In vivo effects of Hb solutions on blood viscosity and rheologic behavior of RBCs: comparison with clinically used volume expanders

Effects of synthetic colloid and crystalloid solutions on hemorheology in vitro and in hemorrhagic shock

Background

Plasma expanders are commonly used in the management of critically ill patients, which may exhibit altered hemorheology. We evaluated the effects of various synthetic colloids and Lactated Ringer’s (LR) solution on hemorheological parameters in vitro and in a rodent hemorrhagic shock model.

Methods

For the in vitro experiments, rat blood was incubated with hydroxyethyl starch (HES) 130/0.4, HES 200/0.5, succinylated gelatine (GEL), or LR at various ratios. The control consisted of blood without dilution. The hemorheological parameters were measured after a 15-min incubation. For the in vivo study, rats were subjected to a severe volume-controlled hemorrhage and were resuscitated using a colloid solution (HES 130/0.4, HES 200/0.5, or GEL) or LR. The hemorheological parameters were measured 2 h after resuscitation.

Results

The GEL significantly elevated the plasma viscosity compared to the other groups. In the in vitro study, GEL and LR accelerated the erythrocyte aggregation. There was no significant difference between HES 130/0.4, HES 200/0.5, and control groups regarding the aggregation amplitude and index. In the in vivo study, the aggregation amplitude increased significantly in the GEL group compared to the HES 130/0.4, HES 200/0.5, LR, and sham groups. There was no significant difference between the groups with respect to the elongation index in vivo.

Conclusions

Hydroxyethyl starch did not change the erythrocyte aggregation compared to the control. GEL significantly accelerates the erythrocyte aggregation and elevates the plasma viscosity compared to hydroxyethyl starch. The in vitro hemorheological measurements most likely provide hints for the in vivo study.

Sodium alginate as viscosity modifier may induce aggregation of red blood cells

Viscosity of blood substitutes is among the important determinants to restore microcirculation. Sodium alginate (SA) is always mentioned as "viscosity modifier" in creating blood substitutes. In the present study, the whole blood was diluted using SA solutions to final hematocrits of 10%, 20%, and 35%, respectively. The whole blood viscosity (WBV) at different shear rates, plasma viscosity (PV), and rheological behavior of red blood cells (RBCs) was studied in vitro. The results show that SA may induce RBCs aggregation in a dose-dependent manner. Furthermore, the effect of SA on RBCs aggregation maybe involve the regulation of microcirculation.

Model analysis of local oxygen delivery with liposome-encapsulated hemoglobin

Liposome-encapsulated hemoglobins (LHs) are comparable to red blood cells (RBCs) in terms of oxygen (O(2))-carrying capacity. The smaller particle size of LHs than of platelets allows their homogeneous dispersion in circulating plasma. In this study, we evaluated the effect of LH transfusion on arterial O(2) delivery through vascular trees by simulation. A mathematical model was established on the basis of the coronary arterial anatomy, the conservation of flow and RBC flux, and Poiseuille's law. The Fåhraeus-Lindqvist, Fåhraeus, and phase separation effects were considered in the model. By assuming steady perfusion, the arterial flow and O(2) delivery were calculated for five model trees undergoing the isovolumic replacement of RBCs (0.3 mg hemoglobin (Hb)/mL) with LHs (0.2 mg Hb/mL) or a plasma volume expander (PVE). The RBC-LH exchange increased both the total flow and the total O(2) flux but had almost no effect on the relative distribution of O(2) flux. In contrast, the RBC-PVE exchange decreased the total O(2) flux and increased the proportion of regions receiving a relatively low O(2) supply. Thus, LH transfusion may compensate for an enhanced bias in RBC-associated O(2) flux under hemodilution and is expected to be beneficial for both total and local O(2) delivery.

