Effect of oxygenated polyethylene glycol decorated hemoglobin on microvascular diameter and functional capillary density in the transgenic mouse model of sickle cell anemia.

Malemide polyethylene glycol-conjugated Hb (MP4OX, Sangart Inc.), a high-affinity low-concentration acellular hemoglobin (P50 = 5 mmHg, 4.3 g/dl) solution, has been shown to optimize microvascular perfusion and target oxygen delivery to anoxic tissue. Microvascular perfusion during an acute hypoxic challenge in a transgenic anemic sickle cell disease mouse model was studied with MP4OX and saline. Arterioles were dilated in both groups. Functional capillary density (FCD) was maintained at a higher level with MP4OX. In conclusion, MP4OX treatment reduced the hypoxia-mediated decline in FCD, an effect in part due to higher arterial pressure resulting in increased microvascular perfusion pressures.

Oxygen: the poison is in the dose.

Cell-free hemoglobin (Hb) has been blamed for a spectrum of problems, including vasoconstriction pancreatitis, myocardial infarction, and pulmonary hypertension in hemolytic anemia, malaria, and sickle cell anemia, and from Hb-based oxygen carriers (HBOCs). Toxicities have been attributed to scavenging of nitric oxide (NO). However, while NO scavenging may explain many in vitro effects, and some effects in animal models and clinical trials with HBOCs, key inconsistencies in the theory require alternative explanations. This review considers the hypothesis that cell-free Hb oversupplies oxygen to tissues, leading to oxygen-related toxicity, possibly through formation of reactive oxygen species and local destruction of NO. Evidence for this hypothesis comes from various sources, establishing that tissue oxygen levels are maintained over very narrow (and low) levels, even at high oxygen consumption. Tissue is normally protected from excessive oxygen by its extremely low solubility in plasma, but introduction of cell-free Hb, even at low concentration, greatly augments oxygen supply, engaging protective mechanisms that include vasoconstriction and ischemia. The requirement to limit oxygen supply by cell-free Hb suggests novel ways to modify it to overcome vasoconstriction, independent of the intrinsic reaction of Hb with NO. This control is essential to the design of a safe and effective cell-free HBOC.

Hemospan: design principles for a new class of oxygen therapeutic.

Hemoglobin-based oxygen carriers have been under development for decades, but safety concerns have prevented commercial approval. Early designs for modified hemoglobins by polymerization or intramolecular cross-linking reactions increased molecular size and decreased oxygen affinity, but all exhibited side effects of vasoconstriction and reduced blood flow. A new strategy has been established by applying principles of oxygen transport to cell-free hemoglobin. Sangart has developed a new oxygen therapeutic, Hemospan, using site-specific, poly(ethylene) glycol conjugation chemistry designed on two principles: (i) increased macromolecular size to prolong intravascular retention time, and (ii) increased oxygen affinity to prevent premature oxygen offloading in arterioles. In contrast to early-generation products, Hemospan infusion maintains normal arteriolar vascular tone and capillary flow. Phase I and Phase II clinical trials have been completed, showing that Hemospan is well-tolerated in humans, with evidence of efficacy to impart hemodynamic stability in surgical patients under anesthesia. Phase III trials in orthopedic surgery have recently completed enrollment in Europe.

Hemospan improves outcome in a model of perioperative hemodilution and blood loss in the rat: comparison with hydroxyethyl starch.

OBJECTIVES: Hemospan (Sangart Inc, San Diego, CA) (MP4) is a hemoglobin-based oxygen carrier consisting of human hemoglobin modified with polyethylene glycol. This study evaluated the effects of MP4 on blood volume, hemodynamics, and metabolic stability in a rat model of hemodilution and hemorrhage. MP4 was compared with hydroxyethyl starch solutions of differing concentrations (ie, HES 260/0.45 and HES 130/0.4). DESIGN: An open-label, randomized comparison of treatments. SETTING: Pharmaceutical industry. PARTICIPANTS: Sprague Dawley rats. INTERVENTIONS: Rats underwent 50% hemodilution with one of the solutions. Control rats were not hemodiluted. Blood volume was determined at baseline and 0, 60, and 120 minutes after exchange. In separate groups, hemodilution and subsequent 60% hemorrhage were examined to determine effectiveness of hemodilution. MEASUREMENTS AND MAIN RESULTS: Endpoints were blood volume after hemodilution and survival, hemodynamics, and acid-base status during hemorrhage. Volume expansion was similar with MP4 (159% of infused volume) and HES 260/0.45 (145%) and less with HES 130/0.4 (104%). The duration of expansion was longest with MP4 (1-2 hours). In the hemorrhage studies, 2-hour survival was 90% with MP4, 50% with controls, and 10% and 0% with HES 260/0.45 and HES 130/0.4, respectively. The severity of hemodynamic and acid-base changes paralleled the survival, with the least disturbance observed in MP4-treated animals. CONCLUSIONS: Hemodilution with MP4 was more effective in maintaining hemodynamic and metabolic stability than starch solutions or no hemodilution before simulated intraoperative hemorrhage. The benefit of MP4 is not ascribed solely to volume expansion. The results suggest that perioperative administration of MP4 may improve outcomes in surgical settings.

