Erythrapheresis Improves Blood Viscosity Compared to Manuel Procedure in Sickle Cell Disease

Blood transfusion is a cornerstone of the treatment in sickle cell disease (SCD). Guidelines and clinical trials indicate their use in several acute and chronic SCD manifestations such as cerebral vasculopathy prevention and acute chest syndrome. Two methods of blood exchange transfusion are available for physicians and patients: manual exchange transfusion (MET) and erythrapheresis, i.e. an automated exchange transfusion (AET). MET consists in a phlebotomy followed by a transfusion while erythrapheresis corresponds to the replacement of only sickle RBCs by healthy RBCs by controlling hematocrit. To our knowledge, no study has compared the impact of these two methods on blood viscosity and the ratio of hematocrit to blood viscosity (HVR); i.e., an index of red blood cell oxygen transport effectiveness (Alexy et al, 2006). Herein we aimed to compare those two procedures in term of biological parameters and blood viscosity, in order to offer new physiological parameters to guide the therapeutic management of SCD patients.

This prospective, monocenter, observational study included sickle cell patients, ≥18 years old, treated by Blood Exchange Transfusion (BET) in our university hospital's Adult Sickle-Cell Referral Center. The primary end point was the change in blood viscosity during the BET procedure. Secondary end-points included the change in HVR, blood viscosity and HVR at the end of the procedure. Blood viscosity was measured after full oxygenation of the blood, at native hematocrit and at a shear rate of 225 s^-1using a cone/plate viscometer (Brookfield DVII+ with CPE40 spindle, Brookfield Engineering Labs, Natick, MA, USA) (Baskurt et al, 2009).

This study was approved by the local Institutional Review Board. All patients gave their signed informed consent for the genetic studies in accordance with the Declaration of Helsinki. All data were rendered anonymous to protect patients' privacy and confidentiality.

Twelve patients in AET group and 31 patients in MET group were included. Thirty-nine patients had a SS genotype, three patients had a S-β⁰ thalassemia genotype (2 AET, 1 MET) and one had a S-β⁺thalassemia genotype (AET). The proportion of hydroxyurea-treated patients was not different between the two groups (20/31 in MET group and 5/12 in the AET group; p=0.17). The BET indication was cerebral vasculopathy in 11/12 and vaso-occlusive crisis in 1/12 in the AET group. BET indications in the MET group were: frequent vaso-occlusive crisis (10/31), severe organ dysfunction or organ transplant (12/31), provisory hydroxyurea interruption due to pregnancy, breastfeeding, paternity desire (4/31) and leg ulcers (3/31).

Differences between groups before BET were only a higher percentage of HbF in the MET group and a higher percentage of HbA in the AET group.

Both AET and MET procedures decreased HbS level, leucocytes and platelets counts, and increased HbA level (p ranging from < 0.01 to < 0.001). The decrease in HbS (p < 0.001), HbF (p < 0.05), HbA2 (p < 0.05), leucocytes (p < 0.001) and platelets (p < 0.001) levels was higher in the AET than in the MET condition. MET caused a significant rise in hematocrit and hemoglobin (p < 0.001). In contrast, AET did not change hematocrit and induced a slight increase in hemoglobin (p < 0.05). The percentage of change in hemoglobin and hematocrit was higher in the MET than in the AET condition (p < 0.01 and p < 0.05, respectively). The median blood viscosity after AET was significantly lower (3.77 cP [3.78-4.25]) compared to before (4.47 cP [3.88-5.22 ]; (p=0.0001)), whereas there was no difference before (4 cP [3.7-4.25]) and after (4.15 cP [3.73-4.88]) BET in the MET group (p=0.11). The percentage of variation in blood viscosity between AET and MET was significantly different (p < 0.01). Both AET and MET significantly increased HVR (p < 0.001 and p < 0.01, respectively) and the percentage of variation was not significantly different between the two procedures (p = 0.138).

Conclusion: Automated exchange transfusion and not manual exchange transfusion improved blood viscosity. This might be due to the control of hematocrit allowed by the automated procedure, while decreasing the HbS percentage. Automated exchange transfusion should be preferred in acute and chronic SCD manifestations to improve blood rheology.