Increased tissue PO2 and decreased O2 delivery and consumption after 80% exchange transfusion with polymerized hemoglobin

Synthetic hemoglobin-based oxygen carriers are an acceptable alternative for packed red blood cells in normothermic kidney perfusion

Normothermic machine perfusion presents a novel platform for pretransplant assessment and reconditioning of kidney grafts. Maintaining the metabolic activity of a preserved graft at physiologic levels requires an adequate oxygen supply, typically delivered by crystalloid solutions supplemented with red blood cells. In this study, we explored the feasibility of using a synthetic hemoglobin-based oxygen carrier (HBOC) in human kidney normothermic perfusion. Fourteen discarded human kidneys were perfused for 6 hours at a mean temperature of 37°C using a pressure-controlled system. Kidneys were perfused with a perfusion solution supplemented with either HBOC (n = 7) or packed red blood cells (PRBC) (n = 7) to increase oxygen-carrying capacity. Renal artery resistance, oxygen extraction, metabolic activity, energy stores, and histological features were evaluated. Throughout perfusion, kidneys from both groups exhibited comparable behavior regarding vascular flow (P = .66), oxygen consumption (P = .88), and reconstitution of tissue adenosine triphosphate (P = .057). Lactic acid levels were significantly higher in kidneys perfused with PRBC (P = .007). Histological findings were comparable between groups, and there was no evidence of histological damage caused by the HBOC. This feasibility experiment demonstrates that a HBOC solution can offer a logistically more convenient off-the-shelf alternative to PRBC in normothermic machine perfusion of human kidneys.

Resuscitation from hemorrhagic shock using polymerized hemoglobin compared to blood

The development of an alternative to blood transfusion to treat severe hemorrhage remains a challenge, especially in far forward scenarios when blood is not available. Hemoglobin level (Hb)-based oxygen (O2) carriers (HBOCs) were developed to address this need. Hemopure (HBOC-201, bovine Hb glutamer-250; OPK Biotech, Cambridge, MA), one such HBOC, has been approved for clinical use in South Africa and Russia. At the time of its approval, however, few studies aimed to understand Hemopure's function, administration, and adverse effects compared to blood. We used intravital microscopy to study the microcirculation hemodynamics (arteriolar and venular diameters and blood flow and functional capillary density [FCD]) and oxygenation implications of Hemopure administration at different Hb concentrations-4, 8, and 12 gHb/dL-compared to fresh blood transfusion during resuscitation from hemorrhagic shock. Experiments were performed in unanesthetized hamsters instrumented with a skinfold window chamber, subjected to hemorrhage (50% of the blood volume), followed by 1-hour hypovolemic shock and fluid resuscitation (50% of the shed volume). Our results show that fluid resuscitation with Hemopure or blood restored systemic and microvascular parameters. Microcirculation O2 delivery was directly correlated with Hemopure concentration, although increased vasoconstriction was as well. Functional capillary density reflected the balance between enhanced O2 transport and reduced blood flow: 12 gHb/dL of Hemopure and blood decreased FCD compared to the lower concentrations of Hemopure (P < .05). The balance between O2 transport and tissue perfusion can provide superior resuscitation from hemorrhagic shock compared to blood transfusion by using a low Hb concentration of HBOCs relative to blood.

Blood substitutes: evolution from noncarrying to oxygen- and gas-carrying fluids

The development of oxygen (O2)-carrying blood substitutes has evolved from the goal of replicating blood O2 transport properties to that of preserving microvascular and organ function, reducing the inherent or potential toxicity of the material used to carry O2, and treating pathologies initiated by anemia and hypoxia. Furthermore, the emphasis has shifted from blood replacement fluid to "O2 therapeutics" that restore tissue oxygenation to specific tissues regions. This review covers the different alternatives, potential and limitations of hemoglobin-based O2 carriers (HBOCs) and perfluorocarbon-based O2 carriers (PFCOCs), with emphasis on the physiologic conditions disturbed in the situation that they will be used. It describes how concepts learned from plasma expanders without O2-carrying capacity can be applied to maintain O2 delivery and summarizes the microvascular responses due to HBOCs and PFCOCs. This review also presents alternative applications of HBOCs and PFCOCs namely: 1) How HBOC O2 affinity can be engineered to target O2 delivery to hypoxic tissues; and 2) How the high gas solubility of PFCOCs provides new opportunities for carrying, dissolving, and delivering gases with biological activity. It is concluded that the development of current blood substitutes has amplified their applications horizon by devising therapeutic functions for O2 carriers requiring limited O2 delivery capacity restoration. Conversely, full, blood-like O2-carrying capacity reestablishment awaits the control of O2 carrier toxicity.

