Diagram shows two commonly used classification systems for truncus
arteriosus. Top row represents the Collett and Edwards system. Type I
truncus arteriosus is characterized by origin of the main pulmonary trunk
from the truncus, which further divides into the right and left pulmonary
arteries; type II is characterized by the separate origin of the right and
left pulmonary arteries from the posterior aspect of the truncus; type III
is characterized by the separate origin of the right and left pulmonary
arteries from the lateral aspect of the truncus; and type IV represents
pseudotruncus (pulmonary atresia with a ventricular septal defect). Bottom
row represents the Van Praagh system. Types A1 and A2 are equivalent to the
Collett and Edwards types I and II, respectively; type A3 is characterized
by atresia of the left or right pulmonary artery, with collateral flow to
the ipsilateral lung; and type A4 is characterized by the presence of an
associated interrupted aortic arch. (Adapted, with permission, from
references 131 and 132.)