Blood flow direction in the lumbar nerve root. The effect of clipping on lumbar nerve root blood flow rates in the region of the nerve root canal was studied experimentally in the hog. Blood flow rate was measured using the hydrogen washout technique. When the entrance zone was clipped with a microvascular clip, blood flow rate of the nerve root was decreased by 37% in comparison with the initial control rate; clipping at the exit zone reduced blood flow rate by 69%. Blood flow direction in the lumbar nerve root within the nerve root canal was found to be predominantly proximal. The current data indicate that the more lateral the impingement of the nerve root occurs, the more ischemic changes are induced. 