Sites of modification of hemospan, a poly(ethylene glycol)-modified human hemoglobin for use as an oxygen therapeutic.

Hemospan is an acellular hemoglobin-based oxygen therapeutic in clinical trials in Europe and the United States. The product is prepared by site-specific conjugation of maleimide-activated poly(ethylene) glycol (PEG, MW approximately 5500) to human oxyhemoglobin through maleimidation reactions either (1) directly to reactive Cys thiols or (2) at surface Lys groups following thiolation using 2-iminothiolane. The thiolation/maleimidation reactions lead to the addition of approximately 8 PEGs per hemoglobin tetramer. Identification of PEG modified globins by SDS-PAGE and MALDI-TOF reveals a small percentage of protein migrating at the position for unmodified globin chains and the remaining as separate bands representing globin chains conjugated with 1 to 4 PEGs per chain. Identification of PEG modification sites on individual alpha and beta globins was made using reverse-phase HPLC, showing a series of alpha globins conjugated with 0 to 3 PEGs and a series of beta globins conjugated with 0 to 4 PEGs per globin. Mass analysis of tryptic peptides from hemoglobin thiolated and maleimidated with N-ethyl maleimide showed the same potential sites of modification regardless of thiolation reaction ratio, with seven sites identified on beta globins at beta8, beta17, beta59, beta66, beta93, beta95, and beta132 and three sites identified on alpha globins at alpha7, alpha16, and alpha40.

Cell-free oxygen carriers: scientific foundations, clinical development, and new directions.

The most significant hurdle to the development of a safe and effective hemoglobin-based oxygen carrier ("blood substitute") is generally thought to be its propensity to cause vasoconstriction in the microcirculation and hypertension. Two theories for this effect are currently being studied: in one, scavenging NO by hemoglobin reduces vasorelaxation; in the other, cell-free hemoglobin oversupplies O2 (a known vasoconstrictor) to vascular walls by facilitated diffusion. While both mechanisms might lead to reduction of local NO concentration, the important distinction between the two is that if the NO scavenging theory is correct, it greatly diminishes the prospects to develop any solution based on free hemoglobin. However, if the O2-oversupply theory is correct, modifications to the hemoglobin molecule can be envisioned that can prevent oversupply and reduce toxicity. This review summarizes the development of Hemospan, a novel modification of human hemoglobin whose design is based on the O2-oversupply theory. Because of its low P50 and increased molecular size, the release of O2 in resistance vessels (arterioles) by Hemospan is restricted, and vasoconstriction is greatly reduced.

Solution structure of poly(ethylene) glycol-conjugated hemoglobin revealed by small-angle X-ray scattering: implications for a new oxygen therapeutic.

Developing protein therapeutics has posed challenges due to short circulating times and toxicities. Recent advances using poly(ethylene) glycol (PEG) conjugation have improved their performance. A PEG-conjugated hemoglobin (Hb), Hemospan, is in clinical trials as an oxygen therapeutic. Solutions of PEG-hemoglobin with two (P5K2) or six to seven strands of 5-kD PEG (P5K6) were studied by small-angle x-ray scattering. PEGylation elongates the dimensions (Hb < P5K2 < P5K6) and leaves the tertiary hemoglobin structure unchanged but compacts its quaternary structure. The major part of the PEG chains visualized by ab initio reconstruction protrudes away from hemoglobin, whereas the rest interacts with the protein. PEGylation introduces intermolecular repulsion, increasing with conjugated PEG amount. These results demonstrate how PEG surface shielding and intermolecular repulsion may prolong intravascular retention and lack of reactivity of PEG-Hb, possibly by inhibiting binding to the macrophage CD163 hemoglobin-scavenger receptor. The proposed methodology for assessment of low-resolution structures and interactions is a powerful means for rational design of PEGylated therapeutic agents.

