Red cell trapping and postischemic renal blood flow. Differences between the cortex, outer and inner medulla.

The distribution of blood flow in the rat kidney after 60 minutes of renal ischemia was studied by single-fiber laser-Doppler flowmetry. Blood flow in superficial cortex and inner medulla was measured with a probe directed towards the kidney surface and exposed papilla, respectively. Outer medullary blood flow was measured with a probe introduced through the renal core. After ischemia the blood flow decreased to 60% of the preischemic value (P less than 0.01) in superficial cortex and to 16% (P less than 0.01) in outer medulla, while inner medullary blood flow increased paradoxically to 125% (P less than 0.01). There was extensive trapping of red blood cells (RBC) in the outer medulla, but not in the inner medulla or cortex. The fractional RBC volume as measured by radiolabeled RBCs was 21% in the inner stripe of the outer medulla, but 2% in this area in a normal kidney. To investigate the influence of RBC trapping on intrarenal distribution of blood flow after ischemia, the hematocrit was reduced from 46% to 31% by isovolemic hemodilution. When performed before ischemia, this maneuver almost completely abolished RBC trapping. In this group blood flow in both outer and inner medulla was almost unchanged after ischemia, while superficial cortical blood flow decreased to 66% (P less than 0.01) of the pre-ischemic value. It is concluded that RBC trapping in the outer medulla causes a large decrease in blood flow in this area and, at the same time, shunting of blood to the inner medulla. In the absence of RBC trapping, blood flow of both outer and inner medulla is well preserved after ischemia.