt
  • Right ventricular enlargement and thinning of the right ventricular free wall with a VSD that is large or in a muscular location
  • Pulmonary artery dilatation with a large shunt fraction and in those with pulmonary hypertension

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

  • Management by intermittent monitoring in horses with small- or medium-sized membranous VSD
  • Palliative care in horses with large membranous or muscular VSD and CHF

Appropriate Health Care!!navigator!!

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

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

  • Horses with small VSDs are safe to continue in unrestricted athletic work provided there is no significant arrhythmias or pulmonary artery dilation
  • Monitor horses with hemodynamically significant VSDs echocardiographically on an annual basis to ensure they are safe to ride and compete. These horses can be used for lower level athletic work
  • Horses with significant pulmonary artery dilatation are no longer safe to ride
  • Affected horses that develop significant arrhythmias often decompensate and are no longer safe to use for athletic performance

Diet!!navigator!!

N/A

Client Education!!navigator!!

  • Regularly monitor cardiac rhythm; any irregularities 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, or cough; if detected, perform a cardiac reexamination
  • Because the defect most likely is heritable, do not breed affected horses

Surgical Considerations!!navigator!!

  • Closure of the VSD would be possible with transvenous umbrella catheters if the umbrella diameter was large enough to close the defect. There are no published reports of this technique being successfully performed in horses to date
  • Surgical closure would require rib resection for the thoracotomy and cardiac bypass, which are not financially feasible or practical for obtaining an equine athlete

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 and rhythm and respiratory rate and effort
  • With defects >2.5 cm in 2 mutually perpendicular planes or peak shunt velocity <4.5 m/s, annual echocardiographic reexaminations are recommended

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

Large VSD—AF; CHF.

Expected Course and Prognosis!!navigator!!

  • Horses with small (≤2.5 cm), restrictive (peak shunt velocity >4.5 m/s), membranous VSDs should have normal performance and life expectancy; they can even race successfully, although not at the top levels
  • Progression of MR in horses with moderate VSDs usually is slow. These horses have normal life expectancy, but usually perform successfully only in less demanding disciplines
  • Horses with large (>4 cm) VSDs that are hemodynamically significant (peak flow velocity ≤3 m/s) have a guarded prognosis; they usually have shortened performance and life expectancy
  • Affected horses with CHF and MR have a guarded to grave prognosis for life. Most affected horses being treated for CHF respond to supportive therapy and transiently improve, but once CHF develops euthanasia of the horse is recommended

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Aortic regurgitation can develop in horses with VSDs and aortic valve prolapse, caused by the lack of aortic root support owing to the location of the VSD
  • MR can develop in horses with significant left atrial and ventricular volume overload secondary to stretching of the mitral annulus, further contributing to left atrial and ventricular volume overload

Age-Related Factors!!navigator!!

Young horses are more likely to be diagnosed.

Pregnancy/Fertility/Breeding!!navigator!!

Do not breed affected horses because of the possibly heritable nature of these defects.

Synonyms!!navigator!!

  • Septal defect
  • Interventricular septal defect

Abbreviations!!navigator!!

  • AF = atrial fibrillation
  • CHF = congestive heart failure
  • MR = mitral regurgitation
  • PMI = point of maximal intensity
  • VSD = ventricular septal defect

Suggested Reading ⬆ ⬇

Hall TL, Magdesian KG, Kittleson MD. Congenital cardiac defects in neonatal foals: 18 cases (1992-2007). J Vet Intern Med 2010;24:206–212.

Reef VB. Heart murmurs in horses: determining their significance with echocardiography. Equine Vet J Suppl 1995;19:71–80.

Reef VB. Evaluation of ventricular septal defects in horses using 2 dimensional and Doppler echocardiography. Equine Vet J Suppl 1995;19:86–95.

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

Author(s) ⬆

Author: Virginia B. Reef

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