Ventricular septal defects

Definition & Classification

Ventricular septal defects (VSDs) are openings in the ventricular septum that vary in size, location, and number. They may be isolated or multiple and can be categorized based on their anatomical position within the septum:

1. Inlet VSD

• Located posteriorly, beneath the crux of the heart.

• The typical offset between the tricuspid valve (TV) and mitral valve (MV) is absent, as they attach at the same level due to the lack of septal tissue.

• Often associated with atrioventricular septal defects (AVSDs), requiring careful evaluation for a primum ASD.

• If both a primum ASD and VSD are present with two separate AV valves, the defect may be classified as a transitional AVSD. In such cases, the VSD often appears small.

• If no primum ASD is present, the defect may be termed an "AV canal type" or simply "inlet VSD."

• Given its potential relationship with AVSDs, careful assessment for a cleft mitral valve is warranted.

2. Muscular VSD

• Located in the trabecular/muscular septum.

• These are the most common small, isolated VSDs seen in otherwise healthy neonates.

• If the defect is larger than tiny, multiple muscular VSDs should be considered.

3. Outlet VSDs

• Membranous (Perimembranous) VSD:

  • Located between the tricuspid valve and aorta.

  • Smaller defects may be associated with secondary complications, such as:

    • Double-chambered right ventricle (DCRV).

    • Subaortic membrane formation.

    • Aortic valve herniation into the VSD, leading to aortic regurgitation.

  • A membranous VSD is a type of outlet VSD.

• Doubly Committed Subarterial VSD (Supracristal VSD)

  • Located beneath both the aortic valve and pulmonary valve.

  • Typically small but often associated with aortic valve herniation into the VSD, which can lead to aortic regurgitation.

  • Also referred to as supracristal VSD (Van Praagh terminology).

• Other Outlet VSDs

  • Located between membranous VSDs and supracristal VSDs.

  • Terminology varies, with some referring to them as "infracristal VSDs".

Physiological Impact of VSDs

A ventricular-level shunt may impose:

1. Pressure overload to the right heart

2. Volume overload to the left heart

3. Both pressure and volume overload

4. Neither, in cases of restrictive VSDs

• Non-Restrictive VSDs:

  • If the VSD size is equal to or larger than the aortic valve annulus, there is no pressure gradient across the defect.

  • The RV pressure equals the LV pressure, indicating a non-restrictive shunt.

• Restrictive VSDs:

  • If a small VSD is present but has a low pressure gradient, alternative causes of RV pressure elevation should be considered, such as:

    • Right ventricular outflow tract (RVOT) obstruction.

    • Pulmonary hypertension (PH).

  • The term "restrictive VSD" typically refers to pressure-restrictive, though a VSD may still impose a significant volume load.

• Right Ventricular (RV) Pressure Overload

  • Leads to right ventricular hypertrophy (RVH) and possibly RV dysfunction.

  • If chronic, Eisenmenger syndrome may develop, with severe pulmonary hypertension and eventual right-to-left shunting through the VSD.

• Left Ventricular (LV) Volume Overload

  • The LV, not the RV, experiences volume overload because the VSD shunt occurs predominantly in systole, directing blood into the RV and pulmonary circulation.

  • This results in increased LV end-diastolic volume (LVEDV), often leading to:

    • Hyperdynamic LV function.

    • LV dilation over time.

    • If prolonged, LV dysfunction may develop.

  • LV end-diastolic dimension (LVEDd) serves as an important marker of volume load and shunt impact.

• Impact on LV Afterload

  • A large VSD reduces LV afterload because the LV ejects blood into both the systemic and pulmonary circulations.

  • Since pulmonary vascular resistance (PVR) is lower than systemic vascular resistance (SVR), the combined resistance is lower than SVR alone.

  • This must be considered when evaluating LV function, as contractility may appear normal despite actual LV dysfunction.

  • A small subset of patients experience significant LV dysfunction postoperatively, due to sudden afterload and preload changes following VSD closure.

