title: Rheologic behavior of sickle and normal red blood cell mixtures in sickle plasma: implications for transfusion therapy.
abstract: BACKGROUND
  Guidelines for transfusion in sickle cell disease usually define an upper hematocrit (Hct) limit of 0.30 to 0.35 to avoid blood hyperviscosity. In vitro viscosity studies of normal (AA) and sickle (SS) red blood cell (RBC) mixtures in buffer appear to confirm that this Hct limit is optimal for oxygen delivery to vascular beds as judged by the ratio of Hct to viscosity, with this ratio often termed "oxygen or RBC transport effectiveness." In the absence of plasma, however, effects due to RBC-RBC interactions mediated by plasma proteins cannot be assessed.
  
  
  METHODS
  To investigate the optimal Hct-to-viscosity ratio of RBCs in plasma, the rheologic effects of Hct (0.20-0.40), the proportion of SS RBCs (0-100%), and shear rate (1-1000/sec) for mixtures of oxygenated and deoxygenated SS and AA RBCs were evaluated in sickle plasma at 37 degrees C.
  
  
  RESULTS
  RBC suspension viscosity was shear-dependent (i.e., viscosity decreased with increasing shear rate) and increased with Hct and proportion of SS RBCs. An "optimal" Hct level (defined as a maximal of the Hct-to-viscosity ratio) was seen only at shear rates above 50/sec. At lower shear rates (e.g., 5/sec), where plasma-mediated RBC-RBC interactions predominate, any increment in Hct was offset by a proportionally greater increase in viscosity, thus leading to a lower Hct-to-viscosity ratio.
  
  
  CONCLUSIONS
  These results indicate the importance of plasma-mediated RBC interactions and suggest that the benefits of transfusion may vary depending on local flow rates (i.e., shear rates) and organ-specific hemodynamics.
authors.affiliation: Department of Physiology and Biophysics, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, USA.
authors.forename: Tamas
authors.initials: T
authors.lastname: Alexy
authors.forename: Eszter
authors.initials: E
authors.lastname: Pais
authors.forename: Jonathan K
authors.initials: JK
authors.lastname: Armstrong
authors.forename: Herbert J
authors.initials: HJ
authors.lastname: Meiselman
authors.forename: Cage S
authors.initials: CS
authors.lastname: Johnson
authors.forename: Timothy C
authors.initials: TC
authors.lastname: Fisher
mesh_terms: D000755:Anemia, Sickle Cell / Q000097:blood*; D001803:Blood Transfusion / Q000379:methods*; D001809:Blood Viscosity; D004912:Erythrocytes; D004913:Erythrocytes, Abnormal; D006400:Hematocrit; D018056:Hemorheology; D006801:Humans; D010100:Oxygen / Q000378:metabolism
chemical_list: D010100:Oxygen