Toxicity and hemodynamic effects after single dose administration of MalPEG-hemoglobin (MP4) in rhesus monkeys.

Maleimide-polyethylene glycol-modified (MalPEG) hemoglobin, 4.3 g/dL (MP4; Hemospan), is a hemoglobin-based oxygen carrier consisting of human hemoglobin (Hb) modified with maleimide polyethylene glycol. This study evaluates the potential toxicity and hemodynamic actions of a single dose of MP4 administered by exchange transfusion to rhesus monkeys. Monkeys were administered MP4 (21 mL/kg, or approximately 30% of estimated blood volume) or an equivalent volume of lactated Ringer's solution (LR). In the toxicity study, blood samples were obtained predose and 3, 7, and 13 days after dosing for clinical chemistry and hematology. Animals were euthanized for complete necropsy and histopathology on day 3 or day 13. A separate group of animals was used for evaluation of arterial pressure, core temperature, and electrocardiogram, by telemetry, for 7 days after dosing with MP4. The results demonstrate no significant toxicity, with only modest, transient elevation of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) on day 3. Mild anemia caused by hemodilution was observed at each time point in both groups, but to a slightly greater degree in the MP4-treated animals. Histologic observations included foamy or vacuolated macrophages in the spleen and marrow of the sternum, rib, and femur, representing the accumulation of test article or a metabolite. In the telemetry study, no changes occurred in arterial pressure, heart rate, or electrocardiogram attributable to administration of MP4 at any time for 7 days after administration. These results demonstrate that MP4 is safe and is without hemodynamic effects when administered as an exchange transfusion of 30% of estimated blood volume.

A quantitative framework for the design of acellular hemoglobins as blood substitutes: implications of dynamic flow conditions.

The delivery of oxygen to tissue by cell-free carriers eliminates intraluminal barriers associated with red blood cells. This is important in arterioles, since arteriolar tone controls capillary perfusion. We describe a mathematical model for O(2) transport by hemoglobin solutions and red blood cells flowing through arteriolar-sized tubes to optimize values of p50, Hill number, hemoglobin molecular diffusivity and concentration. Oxygen release is evaluated by including an extra-luminal resistance term to reflect tissue oxygen consumption. For low consumption (i.e., high resistance to O(2) release) a hemoglobin solution with p50=15 mmHg, n=1, D(HBO2)=3 x 10(-7) cm(2)/s delivers O(2) at a rate similar to that of red blood cells. For high consumption, the p50 must be decreased to 5 mmHg. The model predicts that regardless of size, hemoglobin solutions with higher p50 will present excess O(2) to arteriolar walls. Oversupply of O(2) to arteriolar walls may cause constriction and paradoxically reduced capillary perfusion.

The role of hemoglobin oxygen affinity in oxygen transport at high altitude.

Hemoglobin is involved in the regulation of O(2) transport in two ways: a long-term adjustment in red cell mass is mediated by erythropoietin (EPO), a response to renal oxgyenation. Short-term, rapid-response adjustments are mediated by ventilation, cardiac output, hemoglobin oxygen affinity (P50), barriers to O(2) diffusion, and the control of local microvascular tissue perfusion. The distribution of O(2) between dissolved (PO2) and hemoglobin-bound (saturation) is the familiar oxygen equilibrium curve, whose position is noted as P50. Human hemoglobin is not genetically adapted for function at high altitude. However, more specialized species native to high altitudes (guinea pig and bar-headed goose, for example) seem to have a lower P50 than their sea level counterparts, an adaptation that presumably promotes O(2) uptake from a hypoxic environment. Humans, native to very high altitude either in the Andes or Himalayan mountains, also can increase O(2) affinity, not because of a fundamental difference in hemoglobin structure or function, but because of extreme hyperventilation and alkalosis.

Red cell substitutes.

Oxygen-carrying plasma expanders (blood substitutes) have been sought for over a century. Development of current products is a result of evolution in the understanding of proteins in general, of hemoglobin in particular, and of how cell-free hemoglobin interacts with the control of local blood flow to ensure adequate tissue oxygenation. Hemoglobin-based products are considered in four "generations" corresponding to major improvements. First-generation products consisted of hemoglobin, freed of red cell membranes (stroma-free hemoglobin [SFH]), which was renal toxic and vasoactive. Second-generation products were polymerized with aldehyde reagents to reduce or eliminate the renal toxicity, but the products were heterogeneous and still vasoactive. Third-generation products employed more specific intramolecular crosslinking to eliminate polymerization and promote homogeneity, but they also remained vasoactive. Fourth-generation products are based on a new understanding of the way in which microvascular blood flow is controlled and the influence of O(2) delivery to vascular walls. After more than a century of research, one of these new solutions should find use as an alternative to red cells for transfusion in certain clinical settings.

