Effect of hemoglobin solution on compensation to anemia in the erythrocyte-free primate

The USA Multicenter Prehosptial Hemoglobin-based Oxygen Carrier Resuscitation Trial: scientific rationale, study design, and results

Human polymerized hemoglobin (PolyHeme) is a universally compatible oxygen carrier developed for use when red blood cells are unavailable and oxygen-carrying replacement is needed to treat life-threatening anemia. This multicenter phase III trial assessed survival of patients resuscitated with a hemoglobin-based oxygen carrier starting at the scene of injury. Patients resuscitated with PolyHeme had outcomes comparable to those receiving the standard of care including rapid access to stored red blood cells. Although there were more adverse events in the PolyHeme group compared with control patients receiving blood, the observed safety profile is acceptable for the intended population. The benefit-to-risk ratio of PolyHeme is favorable when blood is needed but is not available or an option.

Current Status of Artificial o2 Carriers

This article describes currently evaluated artificial O2 carriers, summarizes their efficacy, and discusses their side effects, based on and restricted to published data. For compounds in phase III testing, approximately 500 to 1000 patients have been dosed, and similar numbers of control patients have been investigated. For compounds in phase I or II testing, the number of patients dosed is significantly less. Unfortunately, there is a significant amount of nonpublished data, which renders the overall assessment difficult, and the direct comparison among different types of artificial O2 carriers is significantly limited by the virtual nonexistence of studies that directly compare different products.

A compartmental model for oxygen transport in brain microcirculation in the presence of blood substitutes

A compartmental model is developed for oxygen (O(2)) transport in brain microcirculation in the presence of blood substitutes (hemoglobin-based oxygen carriers). The cerebrovascular bed is represented as a series of vascular compartments, on the basis of diameters, surrounded by a tissue compartment. A mixture of red blood cells (RBC) and plasma/extracellular hemoglobin solution flows through the vascular bed from the arterioles through the capillaries to the venules. Oxygen is transported by convection in the vascular compartments and by diffusion in the surrounding tissue where it is utilized. Intravascular resistance and the diffusive loss of oxygen from the arterioles to the tissue are incorporated in the model. The model predicts that most of the O(2) transport occurs at the level of capillaries. Results computed from the present model in the presence of hemoglobin-based oxygen carriers are consistent with those obtained from the earlier validated model (Sharan et al., 1989, 1998a) on oxygen transport in brain circulation in the absence of extracellular hemoglobin. We have found that: (a) precapillary PO(2) gradients increase as PO(2) in the arterial blood increases, P(50 p) (oxygen tension at 50% saturation of hemoglobin with O(2) in plasma) decreases, i.e. O(2) affinity of the extracellular hemoglobin is increased, the flow rate of the mixture decreases, hematocrit decreases at constant flow, metabolic rate increases, and intravascular transport resistance in the arterioles is neglected; (b) precapillary PO(2) gradients are not sensitive to (i) intracapillary transport resistance, (ii) cooperativity (n(p)) of hemoglobin with oxygen in plasma, (iii) hemoglobin concentration in the plasma and (iv) hematocrit when accounting for viscosity variation in the flow; (c) tissue PO(2) is not sensitive to the variation of intravascular transport resistance in the arterioles. We also found that tissue PO(2) is a non-monotonic function of the Hill coefficient n(p) for the extracellular hemoglobin with a maximum occurring when n(p) equals the blood Hill coefficient. The results of the computations give estimates of the magnitudes of the increases in tissue PO(2) as arterial PO(2) increases,P(50 p) increases, flow rate increases, hematocrit increases, hemoglobin concentration in the plasma increases, metabolic rate decreases, the capillary mass transfer coefficient increases or the intracapillary transport resistance decreases.

Evaluation of oxygen extraction ratio as a physiologic transfusion trigger in coronary artery bypass graft surgery patients

BACKGROUND

Approximately 20 percent of all allogeneic blood transfusions are administered in connection with coronary artery bypass graft (CABG) operations. Transfusion practices vary across the country. The whole-body oxygen extraction ratio (O2 ER) reflects the adequacy of the patient's response to acute normovolemic anemia with an O2 ER of approximately 50 percent being shown to be an appropriate transfusion trigger. The present study monitored the O2 ER in patients undergoing CABG and determined if transfusion practices would have been different if an O2 ER > or = 45 percent were used as a transfusion trigger.

STUDY DESIGN AND METHODS

Seventy patients with a postoperative Hct < = 25 percent were the test subjects. Arterial and mixed venous contents were determined before the operation, in the intensive care unit after the operation, and 12 hours after the operation.

RESULTS

There were no deaths. Forty-one patients received allogeneic transfusion. These patients were older, weighed less, and had a preoperative Hct lower than the nontransfused patients. There were no significant differences between transfused and nontransfused patients with respect to postoperative Hct (21.0 +/- 0.4 vs. 22.2 +/- 0.4), cardiac index (2.5 +/- 0.1 vs. 2.7 +/- 0.1), O2 delivery (6.4 +/- 0.3 vs. 6.7 +/- 0.3), O2 consumption (2.5 +/- 0.1 vs. 2.5 +/- 0.1), and O2 ER (38.3 +/- 1.7 vs. 37.5 +/- 1.5). In the transfusion group, 7 of 21 patients had a postoperative O2 ER > or = 45 percent, while 3 of 35 in the nontransfused group had that result.

