FIG 51.12 (A) When viewed externally, this image suggests a small aortopulmonary window (arrow) (B) However, opening the aorta shows that the area indicated by the arrow is no more that the aortic origin of the left pulmonary artery, which has no direct connection with the cavity of the pulmonary trunk The pulmonary trunk feeds only the right pulmonary artery The findings in this case point to the likely explanation for both AP window and anomalous aortic origin of a pulmonary artery (AOPA) based on anomalous formation of the protrusion from the dorsal wall of the aortic sac In terms of nomenclature, it has been frequent, in the past, to find aortic origin of one PA described as “hemitruncus.” This term is inappropriate because the phenotypic feature of the common arterial trunk is the presence of a common ventriculoarterial junction PAs arising directly from the aorta are found in the setting of separate arterial roots (see Figs 51.11 and 51.12) AOPA is a rare anomaly characterized by the origin of a PA directly from the ascending aorta, while the other PA arises from the main PA normally Importantly, in this anomaly there is complete septation of the primitive arterial trunk into an ascending aorta and a main PA, with separate aortic and pulmonary valves Because of variable terminologies used in the past, it is important that this malformation be distinguished from similar, but distinct, anomalies: ■ In ductal origin of a PA, one or both of the PAs arise via an arterial duct from the base of a subclavian or innominate artery or from the underside of the aortic arch, with otherwise normal truncal septation ■ In common arterial trunk, there is absence of truncal septation, such that there is a single semilunar valve (i.e., the truncal valve) while the main PA or both branch PAs arise from the common arterial trunk ■ Persistent fifth aortic arch refers to an anomaly where truncal septation has occurred but one of the branch PAs arises via a fifth aortic arch (a variant of the arterial duct that is subject to similar postnatal constriction) generally from the distal ascending aorta Accordingly, with postnatal constriction, the ostium of the affected PA may become stenotic, whereas in AOPA the affected PA is almost never stenotic and is therefore exposed to systemic pressures ■ Finally, in some cases of PA/VSD and major AP collateral arteries, an entire lung may be occasionally supplied by a large, solitary direct collateral artery that may resemble a PA branch arising from the descending aorta However, an important distinction is that a true branch PA (i.e., a derivative of the proximal sixth aortic arch) cannot arise from the descending aorta, and similarly a direct AP collateral (i.e., a derivative of the primitive intersegmental arteries that arise from the dorsal aorta) cannot arise from the ascending aorta Hence, if a branch PA appears to arise from the ascending aorta with otherwise normal truncal septation present, the most likely diagnoses are either AOPA or a persistent fifth aortic arch, with postnatal constriction of the ostium of the affected PA being a hallmark of the latter condition According to Griffiths36 the initial description of AOPA has been ascribed to Fraentzel in 1868; however, the coexistence of an AP window in this description casts doubt on the veracity of this report as a true case of AOPA One of the earliest reliable reports of AOPA is that of Keane,37 in which a surgically corrected case is described