Effect of maleimide-polyethylene glycol hemoglobin (MP4) on hemodynamics and acid-base status after uncontrolled hemorrhage in anesthetized swine: comparison with crystalloid and blood.

BACKGROUND: Maleimide-polyethylene glycol hemoglobin, 4.3 g/dL (MP4), is a hemoglobin-based oxygen carrier consisting of native human hemoglobin modified with maleimide polyethylene glycol. This study evaluated resuscitation with MP4 after uncontrolled hemorrhage in anesthetized swine, and compared the effects of MP4 alone with those of standard-of-care crystalloid or crystalloid + blood. METHODS: Pigs were anesthetized with isoflurane and hemorrhaged 250 mL by controlled withdrawal, which was followed by a 5-mm tear in the abdominal aorta. Three groups of pigs (n = 7 each) were randomized after aortic tear to receive 500 mL of MP4, unlimited Ringer's acetate (RA), or 2 L of RA + 250 mL of autologous blood. Measurements included arterial and pulmonary artery pressures, central venous and pulmonary capillary wedge pressures, cardiac output, renal blood flow, urine output, arterial and venous blood gases, lactate concentration, and hemoglobin. Animals were monitored for 150 minutes after hemorrhage. RESULTS: Average body weight (22-24 kg) and hemorrhage volume (28-34 mL/kg) were similar in the three groups. The nadir of mean arterial pressure (23-25 mm Hg) and base excess (-3 to -4 mEq/L) after hemorrhage were similar in all groups, indicating equivalent shock in the three groups. Two hours after administration of MP4, arterial pressure and base excess were significantly improved compared with those of animals administered RA or RA + blood, despite a significantly lower volume of infused fluids (MP4 = 36 +/- 2 mL/kg; RA = 224 +/- 28 mL/kg; RA + blood = 110 +/- 10 mL/kg), and a significantly lower total hemoglobin than achieved with RA + blood. Arterial pressure did not rise above baseline values, cardiac output was not diminished, and systemic vascular resistance was unchanged after administration of MP4, indicating the lack of cardiac effects or peripheral vasoconstriction. CONCLUSIONS: These data demonstrate that resuscitation with a small volume of MP4 induces recovery from hemorrhage without vasoconstriction. Overall, the effects of MP4 alone on hemodynamic and acid-base indices appear to be of greater benefit than those observed with a large volume of RA alone or RA + blood.

A multicenter clinical study of the safety and activity of maleimide-polyethylene glycol-modified Hemoglobin (Hemospan) in patients undergoing major orthopedic surgery.

BACKGROUND: Hemospan (Sangart Inc., San Diego, CA), a polyethylene glycol-modified hemoglobin with unique oxygen transport properties, has successfully completed a phase I trial in healthy volunteers. Because adverse events are expected to increase with age, the authors conducted a phase II safety study of Hemospan in elderly patients undergoing elective hip arthroplasty during spinal anesthesia. METHODS: Ninety male and female patients, American Society of Anesthesiologists physical status I-III, aged 50-89 yr, in six Swedish academic hospitals were randomly assigned to receive either 250 or 500 ml Hemospan or Ringer's acetate (30 patients/group) before induction of spinal anesthesia. Safety assessment included vital signs and Holter monitoring from infusion to 24 h, evaluation of laboratory values, and fluid balance. The hypothesis to be tested was that the incidence of adverse events would be no more frequent in patients who received Hemospan compared with standard of care (Ringer's acetate). RESULTS: Three serious adverse events were noted, none of which was deemed related to study treatment. Liver enzymes, amylase, and lipase increased transiently in patients in all three groups. There were no significant differences in electrocardiogram or Holter parameters, but there was a suggestion of more bradycardic events in the treated groups. Hypotension was less frequent in the treated patients compared with controls. CONCLUSIONS: In comparison with Ringer's acetate, Hemospan mildly elevates hepatic enzymes and lipase and is associated with less hypotension and more bradycardic events. The absence of a high frequency of serious adverse events suggests that further clinical trials should be undertaken.

Oxidation and haem loss kinetics of poly(ethylene glycol)-conjugated haemoglobin (MP4): dissociation between in vitro and in vivo oxidation rates.