CONCLUSION

The use of O2 ER as a transfusion trigger as part of a transfusion algorithm could lead to a reduction in allogeneic blood transfusion.

Regional blood flow alterations after bovine fumaryl beta beta-crosslinked hemoglobin transfusion and nitric oxide synthase inhibition

OBJECTIVES

a) To determine whether isovolemic exchange transfusion with cell-free, bovine fumaryl beta beta-crosslinked hemoglobin results in a different pattern of regional blood flow distribution than transfusion with a poor oxygen-carrying, colloidal solution. b) Because of potential nitric oxide scavenging by plasma-based hemoglobin, to determine whether blood flow differences are reduced after nitric oxide synthase inhibition.

DESIGN

A prospective, randomized design with repeated blood flow measurements within groups.

SETTING

Experimental physiology laboratory in a university medical center.

SUBJECTS

Pentobarbital-anesthetized female cats.

INTERVENTIONS

Three groups of eight cats were studied: a) a control group with no transfusion (hematocrit of 32%); b) an anemia group in which exchange transfusion with an albumin-containing solution reduced hematocrit to 18% over a 40- to 50-min period; and c) a group in which cell-free hemoglobin was exchanged transfused to reduce hematocrit to 18%, without a proportional reduction in oxygen-carrying capacity. Bovine hemoglobin was covalently crosslinked intramolecularly between the 81-lysine residues on the beta-subunits to stabilize the tetramer. Regional blood flow was measured by the radiolabeled microsphere technique before transfusion and at 10, 100, and 180 mins from the start of transfusion. At 190 mins, N omega-nitro-L-arginine methyl ester (L-NAME; 10mg/kg) was infused to inhibit nitric oxide synthase and blood flow was measured 30 mins later.

MEASUREMENTS AND MAIN RESULTS

Mean arterial pressure was unchanged in the control and albumin-transfused groups. However, mean arterial pressure increased rapidly in the hemoglobin-transfused group. With hemoglobin transfusion, there were marked reductions in blood flow to the intestines, kidneys and adrenal glands. Administration of L-NAME after hemoglobin transfusion failed to increase arterial pressure or cause further reductions in intestinal, renal, or adrenal blood flow. Administration of L-NAME to the control and albumin-transfused groups increased arterial pressure and reduced intestinal, renal, and adrenal blood flows to values attained with hemoglobin transfusion. In contrast, in skeletal muscle and left ventricle, blood flow rates increased in the albumin-transfused group and were greater than those values found in the control group and hemoglobin-transfused group. The greater flow in the albumin-transfused group persisted after L-NAME administration. There was no difference in renal sodium, potassium, or osmolar excretion, or in urine flow between groups.

CONCLUSIONS

Transfusion with cell-free, bovine crosslinked hemoglobin in cats can selective reductions in blood flow in the intestines, kidneys, and adrenal glands without evidence of renal dysfunction by a mechanism consistent with nitric oxide scavenging. In skeletal and cardiac muscle, the increase in blood flow persisted after nitric oxide inhibition in the albumin group relative to the hemoglobin-transfused group at equivalent hematocrit values. This finding is consistent with compensatory vasoconstriction with hemoglobin transfusion due to improved oxygenation by this oxygen carrier.

The development of hemoglobin solutions as red cell substitutes: hemoglobin solutions

Although the efficacy of hemoglobin-based oxygen carriers was established more than 60 years ago, all prior clinical trials have demonstrated significant toxicity characterized by renal dysfunction, gastrointestinal distress, and systemic vasoconstriction. The mechanisms of these toxicities now appear to be understood. Tetrameric forms of the hemoglobin molecule extravasate from the circulation and interact with endothelial derived relaxing factor, leading to unopposed vasoconstriction. Although numerous efforts are underway to chemically modify the native tetramer, it is likely that all tetrameric forms of the hemoglobin molecule will continue to extravasate. We have focused on developing a polymerized form of hemoglobin that is virtually free of unreacted tetramer. The development and characterization of this polymerized pyridoxylated hemoglobin solution (Poly SFH-P) is described. Clinical trials have been completed successfully in volunteers, and are now underway to assess the safety and efficacy of Poly SFH-P as a clinically useful red cell substitute in the treatment of acute blood loss in the setting of trauma and surgery.

Effects of pyridoxalated hemoglobin polyoxyethylene conjugate and other hemoglobin-related substances on arterial blood pressure in anesthetized and conscious rats

The effects of pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) and other hemoglobin-related substances on arterial blood pressure (ABP) and heart rate (HR) were examined. Infusion of PHP and other hemoglobin-related substances elevated ABP and increased HR. The degree of the increase in ABP and HR did not differ among the groups. There were species' differences in responses to PHP. Pretreatment with various blockers did not abolish the elevation of ABP. The pretreatment of NG-monomethyl-L-arginine dose-dependently reduced the elevation of ABP but did not completely abolish it. PHP also elevated ABP in conscious rats. The magnitude of the ABP elevation was significantly smaller than in the anesthetized rats. Results indicate that inhibition of nitric oxide (NO) might be responsible in part for the elevation of ABP, and the degree of the elevation would be dependent on the degree of contribution of NO to the regulation of ABP.