Overview of artificial O2 carriers

Improved short-term survival with polyethylene glycol modified hemoglobin liposomes in critical normovolemic anemia

OBJECTIVE

To investigate the efficacy of a polyethylene glycol (PEG) modified formulation of liposome-encapsulated hemoglobin (LEH) as an oxygen-carrying blood substitute in the treatment of critical normovolemic anemia.

DESIGN AND SETTING

Prospective, controlled, randomized experimental study in a university research facility.

SUBJECTS

14 anesthetized and mechanically ventilated beagle dogs.

INTERVENTIONS

Animals were splenectomized and hemodiluted by exchange of whole blood for iso-oncotic hetastarch (HES). Target parameter of the hemodilution protocol was the individual critical hemoglobin concentration (Hb(crit)) corresponding with the onset of O(2) supply dependency of total body O(2) consumption. At Hb(crit) animals were randomized to receive a bolus infusion (20[Symbol: see text]ml/kg) of either LEH (n = 7) or normal saline (NS; n = 7). Subsequently animals were observed without further intervention.

MEASUREMENTS AND RESULTS

The primary endpoint was survival time after the completion of treatment; secondary endpoints were parameters of central hemodynamics, O(2) transport and tissue oxygenation. Animals in the LEH group survived significantly longer after completion of treatment (149 +/- 109 vs. 43+/- 56 min). Immediately after treatment LEH-treated animals presented with a more stable cardiovascular condition. After 30 min tissue O(2) tension on the surface of a skeletal muscle was significantly higher in the LEH group (23+/-8 vs. 9 +/- 2 mmHg). Nevertheless, treatment with LEH did not decrease mortality within the observation period.

CONCLUSIONS

In this present experimental study the infusion of a PEG-modified LEH provided adequate tissue oxygenation, hemodynamic stability, and a prolongation of survival time after critical anemia. However, these effects were sustained for only a short period of time.

Alternatives to allogeneic blood transfusions

Inherent risks and increasing costs of allogeneic transfusions underline the socioeconomic relevance of safe and effective alternatives to banked blood. The safety limits of a restrictive transfusion policy are given by a patient's individual tolerance of acute normovolaemic anaemia. latrogenic attempts to increase tolerance of anaemia are helpful in avoiding premature blood transfusions while at the same time maintaining adequate tissue oxygenation. Autologous transfusion techniques include preoperative autologous blood donation (PAD), acute normovolaemic haemodilution (ANH), and intraoperative cell salvage (ICS). The efficacy of PAD and ANH can be augmented by supplemental iron and/or erythropoietin. PAD is only cost-effective when based on a meticulous donation/transfusion plan calculated for the individual patient, and still carries the risk of mistransfusion (clerical error). In contrast, ANH has almost no risks and is more cost-effective. A significant reduction in allogeneic blood transfusions can also be achieved by ICS. Currently, some controversy regarding contraindications of ICS needs to be resolved. Artificial oxygen carriers based on perfluorocarbon (PFC) or haemoglobin (haemoglobin-based oxygen carriers, HBOCs) are attractive alternatives to allogeneic red blood cells. Nevertheless, to date no artificial oxygen carrier is available for routine clinical use, and further studies are needed to show the safety and efficacy of these substances.