Acute decrease in renal microvascular Po2 during acute normovolemic hemodilution

Large differences in the tolerance of organ systems to conditions of decreased O(2) delivery such as hemodilution exist. The kidney receives similar to 25% of the cardiac output and O(2) delivery is in excess of the oxygen demand under normal circumstances. In a rat model of acute normovolemic hemodilution (ANH), we studied the effect of reduced hematocrit on renal regional and microvascular oxygenation. Experiments were performed in 12 anesthetized male Wistar rats. Six animals underwent four steps of ANH ( hematocrit 25, 15, 10, and < 10%). Six animals served as time-matched controls. Systemic and renal hemodynamic and oxygenation parameters were monitored. Renal cortical (c) and outer medullary ( m) microvascular PO(2) ( mu PO2) and the renal venous PO(2) (P(rv)O(2)) were continuously measured by oxygen-dependent quenching of phosphorescence. Despite a significant increase in renal blood flow in the first two steps of ANH, c mu PO(2) and m mu PO(2) dropped immediately. From the first step onward oxygen consumption (VO(2)ren) became dependent on oxygen delivery (DO(2ren)). With a progressive decrease in hematocrit, a significant correlation between mu P(O2) and VO(2ren) could be observed, as well as a PO(2) gap between mu PO(2) and PrvO(2). Furthermore, there was a high correlation between VO(2ren) and RBF over a wide range of flows. In conclusion, the oxygen supply to the renal tissue is becoming critical already in an early stage of ANH due to the combination of increased VO(2ren), decreased DO(2ren), and intrarenal O(2) shunt. This has clinical relevance as recent publications reporting that hemodilution during surgery forms a risk factor for postoperative renal dysfunction.