![]() ![]() BCSH, Br J Haem 2017 176:179Ģ1 Exchange Transfusions Manual exchange: No special equipment required, but slow 1. Transfusing a patient with sickle cell disease to Hgb > g/l may worsen their condition, particularly if the patient is already in a hyperviscous state (dehydrated, low-flow, hypoxic) Target HgbS% may only be safely achievable by removing patient s own blood prior to transfusing = EXCHANGE TRANSFUSIONĢ0 Exchange Transfusions All hospitals that are likely to admit sickle cell disease patients should have staff trained in manual exchange procedures and clearly identified manual exchange procedures, as this can be lifesaving in emergency situations (Grade 1C) Large referral centres managing patients with sickle cell disease should have facilities and trained staff for automated exchange transfusion (Grade 1C). ![]() In most cases, the benefits of transfusing a patient with sickle cell disease will come from decreasing the viscosity of their blood rather than by increasing its oxygen-carrying capacity Goal of transfusion is to decr HgbS%, not incr total Hgb 2. 2006 48ġ5 Hematocrit:Viscosity Ratio vs Hct for Oxygenated Sickle Cell RBCs At high shear blood flow, viscosity increases at a slower rate than Hct over wide-range of Hct values, whether HgbS is 100% or 25% At low shear, however, increased RBC:protein interactions exacerbate viscosity, which therefore increases faster than Hct Alexy T, Transfusion 2006 46:912ġ6 Hematocrit:Viscosity Ratio vs Hct for Deoxygenated Sickle Cell RBCs While oxygenated sickle blood is already more viscous than normal blood, the viscosity increases dramatically when deoxygenated Result is an apparent optimal Hct of 25% at high shear, even if HgbS diluted to 25%, no further benefit in increasing Hct past 30% Even lower Hct may be better at low shear Alexy T, Transfusion 2006 46:912ġ8 Implications of Viscosity Studies In vascular beds with high shear (eg., brain, kidneys, lungs), oxygen delivery may be optimized by increasing the Hct, but with deoxygenated sickle blood there is likely little benefit and possibly harm of transfusing to exceed a Hct of 30%, even if patient s own blood has already been diluted by 75% Moreover, any improvements in oxygen delivery achieved by transfusion in high-shear vascular beds may result in worsened oxygen delivery in low shear bedsġ9 Implications of Viscosity Studies: Rules of Thumb 1. 12 Oxygen Delivery: A Balancing Act Oxygen-carrying capacity Small vessel perfusionġ3 Oxygen Delivery MACROCIRCULATION MICROCIRCULATION Oxygen Delivery = Cardiac Output x Oxygen Carrying Capacity of Blood Flow = pressure x radius 4 x π 8 x tube length x viscosity V = P x r4 x π 8 x l x η DO 2 = CO x CaO 2 Predominantly determined by hematocrit Predominantly determined by Hgb Lower flow = lower oxygen delivery Higher Hgb = More Oxygen Delivery Higher Hgb = Less Oxygen Deliveryġ4 Swerdlow, Hematology Am Soc Hematol Educ Program.
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