Haemoglobin-based oxygen carriers can undergo oxidation of ferrous haemoglobin into a non-functional ferric form with enhanced rates of haem loss. A recently developed human haemoglobin conjugated to maleimide-activated poly(ethylene glycol), termed MP4, has unique physicochemical properties (increased molecular radius, high oxygen affinity and low cooperativity) and lacks the typical hypertensive response observed with most cell-free haemoglobin solutions. The rate of in vitro MP4 autoxidation is higher compared with the rate for unmodified SFHb (stroma-free haemoglobin), both at room temperature (20-22 degrees C) and at 37 degrees C (P<0.001). This appears to be attributable to residual catalase activity in SFHb but not MP4. In contrast, MP4 and SFHb showed the same susceptibility to oxidation by reactive oxygen species generated by a xanthine-xanthine oxidase system. Once fully oxidized to methaemoglobin, the rate of in vitro haem loss was five times higher in MP4 compared with SFHb in the fast phase, which we assign to the beta subunits, whereas the slow phase (i.e. haem loss from alpha chains) showed similar rates for the two haemoglobins. Formation of MP4 methaemoglobin in vivo following transfusion in rats and humans was slower than predicted by its first-order in vitro autoxidation rate, and there was no appreciable accumulation of MP4 methaemoglobin in plasma before disappearing from the circulation. These results show that MP4 oxidation and haem loss characteristics observed in vitro provide information regarding the effect of poly(ethylene glycol) conjugation on the stability of the haemoglobin molecule, but do not correspond to the oxidation behaviour of MP4 in vivo.

Dissociation of local nitric oxide concentration and vasoconstriction in the presence of cell-free hemoglobin oxygen carriers.

Cell-free hemoglobin's (CFH) high affinity for nitric oxide (NO) could limit CFH's use as an oxygen-carrying blood replacement fluid because it scavenges NO, causing vasoconstriction and hypertension. However, the extent to which perivascular NO levels change following intravascular administration of hemoglobin (Hb) with different molecular dimensions correlates with vasoconstrictive responses in the microcirculation is unknown. The study objective was to determine vasoconstrictive effects following bolus infusions of (1) alphaalpha cross-linked Hb; (2) polymerized bovine Hb; or (3) polyethylene glycol-decorated Hb (PEG-Hb), by measurements of in vivo microvessel diameter, blood flow, perivascular NO concentration, and systemic hemodynamic parameters. All CFHs caused reductions in perivascular NO levels, not correlated to microvascular responses. PEG-Hb (largest molecular volume) maintained blood flow, while the others caused vasoconstriction and reduced perfusion. All solutions increased mean arterial pressure due to vasoconstriction and blood volume expansion, except for PEG-Hb, which increased blood pressure due to blood volume expansion and maintenance of cardiac output. In conclusion, perivascular NO reduction is similar for all Hb solutions because NO binding affinities are similar; however, effects on vascular resistance are related to the type of molecular modification, molecular volume, and oxygen affinity.

Enhanced molecular volume of conservatively pegylated Hb: (SP-PEG5K)6-HbA is non-hypertensive.

Recent studies have suggested that the "pressor effect" of acellular Hb is a consequence of perturbation of the macro-and microcirculatory system in multiple ways, and that PEGylation is an effective approach for controlling the same. In an attempt to confirm this concept, a new and simple thiolation mediated, maleimide chemistry-based conservative PEGylation protocol has been developed to conjugate multiple copies of PEG-chains to Hb. This approach combines the high reactivity of maleimides towards thiols with the propensity of iminothiolane to derivatize the epsilon-amino groups of proteins into reactive thiol groups, with conservation of their positive charge. One of the PEGylated products, namely (SP-PEG5K)6-HbA, that carries on an average six copies of PEG5000 chains per Hb, is non-hypertensive in hamster top load and in rat 50% exchange transfusion models. This hexa-PEGylated-Hb has (i) a hydrodynamic volume corresponding to that of an oligomerized Hb of 256kDa, (ii) a molecular radius of approximately 6.8 nm, (iii) high oxygen affinity, (iv) lowered Bohr effect, and (v) increased viscosity and colloidal osmotic pressure. These properties of (SP-PEG5K)6-HbA are consistent with the emerging new paradigms for the design of Hb based oxygen carriers and confirm the concept that the "pressor effect" of Hb is a multifactorial event. The thiolation mediated maleimide chemistry-based PEGylation protocol described here for the generation of (SP-PEG5K)6-Hb is simple, highly efficient, and is carried out under oxy conditions. The results demonstrate that a non-hypertensive PEG-Hb can be generated by conjugation of a lower number of PEG chains than previously reported.

