Adverse HBOC-endothelial dysfunction synergism: a possible contributor to adverse clinical outcomes?

Adverse outcomes in clinical trials on Hemoglobin Based Oxygen Carriers (HBOCs) appear to have occurred more frequently in HBOC treated than in control treated subjects. The differential may be related to many factors, including study complexity and compliance issues. Adverse outcomes also appear to be related to chronic comorbidities in subjects undergoing elective surgery. Frequently occurring comorbidities in these populations are those related to aging, cardiovascular and metabolic disease (hypertension, atherosclerosis, diabetes, etc.). These are highly prevalent among many population subsets. These conditions have been extensively studied and are characterized by dysfunction of important endothelial vasoregulatory mechanisms, including impaired nitric oxide bioavailability, excessive generation of reactive oxygen species (ROS) and possibly enhanced vasoconstrictor mechanisms. Although less extensively studied, HBOCs have properties that may have an important amplifying effect upon mechanisms operating in endothelial dysfunction, by scavenging nitric oxide, generating further excess of ROS which in turn react with nitric oxide, inhibit nitric oxide synthase and possibly stimulate the release of vasoconstrictors such as endothelin. It is likely that amplification of vasoconstrictor effects is not uniformly operative in all vascular beds, and that some protective autoregulatory mechanisms maintain sufficient blood flow in vital organs as long as sufficient vasodilator reserve is available. When the latter is exhausted in the presence of arterial disease with physical obstructions, blood flow to vital organs may become compromised. This paper suggests avenues of further exploration to elucidate whether the combination of HBOC and endothelial dysfunction is a contributing factor in HBOC related adverse outcomes.

Reassessing the risk of hemodilutional anemia: Some new pieces to an old puzzle.

PURPOSE: Clinical studies demonstrate that anemia increases the risk of morbidity and mortality. Tissue hypoxia is an attractive but incompletely characterized candidate mechanism of anemia-induced organ injury. Physiological responses that optimize tissue oxygen delivery (nitric oxide synthase-NOS) and promote cellular adaptation to tissue hypoxia (hypoxia inducible factor-HIF) may reduce the risk of hypoxic organ injury and thereby improve survival during anemia. The presence of vascular diseases would likely impair the efficacy of these physiological mechanisms, increasing the risk of anemia-induced organ injury. In all cases, biological signals that indicate the activation of these adaptive mechanisms could provide an early and treatable warning signal of impending anemia-induced organ injury. Thus, we review the evidence for tissue hypoxia during acute hemodilutional anemia and also explore the novel hypothesis that methemoglobin, a measurable byproduct of increased NOS-derived nitric oxide (NO), may serve as a biomarker for "anemic stress". SOURCE: Published peer-reviewed studies provided the main source of information. Data from experimental studies were reassessed to derive the relationship between hemodilution (reduced hemoglobin concentration) and increased methemoglobin levels. PRINCIPAL FINDINGS: Active physiological mechanisms (sympathetic nervous system) are required to maintain optimal tissue oxygen delivery during hemodilutional anemia. Despite these responses, tissue hypoxia occurs during acute hemodilution, as demonstrated by a decrease in tissue PO(2) and an increase in hypoxic cellular responses (NOS, HIF). Optimal tissue oxygen delivery may be compromised further when cardiovascular responses are impaired. The positive correlation between decreased hemoglobin concentration (Hb) and an increase in methemoglobin levels in acutely anemic animals supports the hypothesis that anemia-induced increases in tissue NOS activity could promote methemoglobin formation. Methemoglobin may be a measurable byproduct of NO-mediated Hb oxidation. CONCLUSIONS: Evidence continues to demonstrate that anemia increases morbidity and mortality, possibly via hypoxic mechanisms. A potential strategy for assessing "anemic stress" was derived from experimental data based on a readily measurable biomarker, methemoglobin. New methods for measuring real-time hemoglobin and methemoglobin levels in patients may provide the basis to translate this idea into clinical practice. Further mechanistic studies are required to determine if the impact of reduced tissue oxygen delivery and activation of hypoxic cellular mechanism can be linked to measurable changes in biomarkers and clinical outcomes in acutely anemic patients.

The intravascular persistence and methemoglobin formation of Hemolink (hemoglobin raffimer) in dogs.

Hemoglobin raffimer (HEMOLINK, Hemosol Inc, Mississauga, Canada) is an o-raffinose cross-linked, purified human hemoglobin-based oxygen therapeutic that is currently being evaluated in late stage clinical trials. It is composed of several molecular weight (MW) species, comprising principally of stabilized tetramers (34-42%) and oligomers (54-62%). The objective of this study was to determine the in vivo circulating half-life (T1/2) of hemoglobin raffimer (Hb raffimer) and of its individual MW components in dogs subjected to a topload infusion of 25% of the estimated blood volume (18 mL/kg). Sampling was done over a 64-hour period that was expected to be equivalent to approximately two-and-half to three half-lives. Methemoglobin (MetHb) levels were also measured at intervals over the same period. The mean circulating half-life of Hb raffimer was 25.4 +/- 3.9 hours. The T1/2 for the individual MW components (determined by size exclusion chromatography) of Hb raffimer was 11 +/- 2 hours for the cross-linked tetramer and 35 +/- 7 hours for the fraction of oligomers. The apparent volume of distribution for Hb raffimer was estimated at 78 mL/kg. There was no difference in the apparent volumes of distribution of the tetrameric and oligomeric components of Hb raffimer. Throughout the course of the experiment (in which MetHb could be measured), plasma MetHb concentration, expressed as a percentage of the total plasma hemoglobin concentration, remained at 10% or less, and the mass concentration of MetHb in plasma remained at about 1 g/L. Thus, in the dog subjected to an estimated 25% topload infusion, the T1/2 of the infused Hb raffimer is approximately one day with the intravascular retention of the individual Hb raffimer components being dependent on the MW. Furthermore, oxidation of Hb raffimer to MetHb is limited under these conditions.