any anomaly in the process in which a developing blood vessel forms an endothelial lumen through which blood will flow; in vertebrates, the lumen of all blood vessels is lined and formed by endothelial cells; in both vasculogenesis and angiogenesis, lumen formation takes place in a cord of endothelial cells and involves a complex mechanism composed of endothelial cell repulsion at the cell-cell contacts within the endothelial cell cords, junctional rearrangement, and endothelial cell shape change