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III/2.4: Tetralogy of Fallot as a special form of DORV
III/2.4.1: General description
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Tetralogy of Fallot was first described as a complex by Etienne Fallot in 1888. Classical elements of the tetralogy include ventricular septal defect (VSD), right ventricular outflow tract stenosis, right shifted and therefore ’overriding’ aorta on top of the septum, and right ventricular hypertrophy. These four malformations can be traced back to one morphological alteration: the infundibular septum shifts to the anterior and left. If atrial septal defect is also present then the diagnosis becomes pentalogy of Fallot.
III/2.4.2: Ventricular septal defect (VSD)
VSD in tetralogy of Fallot is a defect large in size and not stenotic in character, and caused by the dislocation between the left and anterior shifted infundibular septal part and the lower muscular septum (septomarginal trabecula). The VSD is bordered by the anteriorly rotated right (parietal) process of the infundibular septum from superior, the antrior-septal cusp of the tricuspid valve from posterior, the posterior bundle of the septomarginal trabecula from below, and the anterior bundle from the front. The bundle of His pierces the septum at the posterior-inferior part of the defect. Although most of the VSD is sub-aortic, it might extend to the sub-pulmonary area if the infundibular septum is deficient partially or in total. Multiple ventricular septal defects occur in cca. 5% of patients, usually in the muscular septum.
III/2.4.3: Right outflow tract stenosis
The left and anterior shift of the infundibular septum leads to obstruction of the right ventricular outflow tract due to hypoplasia of the right ventricular infundibulum. Strong muscular bundles stretch between the septal process of the infundibular septum and the free wall of the right ventricle thus aggravating the outflow tract stenosis. A localised stenosis called os infundibuli often develops at the inferior border of the infundibular septum. The pulmonary valve almost always take part in the stenosis formation, its cusps are thickened, they may irregularly adhere to the pulmonary trunk wall and they are bicuspid in 58 percent of cases. The annulus of the pulmonary valve is almost always narrower than regularly. Severe stenosis may develop at the level of the left and right pulmonary arteries.
Rarely, the left pulmonary artery originates from the ductus Botalli with no intrapericardial section whatsoever, but the localised stenosis of the initial segment of the right or left pulmonary artery is seen more often. In extreme cases the anterior shift of the infundibular septum leads to complete atresia of the distal part of the right ventricular infundibulum and the pulmonary trunk which stars there. Pulmonary atresia is present in 7 percent of all tetralogy of Fallot cases where – if the ductus Botalli is not patent – so-called major aortopulmonary colletaral arteries (MAPCA) supply blood for the pulmonary circulation with varying efficacy.
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While in default of pulmonary atresia the pulmonary stenosis is usually central and moderate, patients with pulmonary atresia and MAPCA typically develop some kind of peripheral pulmonary stenosis. In cca. 5% of tetralogy of Fallot cases the pulmonary valve cusps are completely missing. In these cases the right ventricular outflow tract stenosis forms at the level of the hypoplastic annulus which leads to severe pulmonary regurgitation in the same time. Sometimes it is accompanied by aneurysm-like dilation of the pulmonary trunk or the right and left pulmonary artery and this may exert pressure on the lower tracheobronchial airway segments.
III/2.4.4: Further disorders: coronary and other structural heart abnormalities
The anterior interventricular coronary branch (left anterior descending artery, LAD) originates from the right coronary artery in 3-5% of tetralogy of Fallot cases. In this case the branch reaches the anterior part of the ventricular septum by crossing the right ventricular outflow tract somewhat below the annulus of the pulmonary trunk valve. Rarely, the whole left coronary system originates from a branch of the only right coronary artery, and it crosses the right ventricular outflow tract to the left. Other significant heart malformations are rare in tetralogy of Fallot, these include e.g. atrial septal defect, patent ductus arteriosus, complete atrioventricular septal defect, multiple ventricular septal defects, or very rarely persistent left superior vena cava, other aberrant origin of the anterior interventricular branch, or abnormal origin of the right or left pulmonary artery.
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