As discussed above, the interventricular communication
found in tetralogy of Fallot exists because of the anterior
and cephalad malalignment of the outlet portion of the
muscular ventricular septum, or of its fibrous remnant
should the outflow cushions fail to muscularise during
embryonic development. The resulting hole is one of a
number of those appropriately described as a malalignment
defect. In four-fifths of Caucasians with such a
defect, the postero-inferior margin of the hole between
the ventricles is formed by an area of fibrous continuity
between the leaflets of the aortic and tricuspid valves, also
involving the remnant of the interventricular portion of
the membranous septum [5]. In these patients, therefore,
the defect is also appropriately classified as being perimembranous
(Figure 2). In the remaining one-fifth of
Caucasian patients, the postero-inferior rim of the defect
is muscular. The muscular bar is formed by continuity of
the postero-inferior limb of the septomarginal trabeculation
with the ventriculo-infundibular fold (Figure 3). The
muscular structure thus formed protects the ventricular
conduction axis during surgical closure of the defect.