Vasoactive hemoglobin solution improves survival in hemodilution followed by hemorrhagic shock

OBJECTIVE

To compare survival after exchange transfusion followed by hemorrhage using: 1) the vasoactive, oxygen-carrying, bovine hemoglobin-based blood substitute Oxyglobin (Biopure, 12.9 g hemoglobin/dL); and 2) the hydroxyethyl starch plasma expander Hextend (high molecular weight and low degree of substitution, 6%).

DESIGN

Comparison between treatments.

SETTING

Laboratory.

SUBJECTS

Awake hamster chamber window model.

INTERVENTIONS

Fifty percent blood volume exchange transfusion followed by a 60% hemorrhage over 1 hr, followed by 1 hr of observation. Measurement of blood gases, mean arterial blood pressure, functional capillary density, arteriolar and venular diameter, and Po2 tension distribution.

MEASUREMENTS AND MAIN RESULTS

Survival with Oxyglobin was 100% and only 50% for the Hextend group. Vasoconstriction was evident in the microcirculation. Mean arterial pressure was higher in the Oxyglobin group. Functional capillary density was significantly reduced, although to a lesser extent by Oxyglobin. There was no difference in microvascular Po2 distribution after 1 hr of shock between groups.

CONCLUSIONS

Higher mean arterial pressure during the initial stages of hemorrhage could be due to vasoconstriction in the Oxyglobin group as compared to the Hextend group. It is concluded that the pressor effect due to a vasoactive oxygen carrier may be beneficial in maintaining perfusion in conditions of severe hemodilution followed by hypovolemia.

Low oxygen-affinity hemoglobin solution increases oxygenation of partially ischemic tissue during acute anemia

BACKGROUND

Maintenance of postsurgical tissue oxygenation depends on the ability of the specific tissue to recruit perfusion and oxygen (O(2)) supply. When native O(2)-carrying capacity is lacking, fluids to improve O(2)-carrying capacity based in hemoglobin (Hb) could prevent partially ischemic tissue hypoxia by increasing O(2) release from the remaining red blood cells.

STUDY DESIGN

Responses to facilitated O(2) transport after exchange transfusion with polymerized bovine Hb (PBH) were studied in a chronic partially ischemic tissue model, induced by large feeding arteriole ligation during hamster window chamber model implantation. PBH effects in microvascular perfusion and tissue oxygenation were studied after exchange transfusion of 40% of animal's blood volume. Experimental groups were defined by the concentration of PBH used, ie, PBH at 13 g/dL (PBH13); PBH at 4 g/dL in albumin solution to matching colloidal osmotic pressure (COP) (PBH4); and no PBH, only albumin solution at matching COP (PBH0).

RESULTS

Restitution of O(2)-carrying capacity with PBH13 increased blood pressure and produced vasoconstriction compared with PBH4 and PBH0. On the other hand, PBH4 maintained blood pressure without substantial vasoconstriction, increased tissue partial pressure of O(2), arteriolar O(2) supply, and extraction to the partially ischemic tissue compared with PBH0 and PBH13.

CONCLUSIONS

Results suggest the existence of an optimal concentration of low O(2)-affinity acellular Hb to increase oxygenation of partially ischemic tissue during anemic conditions.

Hemoglobin encapsulation in vesicles retards NO and CO binding and O2 release when perfused through narrow gas-permeable tubes