Echocardiographic Assessment of Ventricular Septal Defects (VSDs)

1. Define Anatomical Site, Type, and Size

• Best Echocardiographic Windows:

  • Inlet VSD: Best visualized in the apical four-chamber (A4C) view, with posterior angulation to include both atrioventricular (AV) valves.

  • Muscular VSD: Can be seen in any view containing the septum; small defects may require color Doppler for visualization.

  • Membranous (Perimembranous) VSD: Visible in parasternal long-axis (PSLA) view, but best localized in parasternal short-axis (PSSA) view when both the aortic valve (AoV) and tricuspid valve (TV) are present.

  • Other Outlet VSDs: Seen in PSLA view, but best defined in PSSA view when aortic valve, tricuspid valve, and pulmonary valve (PV) are included.

• Size Classification (Relative to Aortic Valve Annulus Diameter):

  • Small: < ½ annular diameter.

  • Moderate: ½ to full annular diameter.

  • Large: > Annular diameter.

• Unusual Variant – Gerbode Defect:

  • A rare left ventricle (LV) to right atrium (RA) shunt, typically involving a membranous VSD.

2. Assess Hemodynamic Impact on Global Cardiac Function

• Left Ventricular Function:

  • Evaluate LV systolic function using fractional shortening (FS, M-mode) and biplane ejection fraction (EF, Simpson’s rule).

• Left Atrial (LA) Size:

  • LA/Ao ratio >1.6 (measured in PSSA) suggests a significant left-to-right shunt.

• Left Ventricular Size:

  • Assess LV end-diastolic dimension (LVEDd) Z-score.

  • Calculate LVED volume from EF (biplane Simpson’s rule).

• Pulmonary Artery (PA) Pressure Estimation:

  • LV-to-RV pressure gradient via Doppler measurement of VSD flow velocity (ensure optimal Doppler alignment).

  • Peak tricuspid regurgitation (TR) velocity to estimate RV and PA systolic pressures.

  • Peak pulmonary regurgitation (PR) velocity to assess mean PA pressure.

3. Assess Shunt Size (Restrictive vs. Non-Restrictive VSDs)

• Qp/Qs measurement (via PW Doppler at LVOT and RVOT) is possible but has significant variability and limited clinical utility.

• More reliable indicator: LVEDd Z-score, which reflects the magnitude of volume overload.

4. Detection of Associated Lesions

• Aortic Valve Prolapse & Aortic Regurgitation (AI):

  • First sign is asymmetry in aortic valve cusps in PSLA view.

  • If suspected, zoom in to assess for AI, which will have a posteriorly directed jet if the right coronary cusp is involved.

  • Also assess in apical five-chamber view.

• Interventricular Septal Aneurysm:

  • A thin, redundant membrane often arises from the VSD margin due to incorporation of TV septal leaflet tissue.

  • May contribute to spontaneous VSD closure.

• Double-Chambered Right Ventricle (DCRV):

  • If a small membranous VSD is present, assess for RV anomalous muscle bundles.

  • Use 2D imaging of the RV cavity and walls in PSSA and subcostal views.

  • Look for anterior wall thickening and dynamic narrowing in systole.

  • Color Doppler may be less useful, as RV flow is already accelerated due to the VSD.

• Subaortic Membrane:

  • Best seen in PSLA and apical four-chamber (A4C) views.

  • Measure the gradient across the membrane using PW Doppler.

  • Assess for associated AI, which may indicate leaflet damage due to turbulent flow.

• Septal Malalignment (Conal Septum Malposition):

  • Anterior malalignment: Causes RVOT obstruction and is associated with Tetralogy of Fallot (TOF).

  • Posterior malalignment: Leads to LVOT obstruction, and is associated with subaortic stenosis, coarctation, or interrupted aortic arch.

• Arch Hypoplasia:

  • Multiple muscular VSDs may be linked to aortic arch hypoplasia in some cases.

Restrictive muscular ventricular septal defect close to the Apex of the left ventricle. Left to right.

Peak systolic gradient is 20-23 mmHg (left to right), indicating that the sPAP is about 20 mmHg less than the systolic systemic blood pressure.

Subcostal views re-demonstrating the muscular VSD

Another Case of Muscular VSD