Pathophysiology and shunt assessment
The presence of a defect causes a pathological left-to-right blood shunt. The clinical presentation directly depends on the shunt volume, which in turn is determined by the size of the opening itself and the ratio of systemic to pulmonary vascular resistance (PVR).
Interestingly, immediately after birth, infants may be asymptomatic: physiologically high PVR during this period limits the shunt volume.
Based on the nature of blood flow and pressure gradient, defects are divided into two groups, which are evaluated using echocardiography:
- Restrictive defects: characterized by high-velocity flow with normal PVR. This means that the pressure gradient between the left (LV) and right (RV) ventricles is preserved.
- Non-restrictive defects: the flow has a low velocity, which indicates equalization of pressure in the LV and RV (they become equal).
Anatomical classification
Patient management largely depends on the anatomical localization of the VSD. The interventricular septum is heterogeneous, and defects can occur in its various parts:
- Perimembranous part: the vast majority of defects (about 80%) are localized here. Many small perimembranous VSDs are capable of spontaneous closure over time.
- Muscular part: divided into three key sections:
- Inlet (inlet).
- Outlet, or subarterial (outlet).
- Mid-muscular (midmuscular) and apical.
- Special forms: doubly committed subarterial VSD and defects combined with an over-riding aorta, for example, in the setting of Tetralogy of Fallot.
Diagnostic imaging methods
Echocardiography (EchoCG) is the primary diagnostic method. However, it allows for only qualitative assessment of the shunt volume (based on relative dilatation of the heart chambers).
For accurate quantitative measurement of the shunt, cardiac MRI (CMR) with phase-contrast velocity mapping is used. MRI is particularly valuable in complex malformations:
- Multi-slice 2D gradient-echo techniques demonstrate dynamic 3D anatomy well, but have low in-plane resolution.
- 3D b-SSFP techniques with isotropic resolution enable precise multiplanar reconstruction and 3D rendering.
- Data acquisition is optimally performed during the diastolic phase, allowing for a detailed study of the defect's anatomy and its relationship with the valvular apparatus.
Key imaging goals
The radiologist faces several critically important tasks:
- Establish the exact location and dimensions of the opening.
- In an outlet (subarterial) defect, determine its relationship to the great vessel (supra-aortic or subpulmonary).
- Quantitatively assess the shunt and ventricular volumes (keeping in mind that LV stroke volume contributes to antegrade pulmonary artery flow in systole).
- Evaluate associated anomalies, including aortic regurgitation.
- In postoperative patients: check the integrity of the implanted surgical patch.
Special attention is required to look for signs of elevated PVR. These include right ventricular hypertrophy, systolic flattening of the interventricular septum, and characteristic notching of the pulmonary artery flow velocity curve.
Briefly: answers to topical questions
What imaging methods are used to detect and evaluate VSD?
Echocardiography, magnetic resonance imaging, and radiography are used to detect and evaluate a ventricular septal defect.
- Echocardiography (EchoCG) is the primary diagnostic method, allowing visualization of the pathological flow from the left ventricle to the right and assessment of blood flow velocity.
- Magnetic resonance imaging (cardiac MRI / CMR) provides accurate 2D and 3D images, and measures the exact shunt volume using phase-contrast velocity mapping.
- Radiography (X-ray) reveals enlargement of the pulmonary artery and increased pulsation of the lung roots.
What associated anomalies can be combined with a ventricular septal defect?
Ventricular septal defect often presents as an associated pathology in other congenital heart and vascular malformations.
- Congenitally corrected transposition of the great arteries (CCTGA) — VSD is the most frequent associated anomaly.
- Double-outlet right ventricle (DORV) — the left ventricle ejects blood through the VSD.
- Congenital anomalies of the aortic arch — almost always combined with a large VSD.
- Pulmonary stenosis (PS) — can be combined with VSD, altering the clinical picture.
What signs are characteristic of VSD on a plain chest radiograph?
A plain chest radiograph in ventricular septal defect reveals signs of increased pulmonary perfusion and vascular changes.
- Pulmonary plethora (Pulmonary plethora) — enlargement of the central and peripheral pulmonary arteries and veins in all lung zones, with an increase in the number and size of vessels.
- Changes in the lung roots — enlargement of the pulmonary artery and increased pulsation of the lung roots are observed.
- Pulmonary arterial hypertension (Pulmonary arterial hypertension) — in late stages, the central pulmonary arteries dilate, while the peripheral ones become smaller than normal.
How is the blood shunt volume quantitatively calculated during cardiac MRI?
Quantitative calculation of the exact blood shunt volume during cardiac magnetic resonance imaging is performed using specialized scanning modes.
- Phase-contrast velocity mapping is the main technique for measuring the exact shunt volume.
- Hemodynamic accounting — when quantitatively assessing the shunt, it must be taken into account that the left ventricular stroke volume contributes to antegrade blood flow in the pulmonary artery during systole.
To search for signs of high pulmonary vascular resistance (PVR) in VSD, remember the triad: RVH (Right ventricular hypertrophy, Flattening of the septum in systole, Notching of the PA flow curve).
Class and exam questions
Why might a newborn with a VSD appear completely healthy?
At birth, pulmonary vascular resistance remains high, which limits left-to-right blood shunting. Symptoms appear later when pulmonary circulation resistance drops.
How does a restrictive VSD differ from a non-restrictive VSD on echocardiography?
In a restrictive defect, high-velocity flow is recorded (pressure gradient is preserved). In a non-restrictive defect, velocities are low as pressures in the right and left ventricles equalize.
Is echocardiography sufficient for accurate shunt volume measurement?
No, EchoCG assesses the shunt only qualitatively (by chamber dilatation). Accurate quantitative measurement requires cardiac MRI with phase-contrast mapping.
What else to study on this topic
- Phase-contrast velocity mapping during cardiac MRI.
- Hemodynamic features and imaging of patent ductus arteriosus (PDA).
- Anatomical picture of Tetralogy of Fallot (VSD and over-riding aorta).
- 3D b-SSFP techniques with isotropic resolution in cardiac imaging.
- Assessment of VSD patch integrity in the postoperative period.
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