title: Anemia increases the risk of renal cortical and medullary hypoxia during cardiopulmonary bypass.
abstract: BACKGROUND
  Anemia is an independent predictor of acute kidney injury (AKI) following cardiopulmonary bypass (CPB), possibly due to inadequate renal oxygen delivery. The objective of this study was to investigate the effects of CPB and anemia on tissue oxygen tension (pO2) and blood flow in the renal cortex and medulla.
  
  
  METHODS
  Rats (n=6/group) underwent 1 hr of normothermic cardiopulmonary bypass (CPB), with target hemoglobin concentrations (Hb) of 10 g/dL (CPB) or 6.5 g/dL (anemia-CPB). Renal blood flow (RBF) and tissue PO2 were measured before, during and after 1 hr of CPB. To confirm the observed differences in renal cortical and medullary PO2, HIF-1α (ODD) luciferase mice were exposed to 8% O2 (hypoxia) and HIF-1α dependent luminescence was measured in the renal cortex and medulla (n=5).
  
  
  RESULTS
  Renal tissue PO2 values decreased initially and returned towards baseline, however, values at the end of CPB. Anemia-CPB resulted in a significant increase in both renal cortical and medullary blood flow, PO2 remained significantly reduced throughout anemia-CPB. Renal medullary HIF-1α-dependent luminescence confirmed a greater degree of hypoxia in the renal medulla.
  
  
  CONCLUSIONS
  During CPB, renal O2 delivery was transiently jeopardized, but recovered after 1 hr. Anemia-CPB resulted in a dramatic and sustained reduction in renal cortical and medullary PO2, which suggests an increased risk of renal hypoxic injury with anemia.
  
  
  CONCLUSIONS
  The clear difference in the degree of hypoxia in the renal cortex and medulla may be useful in understanding the progress of medullary hypoxia during CPB with anemia and the potential development of AKI. Further studies should aim at identifying early markers of medullary hypoxia and potential agents that may decrease the work and O2 consumption in the renal medulla to reduce the risk of hypoxic damage during CPB and anemia.
authors.affiliation: 1Department of Anesthesia, Keenan Research Centre in the Li Ka Shing Knowledge Institute, St. Michael's Hospital, Toronto, Ontario, Canada.
authors.forename: P J
authors.initials: PJ
authors.lastname: Darby
authors.forename: N
authors.initials: N
authors.lastname: Kim
authors.forename: G M T
authors.initials: GM
authors.lastname: Hare
authors.forename: A
authors.initials: A
authors.lastname: Tsui
authors.forename: Z
authors.initials: Z
authors.lastname: Wang
authors.forename: A
authors.initials: A
authors.lastname: Harrington
authors.forename: C D
authors.initials: CD
authors.lastname: Mazer
mesh_terms: D058186:Acute Kidney Injury / Q000209:etiology* / Q000378:metabolism / Q000473:pathology; D000740:Anemia / Q000097:blood / Q000378:metabolism* / Q000473:pathology; D000818:Animals; D002315:Cardiopulmonary Bypass / Q000009:adverse effects*; D015687:Cell Hypoxia / Q000502:physiology; D007672:Kidney Cortex / Q000098:blood supply* / Q000378:metabolism / Q000473:pathology; D007679:Kidney Medulla / Q000098:blood supply* / Q000378:metabolism / Q000473:pathology; D051379:Mice; D010100:Oxygen / Q000097:blood / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012307:Risk Factors
chemical_list: D010100:Oxygen
keywords: anemia; cardiopulmonary bypass; hypoxia; renal medulla; tissue oxygen tension