Development of the outflow tracts with reference to aortopulmonary windows and aortoventricular tunnels.

Although malformations involving the ventricular outflow tracts are often described as conotruncal malformations, there is no consensus as to the lesions included in, or excluded from, this category, reflecting, in part, the current lack of precise definitions of the embryonic truncus and conus. Analysis of development of the outflow tract in terms of proximal, intermediate, and distal components greatly facilitates understanding of the morphology of the aortopulmonary window and aortoventricular tunnels. The aortopulmonary windows reflect failure to close the embryonic aortopulmonary foramen, the space between the distal end of the cushions that divide the lumen of the outflow tract itself and the dorsal wall of the aortic sac. The aortopulmonary tunnels are produced subsequent to abnormal development of the cushions themselves. The distal ends of these cushions excavate to produce the sinuses and leaflets of the arterial valves. The proximal parts of the cushions muscularise to form the subpulmonary infundibulum. The middle part of the cushion mass disappears to provide a tissue plane between the infundibulum and the aortic root. Abnormal formation of this area accounts for the various types of aortoventricular tunnel. In our brief review, we show how the anatomy of these lesions correlates with development of the outflow tract.