Intravenous administration of cell-free Hb induces vasoconstriction and circulatory disorders, presumably because of the intrinsic affinities to endogenous nitric oxide (NO) and carbon monoxide (CO) as vasorelaxation factors and because of the facilitated O2 release that might induce autoregulatory vasoconstriction. We examined these gas reactions when Hb-containing solutions of four kinds were perfused through artificial narrow tubes at a practical Hb concentration (10 g/dl). Purified Hb solution, polymerized bovine Hb (PolyBHb), encapsulated Hb [Hb-vesicles (HbV), 279 nm], and red blood cells (RBCs) were perfused through a gas-permeable narrow tube (25 μm inner diameter) at 1 mm/s centerline velocity. The level of reactions was determined microscopically based on the visible-light absorption spectrum of Hb. When the tube was immersed in NO and CO atmospheres, both NO binding and CO binding of deoxygenated Hb (deoxy-Hb) and PolyBHb in the tube was faster than those of HbV and RBCs, and HbV and RBCs showed almost identical binding rates. When the tube was immersed in a N2 atmosphere, oxygenated Hb and PolyBHb showed much faster O2 release than did HbV and RBCs. PolyBHb showed a faster reaction than Hb because of the lower O2 affinity of PolyBHb than Hb. The diffusion process of the particles was simulated using Navier-Stokes and Maxwell-Stefan equations. Results clarified that small Hb (6 nm) diffuses laterally and mixes rapidly. However, the large-dimension HbV shows no such rapid diffusion. The purely physicochemical differences in diffusivity of the particles and the resulting reactivity with gas molecules are one factor inducing biological vasoconstriction of Hb-based oxygen carriers.

pO(2) and ROS/RNS measurements in the microcirculation in hypoxia

We expose methods for in vivo assessment of oxygen, nitric oxide (NO), and reactive oxygen species (ROS)/reactive nitrogen species (RNS), in the microcirculation during normoxia and hypoxia. We provide an example of the related mechanisms of ROS/RNS and oxygen level in the process of regulating capillary perfusion. Namely, we discuss the real time pO(2) measurements in vivo in the microvessels and tissues of the hamster cheek pouch and window chamber preparations during normoxia and hypoxia, as well as the corresponding changes in ROS/RNS in systemic blood during normoxia and hypoxia under conditions where NO availability is maximally reduced.

Effect of melatonin on the blood oxygen transport during hypothermia and rewarming in rats

PURPOSE

We aimed to study effect of melatonin on the blood oxygen transport during hypothermia and rewarming in rats.

MATERIAL/METHODS

Cold exposure was performed on male rats (body weight 220-270 g, n=48) for 120 minutes under the box water temperature of 19 degrees C; rewarming took the next 120 min, with a mean rate of 0.06 degrees C/min. Melatonin was administered intraperitoneally 30 min before the cold exposure (bolus doses of 0.1, 1 or 10 mg/kg, or 1 mg/kg*day for 4 days). Haemoglobin-oxygen affinity was evaluated by p50 (blood pO2 at its 50% O2 saturation) determined by the "mixing" method at 37 degrees C, pH 7.4 and pCO2 40 mm Hg (p50stand) and at actual pH, pCO2 and temperature (p50act).

RESULTS

After hypothermia and rewarming, the values of p50stand and p50act were 31.5+/-0.28 and 30.2+/-0.61 mm Hg, respectively. The 0.1 mg/kg of melatonin virtually did not change these values, whereas the larger doses increased them. This effect was maximal after the prolonged (4 days) melatonin administration: p50stand rose by 5.4% (p<0.05) and p50act--by 12.9 (p<0.05) compared with rats without the melatonin treatment. Melatonin affected the mechanisms of O2 transport by decreasing the haemoglobin-oxygen affinity (shifting the oxygen dissociation curve of haemoglobin rightwards) and promoting the tissue oxygenation, thereby enhancing the body's resistance to cold.

CONCLUSIONS

The melatonin effect mediated by haemoglobin-oxygen affinity change may be used for the correction of metabolic disorders and the improvement of the body's resistance to low environmental temperature.

Isovolemic exchange transfusion with increasing concentrations of low oxygen affinity hemoglobin solution limits oxygen delivery due to vasoconstriction

