Absent sixth aortic arch: a form of pulmonary atresia.

Pulmonary atresia ischaracterized bydevelopmental abnormalities ofthepulmonary valve or pulmonary arterial tree, withcomplete obstruction toblood flowbythenormal route.We recognize three maintypes, basedonembryology, anatomy, andsuitability forsurgical treatment. Pulmonary atresia asanelement ofamorecomplex lesion such astransposition ofthegreat vessels ortricuspid atresia isclassified undertheprimary abnormality. (1)Pulmonary ValveFusion withIntact Interventricular Septum: embryologically thisisthe extreme formofsimple pulmonary valve stenosis, withcomplete fusion ofthepulmonary valve cusps. Aninteratrial communication ispresent, usually a patent foramen ovale, andbloodreaches thepulmonary arterial tree distal totheobstructed pulmonaryvalve bywayofapatent ductus arteriosus. (2)Extreme Fallot's Tetralogy withPulmonary Atresia: basically theanatomy isthat ofthetetralogy ofFallot, withalarge ventricular septal defect and dextroposed aorta, butthepulmonary valve and sometimes themainpulmonary artery aswell are poorly developed andtotally obstructed. Right andleft branches ofthepulmonary artery arepresent andcommunicate witheachother across themidline.Blood reaches these vessels viaapatent ductus arteriosus orbronchial collateral vessels orboth. Taussig (1947) originally confined this termtocases withafunctioning patent ductus arteriosus, butwe extend ittothose withbronchial collateral vessels as anadditional oralternative source ofbloodsupply to thebranches ofthepulmonary artery.