Introduction
Ventricular septal defects (VSDs) are the commonest congenital heart defect, accounting for 25 to 30 per cent of all congenital heart disease. However, the majority are small defects that either close spontaneously or are of no clinical significance. The clinical implications depend on their size, number and location in the ventricular septum. They occur as isolated lesions or in combination with other anomalies. They are the commonest cardiac defect found in chromosomal abnormalities such as trisomies 13, 18 and 21 and 22q11 deletion, but 95 per cent of VSDs are found in patients with normal chromosomes.
Morphology
From a surgical standpoint, the location of a VSD is important as it defines the approach to closing it and alerts the surgeon as to the location of conduction fibres that may be damaged when closing the VSD. VSDs can most simply be classified as follows (Figures 9.1 to 9.3):
Figure 9.1 3D model of the heart with the free wall of the right atrium and ventricle removed. Arrows show the positions of commonly occurring VSDs.
Figure 9.2 Representation of the ventricular septum viewed from the right ventricular side. The areas of the septum can be approximately divided into three regions: the inlet portion, below the level of the tricuspid valve; the outlet portion, above the level of the tricuspid valve; and the apical trabecular area.
Figure 9.3 The ventricular septum viewed from the right ventricular side showing the positions of commonly found VSDs.
Peri-membranous VSDs.
These occur where a margin of the VSD consists of fibrous continuity between the tricuspid and aortic valves. The conduction bundles run along the inferior rim of the defect. They may be inlet, outlet or inlet-outlet VSDs. They account for about 80 per cent of VSDs, requiring surgical closure in some series. They may cause aortic valve regurgitation secondary to right or non–coronary cusp prolapse into the defect. (They are also called ‘infracristal’ or ‘membranous’.)
Juxta-arterial or Doubly Committed Sub-arterial VSDs.
These occur where the conjoined leaflets of the aortic and pulmonary valves form the rim of the VSD. The conduction bundles are remote from the defect. (They are also called ‘supracristal’, ‘conal’ or ‘infundibular’.) They may also cause aortic insufficiency by right coronary cusp prolapse. They constitute about 5 per cent of VSDs seen in the West but are more common in Asian countries. They do not close spontaneously.
Muscular VSDs.
These are the commonest type of VSD, and most close spontaneously. The rims of the VSD are entirely muscular, and they can occur anywhere within the ventricular septum, being described by their position as inlet, apicotrabecular or outlet defects. They can be multiple. The conduction bundles are remote from the defect, and in the case of the inlet VSD, they run near the superior margin of the defect. Larger defects may require surgical closure, accounting for 5 to 10 per cent of VSDs closed surgically.
Pathophysiology and Natural History
Restrictive defect.
Small defects presenting resistance to flow across the defect, with pulmonary-to-systemic blood flow ratio Qp:Qs < 1.5.
Non-restrictive Defect.
This occurs where the cross-sectional area of the defect is equivalent to or larger than that of the aortic annulus. There is no resistance to flow across the defect, the right and left ventricular pressures approach parity and the Qp:Qs is inversely proportional to the ratio of the pulmonary-to-systemic vascular resistances.
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