Characteristic Patterns of Pulmonary Arterial Supply The potentially complex situation can be simplified by recognizing three major patterns of pulmonary arterial supply: (1) confluent right and left right pulmonary arteries supplied by an arterial duct, (2) confluent intrapericardial pulmonary arteries coexisting with systemic-to-pulmonary collateral arteries, and (3) absence of intrapericardial pulmonary arteries The most favorable arrangement is that in which the right and left pulmonary arteries are confluent and are supplied by an arterial duct (see Fig 36.3) With this pattern, the pulmonary arteries themselves are usually distributed in normal fashion to all the bronchopulmonary segments Such a pulmonary arterial supply is unifocal In the second major pattern, the intrapericardial pulmonary arteries are confluent but coexist with systemic-to-pulmonary collateral arteries (see Fig 36.6) The distribution of the confluent pulmonary arteries themselves is variable, although it is unusual to find them supplying all the bronchopulmonary segments In most instances the blood passing through the confluence supplies two-thirds or less of the pulmonary parenchyma Even in this setting, the ultimate supply to the pulmonary arteries is via the collateral arteries, although the anastomoses with the intrapericardial network can be found at hilar, lobar, or segmental levels (see Fig 36.6) The confluence of the pulmonary arteries itself also varies markedly in size, reflecting the number of the bronchopulmonary segments supplied In this setting the parts of the lung not supplied by the intrapericardial pulmonary arteries are fed directly by systemic-to-pulmonary collateral arteries (isolated supply), with further variation in the number of arteries present and the amount of lung supplied by each artery In most cases the pulmonary arteries and the collateral arteries do not separately supply different portions of a given lung segment, but in some cases the two sets of arterial ramifications can intermingle within the same segment (Fig 36.9) The third typical pattern of arterial supply is encountered when there is absence of the intrapericardial pulmonary arteries In such circumstances, all of the bronchopulmonary segments are supplied by systemic-to-pulmonary collateral arteries, which are designated as isolated supply In the presence of systemic-to-pulmonary collateral arteries, therefore, the keys to complete clinical diagnosis are to establish the course of each artery, to document whether it runs directly into the lung or makes connections with intrapericardial and central pulmonary arteries, and to identify with precision the sites of these anastomoses FIG 36.9 Dissection of the lower lobe of the right lung from a patient with tetralogy and pulmonary atresia The dissected bronchopulmonary segment has a dual arterial supply with the branches of an intrapericardial pulmonary artery (colored red) and the branches of a systemic-topulmonary collateral artery (colored blue) Morphogenesis Much has been written about the morphogenesis of both the ventricular and pulmonary arterial features of TOF/PA, although derived from speculative embryologic concepts and arguably not improving our understanding From the stance of ventricular morphology, the anomaly is readily explained in terms of end-stage TOF, with variation depending on the specific morphology of the subarterial outlets Some cases, in contrast, can be interpreted as representing a common arterial trunk with absence of the intrapericardial pulmonary arteries, such as those in which a solitary trunk is connected to the ventricular mass in the absence of central pulmonary arteries In the setting of absence of the intrapericardial pulmonary arteries and with no evidence of a subpulmonary infundibulum within the right ventricle, the distinction between a common trunk as opposed to an aorta depends on whether, had the intrapericardial pulmonary arteries been present, they would have taken origin from an arterial trunk or directly from the right ventricle This argument cannot be considered hypothetical, since hearts do exist with an atretic pulmonary trunk arising from an arterial trunk, thus showing that the trunk itself was initially a common structure Studies of rats dosed with bisdiamine, furthermore, have revealed that some fetuses developed TOF, whereas others exhibited a common arterial trunk with PA.8 From the standpoint of description, the conundrum is resolved simply by describing the ascending great artery found in the absence of the pulmonary trunk as a solitary arterial trunk rather than an aorta Embryology has also long been used to explain the typical patterns of pulmonary arterial supply Thus the lungs in TOF/PA are supplied either through the confluence of the pulmonary arteries fed by the arterial duct, itself derived from the embryologic sixth aortic arch, or else through systemic-to-pulmonary collateral arteries.9 Initially the developing intrapulmonary arterial plexus is connected to the primitive intersegmental arteries that in turn, are connected to the arteries coursing through the pharyngeal arches, eventually retaining connection to the arteries of the fourth arch.10 It is suggested that when the intrapulmonary plexus eventually achieves its connection to the sixth arch, it loses its connections with the systemic arterial system.6 The systemic-topulmonary collateral arteries are explained on the basis of persistence of the primitive intersegmental arteries, some of which also become bronchial arteries It is argued that these collateral arteries persist only in the absence of the duct, which is the critical connection between the structures derived from the sixth arch and the aortic sac This concept accounts adequately for the majority of cases and offers an excellent working hypothesis In the majority of cases, nonetheless, these developmental considerations aid in providing an explanation of the complexity of the pulmonary arterial supply In essence, the intraparenchymal pulmonary arteries develop along with the lungs They are the final common pathway supplying arterial blood to the pulmonary air sacs This common pathway can be supplied at the hilum, either by the intrapericardial pulmonary arteries fed through the arterial duct, the derivative of the artery to the sixth pharyngeal arch, by the rarer sources of unifocal supply, or else by systemic-to-pulmonary collateral arteries, which are primitive intersegmental arteries These sources of supply can anastomose with different parts of the lungs in the same patient, although usually all the arteries in one lung are supplied either by the duct or else by the systemic-to-pulmonary collateral arteries The common pathway can subsequently be further enhanced by acquired collateral arteries, which reinforce the acinar supply at precapillary level