The hydrodynamic radii of macromolecules and their effect on red blood cell aggregation

The effects of nonionic polymers on human red blood cell (RBC) aggregation were investigated. The hydrodynamic radius (Rh) of individual samples of dextran, polyvinylpyrrolidone, and polyoxyethylene over a range of molecular weights (1,500-2,000,000) were calculated from their intrinsic viscosities using the Einstein viscosity relation and directly measured by quasi-elastic light scattering, and the effect of each polymer sample on RBC aggregation was studied by nephelometry and low-shear viscometry. For all three polymers, despite their different structures, samples with Rh <4 nm were found to inhibit aggregation, whereas those with Rh >4 nm enhanced aggregation. Inhibition increased with Rh and was maximal at approximately 3 nm; above 4 nm the pro-aggregant effect increased with Rh. For comparison, the Rh of 12 plasma proteins were calculated from literature values of intrinsic viscosity or diffusion coefficient. Each protein known to promote RBC aggregation had Rh >4 nm, whereas those with Rh <4 nm either inhibited or had no effect on aggregation. These results suggest that the influence of a nonionic polymer or plasma protein on RBC aggregation is simply a consequence of its size in an aqueous environment, and that the specific type of macromolecule is of minor importance.

Hemodilution With Stoma-Free Hemoglobin at Physiologically Maintained Viscosity Delays the Onset of Vasoconstriction

Solutions of modified cell-free hemoglobin, prepared from outdated red blood cells, have been developed during the past decade to circumvent the increasing need for allogeneic blood. Despite improvements in the safety and efficacy of these solutions, undesirable effects such as an increase in vascular tone leading to hypertension have not been fully resolved, which might hinder their clinical usefulness. To discriminate between the pharmacological and rheological effects of cell-free hemoglobin, we compared the effects of blood/cell-free hemoglobin mixtures of high versus low viscosity on hemodynamics and vascular hindrance, an index of vascular tone, which was normalized for blood viscosity. Anesthetized rats were subjected to 50% exchange transfusion with (1) high-viscosity solutions: whole blood (n=5) or red blood cells mixed with cell-free hemoglobin (Hb-Hv group, n=5); (2) low-viscosity solutions: cell-free hemoglobin (Hb-Lv group, n=5) or human albumin (n=5). Two hours after hemodilution, vascular hindrance remained unchanged in animals transfused with whole blood and albumin. Hb-Lv induced an immediate and sustained increase in vascular hindrance (208%). Conversely, in Hb-Hv animals, the vascular hindrance increase was delayed and smaller (27% to 147%), whereas peripheral resistance increased gradually (94% after 2 hours). Our results demonstrate the beneficial effects of cell-free hemoglobin in the presence of the animals' own red blood cells in maintaining physiological viscosity and limiting vasoconstriction because of the pharmacological properties of cell-free hemoglobin.

Hemoglobin-based oxygen carrier provides heterogeneous microvascular oxygenation in heart and gut after hemorrhage in pigs

BACKGROUND

In this study, the hypothesis was tested that resuscitation with hemoglobin-based oxygen carriers (HBOCs) affects the oxygenation of the microcirculation differently between and within organs. To this end, we tested the influence of the volume of an HBOC on the microcirculatory oxygenation of the heart and the gut serosa and mucosa in a porcine model of hemorrhage.

METHODS

In anesthetized open-chested pigs (n = 24), a controlled hemorrhage (30 mL/kg over 1 hour) was followed by resuscitation with 10, 20, or 30 mL/kg diaspirin-crosslinked hemoglobin (DCLHb) or isovolemic resuscitation with 30 mL/kg of a 6% hydroxyethyl starch solution (HAES). Measurements included systemic and regional hemodynamic and oxygenation parameters. Microvascular oxygen pressures (microPO2) of the epicardium and the serosa and mucosa of the ileum were measured simultaneously by the palladium-porphyrin phosphorescence technique. Measurements were obtained up to 120 minutes after resuscitation.

RESULTS

After hemorrhage, a low volume of DCLHb restored both cardiac and intestinal microPO2. Resuscitation of gut microPO2 with a low volume of DCLHb was as effective as isovolemic resuscitation with HAES. Higher volumes of DCLHb did not restore cardiac microPO2, as did isovolemic resuscitation with HAES, but increased gut microPO2 to hyperoxic values, dose-dependently. Effects were similar for the serosal and mucosal microPo2. In contrast to a sustained hypertensive effect after resuscitation with DCLHb, effects of DCLHb on regional oxygenation and hemodynamics were transient.

