Andersons pediatric cardiology 926

interventricular communication, representing the cranial continuation of the muscular ventricular septum The green disk is the ventricular septal defect, since this is the locus along which the surgeon places the patch to reconnect the aorta to the left ventricle When describing the boundaries of these areas, we concentrate on the margins of the cone as viewed from the right ventricle, since this is the locus along which the surgeon will attach the patch used to repair the malformation.2 This area of putative ventricular septation is therefore defined as the ventricular septal defect It has important variability not only in its roof (see Fig 35.6) but also in its posteroinferior quadrant In about four-fifths of cases, this margin is formed by fibrous continuity between the leaflets of the aortic, mitral, and tricuspid valves (Fig 35.8, left) The defect therefore is not only an outlet defect in this setting but is also perimembranous It is directly comparable to typical perimembranous defects opening to the outlet of the right ventricle in the absence of subpulmonary obstruction (see Fig 35.4) This means that the atrioventricular conduction axis is at risk in this fibrous posteroinferior margin (see Fig 35.8, left) FIG 35.8 Significant differences in the posteroinferior margin of the outlet defect found in the setting of tetralogy of Fallot Left, Arrangement as found in approximately four-fifths of cases, in which the presence of fibrous continuity between the leaflets of the aortic and tricuspid valves makes the defect perimembranous The atrioventricular conduction axis is at direct surgical risk in the posteroinferior corner of the defect (dotted line), with this area often reinforced by the so-called membranous flap Right, Arrangement in the remaining one-fifth of cases Fusion of the caudal limb of the septomarginal trabeculation with the ventriculoinfundibular fold produces a muscular posteroinferior rim, which protects the conduction axis The yellow bars show the septomarginal trabeculation The bundle in the posteroinferior corner is frequently overlaid by a remnant of the interventricular membranous septum, which may, on occasion, become aneurysmal The septal remnant itself, called the membranous flap, is safe tissue for anchorage of sutures when such stitches are placed with care.9 Sutures placed deeply in this area can produce complete heart block It is therefore safer to place sutures through the leaflet of the tricuspid valve, which usually overlaps the membranous flap in this area of the defect Having perforated, the nonbranching atrioventricular bundle enters the left ventricular part of the aortic outflow tract and almost always veers away from the septal crest, the branching atrioventricular bundle being carried on the left ventricular aspect of the septum and staying remote from the septal crest In a minority of hearts, the bundle can branch directly astride the septum (Fig 35.9) Such an arrangement places the bundle at risk if sutures are placed into the crest of the septum FIG 35.9 Left ventricular aspect of a heart in which sutures were placed through the crest of the muscular ventricular septum, which was presumed to be a safe area However, this traumatized the conduction axis, as shown in the inset The white arrow in the inset shows the “bite” taken by the suture on the right ventricular aspect; the red arrowhead points to the hemorrhage in the conduction axis, as seen in the main image The second most common pattern, occurring in about one-fifth of cases, is characterized by interruption of the area of fibrous continuity between the aortic and tricuspid valves by a muscular strap When viewed from its right ventricular aspect, the outlet defect has exclusively muscular rims The strap itself is formed by fusion of the posterocaudal limb of the septomarginal trabeculation with the ventriculoinfundibular fold (see Fig 35.8, right) An intact membranous septum is found between the muscular strap and the remaining septal structures Since the atrioventricular conduction axis runs posteroinferior to the membranous septum, the muscular fold, together with the membranous septum itself, separates the conduction tissues from the crest of the ventricular septum As already discussed, a third variety of defect is characterized by the presence of a fibrous rather than a muscular outlet septum (see Fig 35.6) These defects are doubly committed and juxta-arterial They are by far the least common variants found in white populations but are more common in the Far East and South America The defect exists because of failure of formation of a completely muscular subpulmonary infundibulum Such defects can be found with fibrous continuity between the leaflets of the aortic and tricuspid valves, making them also perimembranous (see Fig 35.6) More often there is a muscular posteroinferior rim, comparable to the muscular structure seen in the right panel of Fig 35.8 If present, the muscular structure will protect the atrioventricular conduction axis Patients with tetralogy, of necessity, possess one variant of the defects already described, all of which open to the right ventricle between the limbs of the septomarginal trabeculation Additional defects can be found elsewhere in the septum Inlet defects are particularly important, whether they are muscular inlet defects, defects associated with straddling and overriding of the tricuspid valve, or the ventricular component of an atrioventricular septal defect associated with a common atrioventricular junction The septal defect is almost always large in tetralogy, approximating in size to the diameter of the aortic root It can become restrictive owing to the presence of accessory fibrous tissue tags formed at the margins of the defect Such tags may be derived in part from the tricuspid valve or extend from attachment of the tension apparatus of the mitral valve across the left ventricular aspect of the defect Narrowing of the Subpulmonary Infundibulum