Ventricular Septal Defects

VSD of the AV Canal Type.

The septum of the AV canal (see Fig. 16.1 , component 1) is completely absent in the complete form of common AV canal.

In the incomplete form of common AV canal, the septum of the AV canal is absent above the leaflets of the AV valve(s) but present below the leaflets of the common AV valve. In other words, typically there is no VSD of the AV canal type in the incomplete form of common AV canal. Also typically, there is a cleft in the anterior leaflet of the mitral valve.

The incomplete form of common AV canal is also known as an ostium primum defect with a cleft anterior mitral leaflet. Accurately speaking, an ostium primum defect is an incomplete AV septal defect, not an atrial septal defect (ASD). The posterosuperior margin of an ostium primum defect is the anteroinferior margin of the atrial septum. Ostium primum defects have often been called, inaccurately, ostium primum atrial septal defects because the associated shunt (left to right, or right to left) is above the leaflets of the AV valve(s)—like an ASD. However, it is helpful to know that many different types of shunt occur above the leaflets of the AV valve(s). Ostium primum defects and ASDs are just two of many different anatomic types of abnormal communication that can occur above the AV valve(s).

Straddling tricuspid valve also has a VSD of the AV canal type. Typically, the right ventricular sinus (body or inflow tract) is underdeveloped compared with the left ventricular sinus. Consequently, the muscular ventricular septum (see Fig. 16.1 , component 2) is located well to the right of the normally located atrial septum. The muscular ventricular septum lies beneath approximately the middle of the normally located tricuspid orifice. This ventriculoatrial septal malalignment results in a straddling tricuspid valve with biventricular insertions of the tricuspid tensor apparatus, a VSD of the AV canal type between components 1 and 2 (see Fig. 16.1 ), and often with no septal defect above the level of the AV valves.

A defect in the septum of the AV canal (see Fig. 16.1 , component 1) is also known as an AV septal defect. AV septal defects occur both with and without common AV canal. An AV septal defect is an integral part of common AV canal, both complete and partial forms.

But an AV septal defect also occurs without common AV canal, as in a Gerbode defect. There is a left ventricle (LV)–to–right atrium (RA) shunt through a defect in the AV portion of the membranous ventricular septum, predominantly above the tricuspid valve. Megarity et al published a well-documented case of double-outlet right ventricle (DORV) with no VSD and with an LV-to-RA shunt. This patient did not have a common AV canal.

We avoid the term AV septal defect because of its confused usage. Some of our colleagues use this designation when they mean common AV canal. For clarity, we prefer the terms common AV canal and LV-to-RA shunt, because these terms make their different meanings clear.

We also avoid the designation inlet septal defect because this term is not specific. This term may apply to component 1, or to component 2, or to the component 1-2 junction (see Fig. 16.1 ). We prefer anatomically specific diagnoses in the interests of accuracy and clarity: defect in the septum of the AV canal, defect in the muscular ventricular septum, or defect at the junction of these two septal components. The latter diagnoses are anatomically accurate and their meanings are clear.

Muscular VSDs.

Muscular VSDs are openings in the muscular ventricular septum, also known as the ventricular sinus septum. Muscular VSDs, which is what these defects have long been called, involve component 2 (see Fig. 16.1 ).

Mid-muscular VSDs are often located at the junction of components 2 and 3 (see Fig. 16.1 ). From the right ventricular aspect, such apertures are seen slightly above or slightly below the septal band (see Fig. 16.1A , component 3). From the left ventricular aspect, mid-muscular VSDs typically are found at the junction of the smooth or nontrabeculated portion of the ventricular septum superiorly (see Fig. 16.1B , component 3) and the finely trabeculated part of the ventricular septum inferiorly (see Fig. 16.1B , component 2). These mid-muscular VSDs are sometimes called trabecular defects. Why? Because they are close to the septal band, as noted earlier. Some of our colleagues call the septal band the trabecular septomarginalis or the septomarginal trabeculation. The term trabecular septomarginalis was introduced in 1911 by Tandler for the structure that we now call the moderator band. Trabecula septomarginalis (Latin) means “little beam” ( trabecular ) that runs from the septum (septo) to the acute margin of the RV (marginalis). So, the trabecular septomarginalis , or septomarginal trabeculation, means the moderator band, not the septal band. But those who revived Tandler’s term , trabecular septomarginalis, being unfamiliar with Latin, did not understand what Tandler’s term really meant. So, they applied trabecular septomarginalis and its English equivalent septomarginal trabeculation to the septal band and the moderator band.

The foregoing is intended as an explanation, not as a criticism. The history of language is full of similar changes. Indeed, in terminology, the only constant we know is change. But it should be slow change, and necessary change, to minimize confusion and linguistic errors. What do we think is necessary terminologic change?

• 1.

  If there is no extant term, the introduction of a designation may be necessary.

• 2.

  If the extant term (or terms) is (are) inaccurate, a new and accurate designation may be necessary.

The foregoing explains why mid-muscular VSDs are called trabecular VSDs by some of our colleagues: trabecular comes from trabecula septomarginalis, or septomarginal trabeculations, that is, from the renaming of the septal band by some of our colleagues (see Fig. 16.1A ).

Muscular VSDs can be found in many different locations within component 2, and at its junctions with components 1 and/or 3 (see Fig. 16.1 ). Mid-muscular VSDs do not involve component 3, as seen from the right ventricular aspect (see Fig. 16.1A ). Instead, mid-muscular VSDs lie behind and to the left of the septal band.

Ironically, the muscular ventricular septum (see Fig. 16.1A–B , component 2) is trabeculated. The right ventricular septal surface is coarsely trabeculated (see Fig. 16.1A , component 2), whereas the left ventricular surface (see Fig. 16.1B , component 2) is finely trabeculated. Thus, although component 3 is not trabeculated per se (see Fig. 16.1A–B ), component 2 is coarsely trabeculated on the right ventricular side and finely trabeculated on the left ventricular side (see Fig. 16.1A–B ).

Infundibuloventricular Defects.

Infundibuloventricular defects lie between the infundibular (or conal) septum above (see Fig. 16.1A , component 4, and B) and the ventricular septum below (see Fig. 16.1A–B , components 1, 2, and 3). Infundibuloventricular defects are also known as conoventricular VSDs. The infundibular or conal septum (see Fig. 16.1A–B component 4) is the “lid” that normally fits on top of and seals the ventricular septal complex :

• 1.

  the septum of the AV canal (see Fig. 16.1A–B , component 1), the top of which in the normally formed heart is called the membranous septum;

• 2.

  the muscular ventricular sinus septum (see Fig. 16.1A–B component 2); and

• 3.

  the septal band (see Fig. 16.1A , component 3), and the proximal infundibular or outflow tract septum (see Fig. 16.1B , component 3).

Thus, the ventricular septal complex is a tetralogy: components 1 to 4, inclusive (see Fig. 16.1A–B ). To achieve a normal heart, each of these four septal components must be normally formed, normally aligned, and normally connected.

Infundibular Septal Defects.

Infundibular septal defects result from an anomaly of, or within, component 4 (see Fig. 16.1A–B ). These malformations are also known as conal septal defects.