FIG 32.1 Situation encountered in tetralogy of Fallot, when the aortic trunk overrides the crest of the muscular ventricular septum There are three important planes within the cone of space subtended beneath the leaflets of the overriding aortic valve, which represents the channel between the ventricles The left ventricular margin, shown in yellow, is the effective outlet for the left ventricle The cranial continuation of the long axis of the ventricular septum, shown in green, is the geometric interventricular communication The right ventricular margin, shown in red, is the area closed surgically to connect the aorta with the left ventricle This is the area that, in tetralogy of Fallot, is described as the ventricular septal defect FIG 32.2 Arrangement of the channel between the ventricles in the setting of double-outlet right ventricle This channel, in tetralogy of Fallot, represents the outlet for the left ventricle (LV), which is now the interventricular communication It extends between the crest of the ventricular setpum and the inner heart curvature (or ventriculoinfundibular fold) In double-outlet right ventricle, the outlet septum is exclusively a right ventricular structure, as it is in tetralogy of Fallot It is the area between the crest of the ventricular septum and the outlet septum, shown by the oval with dotted borders, which is equivalent to the ventricular septal defect as described in tetralogy (see Chapter 35) Most textbooks, nonetheless, continue to describe the interventricular communication in this setting as the ventricular septal defect This chapter is concerned with the situation in patients with concordant atrioventricular and ventriculoarterial connections Our approach to definition and classification is based on several anatomic principles (Box 32.1) We first define the margins of the area chosen to represent the defect Having defined this area, our second principle is to account for all its various anatomic features We take the stance that the most important features are its boundaries, since these features determine the phenotypic categorization We recognize the necessity also of describing the position of the hole relative to the landmarks and components of the ventricular septum, including the surgically significant atrioventricular conduction axis If present, we take account of any malalignment between the different septal components Attention must also be given to the size of the channel chosen to represent the defect These principles also underscore the categorization that will appear in the 11th iteration of the International Classification of Congenital Heart Disease established by the World Health Organization, known as ICD-11 In this categorization, nonetheless, the geographic location of the channels is used as the primary feature, with the phenotypic variants described as secondary features Box 32.1 Criteria for Classification of a Ventricular Septal Defect Define the area of the defect Define the boundaries of the defect: perimembranous, muscular, or doubly committed and juxtaarterial ■ The relation of the defect to the atrioventricular conduction axis ■ The relation of the defect to the atrioventricular valves ■ The relation of the defect to the arterial valves Define the geographic location of the defect relative to the right ventricular aspect of the ventricular septum: opening to the inlet, apical trabecular, or outlet parts of the right ventricle Define the presence of septal malalignment: subpulmonary outlet septum, or atrial and muscular ventricular septal malalignment Define the size of the defect What Is the Defect? When there is a simple hole, punched as it were into the substance of the muscular ventricular septum or representing divorce of the muscular septum from the freestanding subpulmonary infundibulum, there is no problem in defining its margins or in agreeing that these margins are exclusively muscular (Fig 32.3, left) The majority of channels, however, have the crest of the muscular ventricular septum as one of the margins, but overridden to some extent by the leaflets of the arterial or atrioventricular valves or these valves in combination (see Fig 32.3, right) In the latter situation, the areas forming the right ventricular border are of greatest practical importance, since it is around these margins that the surgeon will usually place a patch to restore septal integrity We therefore define this area as the “ventricular septal defect.” The differences in the anatomic make-up of the margins of this area, as viewed from the morphologically right ventricle, provide the means of differentiating between the phenotypes