图示永存动脉干，见于闭合不全以及动脉干和动脉圆锥(正常情况下将主动脉和肺动脉分开)的旋脊下降不够。一旦螺旋形隔板下降不全，主动脉和肺动脉干的血流将不能分开。这造成左右心室的动脉骑跨。永存动脉干常常伴发膜性室间隔缺损。
The diagram above depicts the findings with a persistent truncus arteriosus. This occurs when there is failure of fusion and descent of the spiral ridges of the truncus and conus that would ordinarily divide into aorta and pulmonic trunck respectively. When the spiral septum fails to completely descend, the aortic and pulmonic trunks are left undivided at their outflow.The truncus overrides both ventricles.The persistent truncus is always accompanied by a membranous ventricular septal defect.