CONCLUSION

This study showed that a low volume of DCLHb was effective in resuscitation of the microcirculatory oxygenation of the heart and gut back to control levels. Increasing the volume of DCLHb did not cause an additional increase in heart microPO2, but caused hyperoxic microvascular values in the gut to be attained. It is concluded that microcirculatory monitoring in this way elucidates the regional behavior of oxygen transport to the tissue by HBOCs, whereas systemic variables were ineffective in describing their response.

Hemoglobin solutions and tissue oxygenation

Oxygen (O2) delivery to tissues plays an important role in determining microcirulatory autoregulatory responses. The balance between O2 delivery by whole blood and tissue O2 consumption likely has evolved based on regulatory processes designed to accommodate the encapsulation of hemoglobin (Hb) within red blood cells (RBCs). The hemodynamic, rheologic, and physical properties of blood, or an alternate O2-carrying solution, can have important consequences for O2 delivery to tissue. The development of acellular hemoglobin-based oxygen carriers (HBOC) requires reassessment of the O2 loading and unloading charactistics of Hb. the effects of altering the rheologic properties of blood, and the impact of these changes on microcirculatory autoregulation and tissue oxygenation. A variety of experimental and clinical studies have demonstrated beneficial effects of HBOCs. However, mechanisms responsible for HBOC-facilitated, O2-dependent autoregulatory changes in the microcirculation have not been completely elucidated.

Volume expansion with modified hemoglobin solution, colloids, or crystalloid after hemorrhagic shock in rabbits: effects in skeletal muscle oxygen pressure and use versus arterial blood velocity and resistance

Therapeutic goals for hemorrhagic shock resuscitation are the increase of cardiac output and oxygen delivery. The possibility exists that because of microcirculatory effects, different volume expanders result in different tissue oxygen delivery and oxygen use. In a rabbit model of resuscitation from hemorrhagic shock (50% blood loss), we compared the effects of an hemoglobin-based O2-carrying solution (HbOC) with those elicited by albumin, hydroxyethyl starch (HES), or saline on systemic hemodynamics, skeletal muscle O2 pressure (PtiO2), and interstitial concentration of lactate (LACi) through the combined implantation of a microdialysis probe and a sensitive O2 electrode into the hind limb. Hemorrhagic shock induced a 50% decrease in mean arterial pressure (MAP), femoral artery blood flow (BF), and PtiO2. After resuscitation, there were statistically significant differences among the volume expanders. The increase in MAP was faster with HbOC and colloids, and slower with saline, mainly obtained by vasoconstriction for HbOC and by increased BF with albumin and HES. The maximum MAP values were significantly higher for HbOC compared with the other volume expanders. HbOC and colloids induced a faster increase in PtiO2 as compared with saline, but maximum PtiO2 values were not different among the volume expanders. Tissue oxygen use as estimated by LACi increased transiently at the beginning of volume expansion with similar maximum values. Animals resuscitated with saline had significantly higher LACi concentrations after the onset of volume expansion as compared with HbOC but not with colloids. Our results demonstrate that there are measurable differences in MAP and BF upon resuscitation with the four different solutions and there is a slower increase in tissue PtiO2 with saline than with colloids associated with significantly increased LACi consistent with delayed reoxygenation upon resuscitation with saline.