MalPEG-hemoglobin (MP4) improves hemodynamics, acid-base status, and survival after uncontrolled hemorrhage in anesthetized swine.

OBJECTIVES: MalPEG-hemoglobin, 4 g/dL (MP4), is a hemoglobin-based oxygen carrier with a low hemoglobin concentration, low P50 (oxygen half-saturation pressure of hemoglobin), high colloid osmotic pressure, and high viscosity. This study evaluated resuscitation with MP4 in anesthetized swine hemorrhaged 250 mL by controlled withdrawal, followed by a 5-mm tear in the abdominal aorta. DESIGN: Randomized, unblinded. SETTING: Academic animal laboratory. SUBJECTS: Anesthetized male and female Swedish Landrace pigs. INTERVENTIONS: Four groups of pigs (n = 7 each) were randomized after hemorrhage by aortic tear to receive 250 mL of MP4, Ringer's acetate, 10% pentastarch, or 4 g/dL of stroma-free hemoglobin, followed by aortic repair and transfusion of 250 mL of autologous blood. End points were 20-hr survival, hemodynamic variables, and acid-base status. MEASUREMENTS AND MAIN RESULTS: Measurements included continuous aortic, pulmonary arterial, and central venous pressures, cardiac output by thermodilution, arterial and venous blood gases; electrolytes; lactate; base excess; oxygen delivery, consumption, and extraction ratio; hematocrit; hemoglobin; and urine output. Body weight (24-27 kg) and hemorrhage volume (26-33 mL/kg) were similar in the four groups. The nadir of mean arterial pressure (22-28 mm Hg) and the increase in lactic acid (5-8 mEq/L) after hemorrhage were similar in all groups, indicating equivalent shock in the four groups. Survival was greatest in the MP4-treated animals (six of seven) compared with Ringer's acetate (two of seven), 10% pentastarch (one of seven), and stroma-free hemoglobin (two of seven) and was accompanied by an improved recovery of arterial pressure, cardiac output, and lactate. Total hemoglobin concentration was equivalent in all groups. Arterial pressure did not increase above baseline values, and systemic vascular resistance was unchanged following administration of MP4, indicating the lack of peripheral vasoconstriction. Mortality in Ringer's acetate, stroma-free hemoglobin, and 10% pentastarch treated animals was associated with deteriorating acid-base status, low urine output, and hyperkalemia. CONCLUSION: These data demonstrate that restoration of oxygen delivery with a small volume of MP4 yields significant recovery from hemorrhage without systemic vasoconstriction.

Extreme hemodilution with PEG-hemoglobin vs. PEG-albumin.

Isovolemic hemodilution to 11% systemic hematocrit was performed in the hamster window chamber model using 6% dextran 70 kDa (Dx 70) and 5% human serum albumin (HSA). Systemic and microvascular effects of these solutions were compared with polyethylene glycol (PEG)-conjugated 5% albumin (MPA) and PEG-conjugated 4.2% Hb (MP4). These studies were performed for the purpose of comparing systemic and microvascular responses of PEG vs. non-PEG plasma expanders and similar oxygen-carrying vs. noncarrying blood replacement fluids. Mean arterial blood pressure was statistically significantly reduced for all groups compared with baseline (P < 0.05), HSA, MPA, and MP4 higher than Dx 70 (P < 0.05). MP4 and MPA had a significantly higher cardiac index than HSA and Dx 70, in addition to a positive base excess. Microvascular blood flow and capillary perfusion were significantly higher for the PEG compounds compared with HSA and Dx 70. Intravascular PO2 for MP4 and MPA was higher in arterioles (P < 0.05) compared with HSA and Dx 70, but there was no difference in either tissue or venular PO2 between groups. Total Hb in the MP4 group was 4.8 +/- 0.4 g/dl, whereas the remaining groups had a range of 3.6-3.8 g/dl. The hemodilution results showed that PEG compounds maintained microvascular conditions with lower concentrations than conventional plasma expanders. Furthermore, microvascular oxygen delivery and extraction in the window chamber tissue were significantly higher for the PEG compounds. MP4 was significantly higher than MPA (P < 0.05) and was not statistically different from baseline, an effect due to the additional oxygen release to the tissue by the Hb MP4.