O2-carrying fluids based on hemoglobin (Hb) are in various stages of clinical trials to determine their suitability as O2-carrying plasma expanders. Polymerized Hb solutions are characterized by their vasoactivity, low O2 affinity, oncotic effect, prolonged shelf life, and stability. Physiological responses to facilitated O2 transport after exchange transfusion with polymerized bovine Hb (PBH) were studied in the hamster window chamber model during acute moderate anemia to determine how PBH affects microvascular perfusion and tissue oxygenation. The anemic state [29% hematocrit (Hct)] was induced by hemodilution with a plasma expander (70 kDa dextran). After hemodilution, animals were randomly assigned to different exchange transfusion groups. Study groups were based on the concentration of PBH used, namely: PBH at 13 g Hb/dl [PBH13], PBH diluted to 8 (PBH8) or 4 (PBH4) g Hb/dl in albumin solution at matching colloidal osmotic pressure (COP), and no PBH (only albumin solution) at matching COP (PBH0). Measurement of systemic parameters, microvascular hemodynamics, capillary perfusion, and intravascular and tissue O2 levels was performed at 18% Hct. Restitution of O2-carrying capacity with PBH13 increased arterial pressure and triggered vasoconstriction, low perfusion, and high peripheral resistance. PBH4 and PBH0 exhibited lower arterial pressures compared with PBH13. Exchange transfused animals with PBH8 and PBH4 better maintained perfusion and functional capillary density than PBH13. Blood gas parameters and acid-base balance were recovered proportional to microvascular perfusion. Arterial O2 tensions were improved with PBH4 and PBH8 by preventing O2 precapillary release and increasing O2 reserve. Further studies to establish PBH optimal dosage, efficacy, safety, and its effect on outcome are indicated before Hb-based O2-carrying blood substitutes are implemented in routine practice.

Balance between vasoconstriction and enhanced oxygen delivery

BACKGROUND

Hemoglobin (Hb) solutions are potential alternatives to blood transfusion when native oxygen (O(2))-carrying capacity is lacking. Polymerized bovine Hb (PBH) solutions are characterized by its vasoactivity, low O(2) affinity, oncotic effect, prolonged shelf life, and stability. Responses to facilitated O(2) transport, after exchange transfusion with PBH, were studied in the hamster window chamber model during acute extreme anemia to determine how PBH affects microvascular perfusion and tissue oxygenation.

STUDY DESIGN AND METHODS

An anemic state was induced by hemodilution with a plasma expander (70-kDa dextran). After hemodilution, animals were randomly assigned to exchange transfusion groups based on the concentration of PBH used, namely, PBH at 13 g(Hb) per dL (PBH13), PBH diluted to 8 or 4 g(Hb) per dL in albumin solution at matching colloidal osmotic pressure (COP; PBH8 and PBH4), and no PBH only albumin solution at matching COP (PBH0). Measurement of systemic variables, microvascular hemodynamics, capillary perfusion, and intravascular and tissue O(2) levels was performed at 11 percent Hct.

RESULTS

Restitution of O(2)-carrying capacity with PBH13 restored higher arterial pressure and triggered vasoconstriction, low perfusion, and higher peripheral resistance. Groups PBH4 and PBH0 had lower arterial pressures than Group PBH13. Groups PBH4 and PBH8 maintained higher perfusion and functional capillary density than Groups PBH13 and PBH0. In all groups, blood gas variables and acid-base balance were recovered proportional to microvascular perfusion. Arterial O(2) tensions were improved for Groups PBH4 and PBH8 by preventing O(2) precapillary release and increasing O(2) reserve.

CONCLUSION

Further studies to establish acellular Hb optimal dosage, efficacy, safety, and effects on outcome are indicated before these solutions are implemented in routine practice.

Polymerized bovine hemoglobin decreases oxygen delivery during normoxia and acute hypoxia in the rat

Hemoglobin-based oxygen carriers (HBOC) have been primarily studied for blood loss treatment. More recently infusions of HBOC in euvolemic subjects have been proposed for a wide variety of potential therapies in which increased tissue oxygenation would be beneficial. However, compared with the exchange transfusion models to study blood loss, less is known about HBOC oxygen delivery and vasoacitvity when it is infused in euvolemic subjects. We hypothesized that HBOC [polymerized bovine hemoglobin (PBvHb)] infusion creating hypervolemia would increase oxygen delivery to tissues during acute global hypoxia. Vascular oxygen content and hemodynamics were determined after euvolemic rats were infused with 3 ml of either lactated Ringer or PBvHb solution (13 g/dl, 1.3 g/kg) during acute hypoxia (FIO2 = 10%, 4 h) or normoxia (FIO2 = 21%) exposure. Our data demonstrated that compared with Ringer-infused animals, in hypoxia and normoxia, PBvHb treatment improved oxygen content but raised mean arterial pressure, lowered stroke volume, heart rate, and cardiac index, which resulted in a net reduction in blood flow and oxygen delivery to the tissues. The PBvHb vasoactive effect was similar in magnitude and direction as to the Ringer-infused animals treated with a nitric oxide synthase inhibitor nitro-l-arginine, suggesting the PBvHb effect is mediated via nitric oxide scavenging. We conclude that infusion of PBvHb is not likely to be useful in treating global hypoxia under these conditions.