The effects of hemodilution with Hemolink upon hemodynamics and blood flow distribution in anesthetized dogs

Hemoglobin based oxygen-carrying solutions (HBOCs) as hemoglobin replacement therapeutics are being tested for clinical use. Some of these products are associated with elevations in both systemic and pulmonary vascular resistances but their effect on the distribution of blood flow to major organs in larger animals have not been extensively described. We tested two formulations of o-raffinose cross-linked human hemoglobin, Hemolink (frozen-Hemolink-1 and refrigerated Hemolink-2) and compared them to Pentaspan, a colloid volume expander in extensive clinical use. Cardiovascular measurements and the distribution of blood flow (radionuclide-labeled microspheres) to the major organs were determined in Beagle dogs (n = 5 per group). After baseline measurements, either Hemolink-1, Hemolink-2, or Pentaspan was exchange transfused in an isovolemic manner (resulting in hematocrit reduction to approximately 20-25%); measurements were made 30, 60, 120 and 180 min post-exchange. There was no significant difference in cardiac output, mean arterial pressures and systemic or pulmonary vascular resistances after exchange in any of the three groups. Myocardial blood flow increased in all three groups post-exchange but the increase was more sustained in the Hemolink groups. Endocardial/epicardial flow ratios were also maintained after exchange in all groups. Thus, Hemolink is ideally suited for volume replacement when used in conjunction with acute normovolemic hemodilution because under these circumstances, the adverse hemodynamic effects are alleviated while extra hemoglobin is added to the blood.

The effects of hemodilution with Hemolink upon hemodynamics and blood flow distribution in anesthetized dogs

Hemoglobin based oxygen-carrying solutions (HBOCs) as hemoglobin replacement therapeutics are being tested for clinical use. Some of these products are associated with elevations in both systemic and pulmonary vascular resistances but their effect on the distribution of blood flow to major organs in larger animals have not been extensively described. We tested two formulations of o-raffinose cross-linked human hemoglobin, Hemolink (frozen Hemolink-1 and refrigerated Hemolink-2) and compared them to Pentaspan, a colloid volume expander in extensive clinical use. Cardiovascular measurements and the distribution of blood flow (radionuclide-labeled microspheres) to the major organs were determined in Beagle dogs (n=5 per group). After baseline measurements, either Hemolink-1, or Hemolink-2, or Pentaspan was exchange transfused in an isovolemic manner (resulting in hematocrit reduction to approximately 20-25%); measurements were made 30, 60, 120 and 180 min post-exchange. There was no significant difference in cardiac output, mean arterial pressures and systemic or pulmonary vascular resistance after exchange in any of the three groups. Myocardial blood flow increased in all three groups post-exchange but the increase was more sustained in the Hemolink groups. Endocardial/epicardial flow ratios were also maintained after exchange in all groups. Thus, Hemolink is ideally suited for volume replacement when used in conjunction with acute normovolemic hemodilution because under these circumstances, the adverse hemodynamic effects are alleviated while extra hemoglobin is added to the blood.

O-raffinose-polymerised haemoglobin. A biochemical and pharmacological profile of an oxygen carrier

Haemoglobin-based oxygen carriers (HBOCs) represent an interesting class of blood substitutes which are undergoing advanced clinical trials. The therapeutic goal of these compounds is to avoid or reduce blood transfusion in different surgical and medical situations of acute haemoglobin deficiency. Their main advantages include availability in large volumes, storage for prolonged periods, rapid administration (without typing and cross matching) and sterilisation by pasteurisation. Their main known disadvantages are reduced circulation half-life, haemodynamic and gastrointestinal perturbations, probably related to nitric oxide (NO) scavenging, free radical induction, and alterations of biochemical and haematological parameters (increases in liver enzymes levels, platelet aggregation). Cell-free o-raffinose cross-linked and oligomerised human haemoglobin (O-r-poly-Hb) (Hemolink, Hemosol, Canada) is a modified haemoglobin with molecular weight ranging from 32- > 500 kDa. Its affinity for oxygen appears lower than normal blood and an n (Hill coefficient) value of about 1 indicates a very low degree of co-operativity. Probably related to the low O2 affinity value and to the high molecular weight, O-r-poly-Hb has been shown to induce lesser haemodynamic perturbations than other first generation modified haemoglobins. This HBOC is in Phase III clinical trials in cardiac and orthopaedic surgery for perioperative haemodilution, at doses from 25 g (250 ml)-100 g (1000 ml).