Red Cell Exchange

Sickle Cell Disease

• The main indication for red cell apheresis is exchange transfusion in sickle cell disease (SCD).
• The main aim of exchange transfusion in SCD is to reduce the %HbS and increase the haemoglobin concentration, so as to increase the oxygen-carrying capacity while minimising the effect of transfusion on blood viscosity.
  • The risk of hyperviscosity at any point is determined by the %HbS and the haematocrit.
  • The maximum oxygen transport is at Hb concentrations of 10-11g/dL because of the higher viscosity of sickle cells.
  • Simple transfusion leads to a rise in haematocrit; any increment in oxygen-carrying capacity achieved by this is offset by the concurrent increase in blood viscosity.
  • In contrast, exchange transfusion removes HbS and reduces blood viscosity.
  • Hence, when the pre-transfusion Hb is close to the steady state, or high for other reasons (e.g. SC disease), exchange transfusion is preferred.
• Indications for exchange transfusion:
  • Acute ischaemic stroke: target Hb 10g/dL (not higher due to risk of hyperviscosity) and %HbS <30%. Avoid hypovolaemia during the procedure.
  • Severe acute chest syndrome: target Hb 10-11g/dL and %HbS <30-40%.
    • Exchange transfusion is also recommended for patients with ACS who fail to respond to simple transfusion, or for those whose Hb is >9g/dl, who have less leeway for simple transfusion.
  • Although chronic transfusion programs to reduce %HbS to <30% are recommended for primary stroke prophylaxis in children with TCD velocities >200cm/s and for secondary stroke prophylaxis in children and adults, exchange transfusion can also be used to minimise complications of iron overload.
  • Refractory priapism which has failed penile aspiration/irrigation, intracavernosal sympathomimetics and shunt procedures.
  • Acute multi-organ failure syndrome (may be a complication of severe painful crisis).
  • Mesenteric syndrome (sickling and sequestration in mesenteric vascular bed, liver and lungs). Often complicated by acute chest syndrome.
  • Severe sepsis. 
  • Acute intrahepatic cholestasis.
  • High-risk elective surgery and, by extrapolation, possibly prior to urgent surgery.
• Red cell component requirement for exchange transfusion:
  • Less than 7 days old
  • ABO, extended Rh- (c, C, D, e, E) and Kell-matched
  • Antigen-negative for any existing allo-antibodies
  • Sickle negative
  • IAT compatible at 37°C 
• Red cell exchange may be carried out via apheresis or manually. Apheresis has a number of advantages over manual exchange, including:
  • More effective at preventing iron overload.
  • Faster HbS reduction during acute events (plasma, platelets and white cells are returned to the patient).
  • Longer gaps in transfusion interval for patients on a chronic transfusion program (up to 6-weekly intervals).
• Apheresis requires trained operators; hence, in emergency settings, manual exchange transfusions may be more practical.
  • Each apheresis procedure typically requires 8-10 red cell units for an adult on a long-term programme. 
• Manual exchange:
  • Aims to exchange approximately one-third of the patient’s blood volume to achieve ~30% HbA.
  • Should be done isovolaemically, rermoving a larger volume of blood than transfused.
  • The difference should be made up with normal saline.
  • Typically, exchanges require removal of 4 red cell units and transfusion of 3 units of red cells.
  • This will increase the Hb by 1-2g/dL.
  • As this procedure can be life-saving in emergencies, it is recommended that all hospitals likely to admit SCD patients have staff trained in manual exchange procedures and have clearly identified manual exchange protocols.
• Risks of red cell exchange:
  • Hypocalcaemia.
  • Dilutional thrombocytopaenia.
  • Fluid shifts and hypovolaemia – >15% of the patient’s total blood volume exists ex vivo. Large shifts may be minimised by priming the apheresis machine with donor blood.
  • Initial top-up transfusion should be used for patients with severe anaemia or a significant drop from the steady-state haemoglobin.

Other Indications for Red Cell Exchange

• Severe malaria (>10% parasitaemia and manifestations of severe disease, or >30% parasitized cells) or babesiosis, to reduce the parasite load.
• Following transfusion of RhD-positive blood to RhD-negative women of childbearing age or children with anti-D immunoglobulin intravenously, to reduce the risk of alloimmunisation.
• Automated erythrocytapheresis in polycythaemia vera or hereditary haemochromatosis.
  • May be preferable to large-volume venesection in certain patients, e.g. haemodynamically-unstable, elective pre-operative reduction when cytoreductive therapy has been sub-optimal, or for emergency pre-operative preparation.