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1 Department of Physiology, Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands; Department of Anesthesiology and Critical Care, University Hospital Tuebingen, Tuebingen, Germany
2 Department of Physiology, Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands; Department of Anesthesiology, Erasmus Medical Center, University Rotterdam, Netherlands
3 Department of Anesthesiology and Critical Care, University Hospital Tuebingen, Tuebingen, Germany
4 Department of Physiology, Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands
* To whom correspondence should be addressed. E-mail: t.johannes{at}amc.uva.nl.
Background: Large differences in the tolerance of organ systems to conditions of decreased O2-delivery such as hemodilution exist. The kidney receives approximately 25% of the cardiac output and O2-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. Methods: 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 PO2 (µPO2) and the renal venous PO2 (PrvO2) were continuously measured by oxygen-dependent quenching of phosphorescence. Results: Despite a significant increase in renal blood flow (RBF) in the first two steps of ANH, cµPO2 and mµPO2 dropped immediately. From the first step onwards VO2ren (oxygen consumption) became dependent on DO2ren (oxygen delivery). With a progressive decrease in hematocrit a significant correlation between µPO2 and VO2ren could be observed, as well as a PO2-gap between µPO2 and PrvO2. Furthermore there was a high correlation between VO2ren and RBF over a wide range of flows. Conclusions: The oxygen supply to the renal tissue is becoming critical already in an early stage of ANH due to the combination of increased VO2ren, decreased DO2ren and intra-renal O2-shunt. This has clinical relevance as recent publications reporting that hemodilution during surgery forms a risk factor for postoperative renal dysfunction.
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