Microvascular experimental evidence on the relative significance of restoring oxygen carrying capacity vs. blood viscosity in shock resuscitation

The development of volume replacement fluids for resuscitation in hemorrhagic shock comprises oxygen carrying and non carrying fluids. Non oxygen carrying fluids or plasma expanders are used up to the transfusion trigger, and upon reaching this landmark either blood, and possibly in the near future oxygen carrying blood substitutes, are used. An experimental program in hemorrhagic shock using the hamster chamber window model allowed to compare the relative performance of most fluids proposed for shock resuscitation. This model allows investigating simultaneously the microcirculation and systemic reactions, in the awake condition, in a tissue isolated from the environment. Results from this program show that in general plasma expanders such as Ringer's lactate and dextran 70 kDa do not sufficiently restore blood viscosity upon reaching the transfusion trigger, causing microvascular collapse. This is in part restored by a blood transfusion, independently of the oxygen carrying capacity of red blood cells. These results lead to the proposal that effective blood substitutes must be designed to prevent microvascular collapse, manifested in the decrease of functional capillary density. Achievement of this goal, in combination with the increase of oxygen affinity, significantly postpones the need for a blood transfusion, and lowers the total requirement of restoration of intrinsic oxygen carrying capacity.

Po2 profiles near arterioles and tissue oxygen consumption in rat mesentery

A scanning phosphorescence quenching microscopy technique, designed to prevent accumulated O2 consumption by the method, was applied to Po2 measurements in mesenteric tissue. In an attempt to further increase the accuracy of the measurements, albumin-bound probe was topically applied to the tissue and an objective-mounted pressurized bag was used to reduce the oxygen transport bypass through the thin layer of fluid over the mesentery. Po2 was measured at multiple sites perpendicular to the blood/wall interface in the vicinity of 84 mesenteric arterioles (7–39 μm in diameter) at distances of 5, 15, 30, 60, 120, and 180 μm in seven anesthetized Sprague-Dawley rats, thereby creating Po2 profiles. Interstitial Po2 above and immediately beside arterioles was found to agree with known intravascular values. No significant difference in Po2 profiles was found between small and large arterioles, indicating a small longitudinal Po2 gradient in the precapillary mesenteric microvasculature. In addition, the Po2 profiles were used to calculate oxygen consumption in the mesenteric tissue (56–65 nl O2·cm−3·s−1). Correction of these values for contamination with ambient oxygen yielded an oxygen consumption rate of 60–68 nl O2·cm−3·s−1, the maximal limit for consumption in the mesentery. The results were compared with measurements made by other workers in regard to the employed techniques.

Influence of intramolecular cross-links on the molecular, structural and functional properties of PEGylated haemoglobin

The influence of intramolecular cross-links on the molecular, structural and functional properties of PEGylated {PEG [poly(ethylene glycol)]-conjugated} haemoglobin has been investigated. The sites and the extent of PEGylation of haemoglobin by reductive alkylation are not influenced by the presence of an alphaalpha-fumaryl cross-link at Lys-99(alpha). The propylated hexaPEGylated cross-linked haemoglobin, (propyl-PEG5K)(6)-alphaalpha-Hb, exhibits a larger molecular radius and lower colloidal osmotic pressure than propylated hexaPEGylated non-cross-linked haemoglobin, (propyl-PEG5K)(6)-Hb. Perturbation of the haem microenvironment and the alpha1beta2 interface by PEGylation of haemoglobin is reduced by intramolecular cross-linking. Sedimentation velocity analysis established that PEGylation destabilizes the tetrameric structure of haemoglobin. (Propyl-PEG5K)(6)-Hb and (propyl-PEG5K)(6)-alphaalpha-Hb sediment as stable dimeric and tetrameric molecules, respectively. The betabeta-succinimidophenyl PEG-2000 cross-link at Cys-93(beta) outside the central cavity also influences the molecular properties of haemoglobin, comparable to that by the alphaalpha-fumaryl cross-link within the central cavity. However, the influence of the two cross-links on the oxygen affinity of PEGylated haemoglobin are very distinct, indicating that the high oxygen affinity of PEGylated haemoglobin is not a direct consequence of the dissociation of the haemoglobin tetramers into dimers. alphaalpha-Fumaryl cross-linking is preferred to modulate both oxygen affinity and molecular properties of PEGylated haemoglobin, and cross-linking outside the central cavity could only modulate molecular properties of PEGylated haemoglobin. It is suggested that PEGylation induces a hydrodynamic drag on haemoglobin and this plays a role in the microcirculatory properties of PEGylated haemoglobin.

