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.