Can the effects of vasoactivity of molecular hemoglobin-based plasma expanders be ignored?

Replacing the Transfusion of 1-2 Units of Blood with Plasma Expanders that Increase Oxygen Delivery Capacity: Evidence from Experimental Studies

At least a third of the blood supply in the world is used to transfuse 1–2 units of packed red blood cells for each intervention and most clinical trials of blood substitutes have been carried out at this level of oxygen carrying capacity (OCC) restoration. However, the increase of oxygenation achieved is marginal or none at all for molecular hemoglobin (Hb) products, due to their lingering vasoactivity. This has provided the impetus for the development of “oxygen therapeutics” using Hb-based molecules that have high oxygen affinity and target delivery of oxygen to anoxic areas. However it is still unclear how these oxygen carriers counteract or mitigate the functional effects of anemia due to obstruction, vasoconstriction and under-perfusion. Indeed, they are administered as a low dosage/low volume therapeutic Hb (subsequently further diluted in the circulatory pool) and hence induce extremely small OCC changes. Hyperviscous plasma expanders provide an alternative to oxygen therapeutics by increasing the oxygen delivery capacity (ODC); in anemia they induce supra-perfusion and increase tissue perfusion (flow) by as much as 50%. Polyethylene glycol conjugate albumin (PEG-Alb) accomplishes this by enhancing the shear thinning behavior of diluted blood, which increases microvascular endothelial shear stress, causes vasodilation and lowering peripheral vascular resistance thus facilitating cardiac function. Induction of supra-perfusion takes advantage of the fact that ODC is the product of OCC and blood flow and hence can be maintained by increasing either or both. Animal studies suggest that this approach may save a considerable fraction of the blood supply. It has an additional benefit of enhancing tissue clearance of toxic metabolites.

Vital organ tissue oxygenation after serial normovolemic exchange transfusion with HBOC-201 in anesthetized swine

This study determined the effects of serial, normovolemic, stepwise exchange transfusions with either 6% human serum albumin (HSA) or the hemoglobin-based oxygen carrier, HBOC-201, on tissue oxygenation of the heart, brain, and kidney in intact anaesthetized pigs. Exchange transfusions to 10%, 30%, and 50% of the pigs' total blood volume were completed at a withdrawal rate of 1.0 mL·kg(-1)·min(-1) followed by an infusion rate of 0.5 mL·kg(-1)·min(-1) of HBOC-201 or iso-oncotically matched 6% HSA. Measurements included invasive systemic hemodynamic (blood pressures, left ventricular end-diastolic pressure), hematolic (hemoglobin, hematocrit, methemoglobin), acid-base (pH, PCO2), and biochemistry (serum lactate) measurements. Brain and kidney tissue oxygenation (tPO2) was determined by electron paramagnetic resonance and heart tPO2 by O2 sensitive fiberoptic probe. The main results demonstrated that tPO2 after HBOC-201 remained stable despite significant decreases in hematocrit and changing hemodynamics. In vivo tPO2 measurements (heart tPO2 average ≥22 mmHg, brain tPO2 average ≥8 mmHg, and kidney tPO2 average ≥10 mmHg) were maintained in all groups at all times. Blood pressures were 20 to 30 mmHg higher after HBOC-201 compared with HSA controls. Heart rate and left ventricular end-diastolic pressure were not different among treatment groups. In conclusion, the administration of HBOC-201 maintained tPO2 in three vital organs after profound hemodilution.

Glutaraldehyde-polymerized bovine hemoglobin and phosphodiesterase-5 inhibition

OBJECTIVE

Hemoglobin-based oxygen carriers (HBOC) of several types scavenge nitric oxide from the vasculature resulting in vasoconstriction and hypertension, both systemic and pulmonary. Phosphodiesterase-5 (PDE5) inhibitors promote nitric oxide activity and enhance vasodilation. The purpose of this study was to determine whether combined therapy of glutaraldehyde-polymerized bovine hemoglobin (HBOC) with a PDE5 inhibitor would counter the negative hemodynamic consequences of HBOC therapy alone, resulting in improved hemodynamics and oxygen delivery.

DESIGN

A controlled, experimental study.

SETTING

A research laboratory at a university.

SUBJECTS

Conscious male Sprague-Dawley rats.

INTERVENTIONS

Glutaraldehyde-polymerized bovine hemoglobin (HBOC), sildenafil (PDE5 inhibitor), and lactated Ringer's solution (control).

MEASUREMENTS AND MAIN RESULTS

Infusion of the HBOC resulted in significant (p < 0.05) systemic and pulmonary vasoconstriction, with reduced cardiac output and reduced oxygen delivery to the periphery. Infusion of lactated Ringer's demonstrated no changes in the measured variables. Infusion of sildenafil alone reduced systemic and pulmonary artery blood pressure, while maintaining cardiac output and oxygen delivery. Combined HBOC and sildenafil infusion resulted in stable systemic blood pressure, cardiac output, and oxygen delivery. However, the addition of sildenafil to HBOC did not fully ameliorate the pulmonary vasoconstriction caused by HBOC.

CONCLUSION

The HBOC used in this study resulted in pulmonary and systemic hypertension, reduced cardiac output, and oxygen delivery. These negative consequences of HBOC treatment can be largely overcome by combing HBOC treatment with a PDE5 inhibitor (sildenafil). Thus, these data support the continued investigation of combined HBOC and PDE5 inhibitor treatment in circumstances in which HBOC therapy is being considered.