Early resuscitation with polymerized bovine hemoglobin reverses acidosis, but not peripheral tissue oxygenation, in a severe hamster shock model

Awake hamsters equipped with the dorsal window chamber preparation were subjected to hemorrhage of 50% of the estimated blood volume. Initial resuscitation (25% of estimated blood volume) with polymerized bovine hemoglobin (PBH) or 10% hydroxyethyl starch (HES) occurred in concert with an equivolumetric bleeding to simulate the early, prehospital setting (exchange transfusion). Resuscitation (25% of estimated blood volume) without bleeding was performed with PBH, HES, or autologous red blood cells (HES-RBCs). Peripheral microcirculation, tissue oxygenation, and systemic hemodynamic and blood gas parameters were assessed. After exchange transfusion, base deficit was -8.6 +/- 3.7 mmol/L (PBH) and -5.1 +/- 5.3 mmol/L (HES) (not significant). Functional capillary density was 17% +/- 6% of baseline (PBH) and 31% +/- 11% (HES) (P < 0.05) and arteriolar diameter 73% +/- 3% of baseline (PBH) and 90% + 5% (HES) (P < 0.01). At the end, hemoglobin levels were 3.7 +/- 0.3 g/dL with HES, 8.2 +/- 0.6 g/dL with PBH, and 10.4 +/- 0.8 g/dL with HES-RBCs (P < 0.01 HES vs. PBH and HES-RBCs, P < 0.05 PBH vs. HES-RBCs). Base excess was restored to baseline with PBH and HES-RBCs, but not with HES (P < 0.05). Functional capillary density was 46% +/- 5% of baseline (PBH), 62% + 20% (HES-RBCs), and 36% +/- 19% (HES) (P < 0.01 HES-RBCs vs. HES). Peripheral oxygen delivery and consumption was highest with HES-RBCs, followed by PBH (P < 0.05 HES-RBCs vs. PBH, P < 0.01 HES-RBCs and PBH vs. HES). In conclusion, the PBH led to a correction of base deficit comparable to blood transfusion. However, oxygenation of the peripheral tissue was inferior with PBH. This was attributed to its negative impact on the peripheral microcirculation caused by arteriolar vasoconstriction.

Plasma viscosity regulates systemic and microvascular perfusion during acute extreme anemic conditions

The hamster window chamber model was used to study systemic and microvascular hemodynamic responses to extreme hemodilution with low- and high-viscosity plasma expanders (LVPE and HVPE, respectively) to determine whether plasma viscosity is a factor in homeostasis during extreme anemic conditions. Moderated hemodilution was induced by two isovolemic steps performed with 6% 70-kDa dextran until systemic hematocrit (Hct) was reduced to 18% ( level 2). In a third isovolemic step, hemodilution with LVPE (6% 70-kDa dextran, 2.8 cP) or HVPE (6% 500-kDa dextran, 5.9 cP) reduced Hct to 11%. Systemic parameters, cardiac output (CO), organ flow distribution, microhemodynamics, and functional capillary density, were measured after each exchange dilution. Fluorescent-labeled microspheres were used to measure organ (brain, heart, kidney, liver, lung, and spleen) and window chamber blood flow. Final blood and plasma viscosities after the entire protocol were 2.1 and 1.4 cP, respectively, for LVPE and 2.8 and 2.2 cP, respectively, for HVPE (baseline = 4.2 and 1.2 cP, respectively). HVPE significantly elevated mean arterial pressure and CO compared with LVPE but did not increase vascular resistance. Functional capillary density was significantly higher for HVPE [87% (SD 7) of baseline] than for LVPE [42% (SD 11) of baseline]. Increases in mean arterial blood pressure, CO, and shear stress-mediated factors could be responsible for maintaining organ and microvascular perfusion after exchange with HVPE compared with LVPE. Microhemodynamic data corresponded to microsphere-measured perfusion data in vital organs.

