Balance between vasoconstriction and enhanced oxygen delivery

BACKGROUND: Hemoglobin (Hb) solutions are potential alternatives to blood transfusion when native oxygen (O-2)-carrying capacity is lacking. Polymerized bovine Hb (PBH) solutions are characterized by its vasoactivity, low O-2 affinity, oncotic effect, prolonged shelf life, and stability. Responses to facilitated O-2 transport, after exchange transfusion with PBH, were studied in the hamster window chamber model during acute extreme anemia to determine how PBH affects microvascular perfusion and tissue oxygenation. STUDY DESIGN AND METHODS: An anemic state was induced by hemodilution with a plasma expander (70-kDa dextran). After hemodilution, animals were randomly assigned to exchange transfusion groups based on the concentration of PBH used, namely, PBH at 13 g(Hb) per dL (PBH13), PBH diluted to 8 or 4 g(Hb) per dL in albumin solution at matching colloidal osmotic pressure (COP; PBH8 and PBH4), and no PBH only albumin solution at matching COP (PBH0). Measurement of systemic variables, microvascular hemodynamics, capillary perfusion, and intravascular and tissue O-2 levels was performed at 11 percent Hct. RESULTS: Restitution of O-2-carrying capacity with PBH13 restored higher arterial pressure and triggered vasoconstriction, low perfusion, and higher peripheral resistance. Groups PBH4 and PBH0 had lower arterial pressures than Group PBH13. Groups PBH4 and PBH8 maintained higher perfusion and functional capillary density than Groups PBH13 and PBH0. In all groups, blood gas variables and acid-base balance were recovered proportional to microvascular perfusion. Arterial O-2 tensions were improved for Groups PBH4 and PBH8 by preventing O-2 precapillary release and increasing O-2 reserve. CONCLUSION: Further studies to establish acellular Hb optimal dosage, efficacy, safety, and effects on outcome are indicated before these solutions are implemented in routine practice.