Blood viscosity maintains microvascular conditions during normovolemic anemia independent of blood oxygen-carrying capacity

Responses to exchange transfusion with red blood cells (RBCs) containing methemoglobin (MetRBC) were studied in an acute isovolemic hemodiluted hamster window chamber model to determine whether oxygen content participates in the regulation of systemic and microvascular conditions during extreme hemodilution. Two isovolemic hemodilution steps were performed with 6% dextran 70 kDa (Dex70) until systemic hematocrit (Hct) was reduced to 18% ( Level 2). A third-step hemodilution reduced the functional Hct to 75% of baseline by using either a plasma expander (Dex70) or blood adjusted to 18% Hct with all MetRBCs. In vivo functional capillary density (FCD), microvascular perfusion, and oxygen distribution in microvascular networks were measured by noninvasive methods. Methylene blue was administered intravenously to reduce methemoglobin (rRBC), which increased oxygen content with no change in Hct or viscosity from MetRBC. Final blood viscosities after the entire protocol were 2.1 cP for Dex70 and 2.8 cP for MetRBC (baseline, 4.2 cP). MetRBC had a greater mean arterial pressure (MAP) than did Dex70. FCD was substantially higher for MetRBC [82 (SD 6) of baseline] versus Dex70 [38 (SD 10) of baseline], and reduction of methemoglobin to oxyhemoglobin did not change FCD [84% (SD 5) of baseline]. Po2levels measured with palladium-meso-tetra(4-carboxyphenyl)porphyrin phosphorescence were significantly changed for Dex70 and MetRBC compared with Level 2 (Hct 18%). Reduction of methemoglobin to oxyhemoglobin partially restored Po2to Level 2. Wall shear rate and wall shear stress decreased in arterioles and venules for Dex70 and did not change for MetRBC or rRBC. Increased MAP and shear stress-mediated factors could be the possible mechanisms that improved perfusion flow and FCD after exchange for MetRBC. Thus the fall in systemic and microvascular conditions during extreme hemodilution with low-viscosity plasma expanders seems to be, in part, from the decrease in blood viscosity independent of the reduction in oxygen content.

MICROVASCULAR PERSPECTIVE OF OXYGEN-CARRYING AND -NONCARRYING BLOOD SUBSTITUTES

The development of an alternative to natural blood has evolved from the initial goal of replicating blood properties to the current objective of formulating a fluid that can be used to replace blood while preserving microvascular function and delivering oxygen. The properties of this fluid are counterintuitive and different from blood because it has high viscosity, oxygen affinity, and a low oxygen carrier concentration when compared with blood. The optimal oxygen carrier devised presently is poly-ethylene-conjugated human hemoglobin, a material demonstrated to be vasoinactive and void of the toxicities present in previous hemoglobin formulations. A feature of this material is that it is effective in small quantities, and therefore amplifies the equivalent supply of blood derived from blood donations.

Nitric oxide regulation of microvascular oxygen exchange during hypoxia and hyperoxia

The objective of this work was to test the hypothesis that the limitation of nitric oxide (NO) availability accentuates microvascular reactivity to oxygen. The awake hamster chamber window model was rendered hypoxic and hyperoxic by ventilation with 10 and 100% oxygen. Systemic and microvascular parameters were determined in the two conditions and compared with normoxia in a group receiving the NO scavenger nitronyl nitroxide and a control group receiving only the vehicle (saline). Mean arterial blood pressure did not change with different gas mixtures during infusion of the vehicle, but it increased significantly in the NO-depleted group. NO scavenging increased the reactivity of microvessels to the changed oxygen supply, causing the arteriolar wall to significantly increase oxygen consumption. Tissue Po2 was correspondingly significantly reduced during NO scavenger infusion. The present findings support the hypothesis that microvascular oxygen consumption is proportional to oxygen-induced vasoconstriction. The effect of oxygen on vascular tone is modulated by NO. As a consequence, NO acts as a regulator of the vessel wall oxygen consumption. The vessel wall consumes oxygen in proportion to the local Po2, and an impairment of NO availability renders the circulation more sensitive to changes in the oxygen supply.

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.