Hemoglobin ß93 Cysteine Is Not Required for Export of Nitric Oxide Bioactivity From the Red Blood Cell.

BACKGROUND: Nitrosation of a conserved cysteine residue at position 93 in the hemoglobin ß chain (ß93C) to form S-nitroso (SNO) hemoglobin (Hb) is claimed to be essential for export of nitric oxide (NO) bioactivity by the red blood cell (RBC) to mediate hypoxic vasodilation and cardioprotection. METHODS: To test this hypothesis, we used RBCs from mice in which the ß93 cysteine had been replaced with alanine (ß93A) in a number of ex vivo and in vivo models suitable for studying export of NO bioactivity. RESULTS: In an ex vivo model of cardiac ischemia/reperfusion injury, perfusion of a mouse heart with control RBCs (ß93C) pretreated with an arginase inhibitor to facilitate export of RBC NO bioactivity improved cardiac recovery after ischemia/reperfusion injury, and the response was similar with ß93A RBCs. Next, when human platelets were coincubated with RBCs and then deoxygenated in the presence of nitrite, export of NO bioactivity was detected as inhibition of ADP-induced platelet activation. This effect was the same in ß93C and ß93A RBCs. Moreover, vascular reactivity was tested in rodent aortas in the presence of RBCs pretreated with S-nitrosocysteine or with hemolysates or purified Hb treated with authentic NO to form nitrosyl(FeII)-Hb, the proposed precursor of SNO-Hb. SNO-RBCs or NO-treated Hb induced vasorelaxation, with no differences between ß93C and ß93A RBCs. Finally, hypoxic microvascular vasodilation was studied in vivo with a murine dorsal skin-fold window model. Exposure to acute systemic hypoxia caused vasodilatation, and the response was similar in ß93C and ß93A mice. CONCLUSIONS: RBCs clearly have the fascinating ability to export NO bioactivity, but this occurs independently of SNO formation at the ß93 cysteine of Hb.

Posttransfusion Increase of Hematocrit per se Does Not Improve Circulatory Oxygen Delivery due to Increased Blood Viscosity.

BACKGROUND: Blood transfusion is used to treat acute anemia with the goal of increasing blood oxygen-carrying capacity as determined by hematocrit (Hct) and oxygen delivery (DO2). However, increasing Hct also increases blood viscosity, which may thus lower DO2 if the arterial circulation is a rigid hydraulic system as the resistance to blood flow will increase. The net effect of transfusion on DO2 in this system can be analyzed by using the relationship between Hct and systemic blood viscosity of circulating blood at the posttransfusion Hct to calculate DO2 and comparing this value with pretransfusion DO2. We hypothesized that increasing Hct would increase DO2 and tested our hypothesis by mathematically modeling DO2 in the circulation. METHODS: Calculations were made assuming a normal cardiac output (5 L/min) with degrees of anemia ranging from 5% to 80% Hct deficit. We analyzed the effects of transfusing 0.5 or more units of 300 cc of packed red blood cells (PRBCs) at an Hct of 65% and calculated microcirculatory DO2 after accounting for increased blood viscosity and assuming no change in blood pressure. Our model accounts for O2 diffusion out of the circulation before blood arriving to the nutritional circulation and for changes in blood flow velocity. The immediate posttransfusion DO2 was also compared with DO2 after the transient increase in volume due to transfusion has subsided. RESULTS: Blood transfusion of up to 3 units of PRBCs increased DO2 when Hct (or hemoglobin) was 60% lower than normal, but did not increase DO2 when administered before this threshold. CONCLUSIONS: After accounting for the effect of increasing blood viscosity on blood flow owing to increasing Hct, we found in a mathematical simulation of DO2 that transfusion of up to 3 units of PRBCs does not increase DO2, unless anemia is the result of an Hct deficit greater than 60%. Observations that transfusions occasionally result in clinical improvement suggest that other mechanisms possibly related to increased blood viscosity may compensate for the absence of increase in DO2.

Microhemodynamic aberrations created by transfusion of stored blood.

BACKGROUND: Human red blood cells (RBCs) can be stored for up to 42 days under controlled conditions. Physical and chemical changes occur during RBC storage, altering their function. This study links stored cell mechanical changes with hemodynamic functional alterations upon transfusion. STUDY DESIGN AND METHODS: Mechanical properties of fresh and stored RBCs were evaluated in vitro. Their transfusion effects were evaluated in vivo using intravital microscopy of the rat's cremaster muscle preparation. Rats were hemodiluted to 30% hematocrit, to mimic an anemic state before transfusion, and then exchange-transfused with fresh or stored cells. RESULTS: In vitro studies on rheology and oxygen affinity of stored cells confirmed previously published results. Storage was found to modify static and dynamic RBC mechanic behavior. After transfusion, systemic hemodynamics were similar for fresh and stored cells; however, microvascular hemodynamics were drastically affected by stored cells. Stored cells reduced blood flow and oxygen delivery. Additionally, the presence of stored cells in circulation affected cell-to-cell and cell-to-wall interactions and affected cell hydrodynamics. Stored cells disrupted the RBC cell-free layer and wall shear stress signals. CONCLUSION: The reduced cell deformability due to RBC "storage lesions" caused pathologic changes in microvascular hemodynamics, endothelial cell mechanotransduction, and RBC dynamics. Thus, the mechanical changes of blood-banked cells can limit transfusion ability to achieve its intended goal.

Effects of fibrinogen concentrate after shock/resuscitation: a comparison between in vivo microvascular clot formation and thromboelastometry*.

OBJECTIVES: Dilutional coagulopathy after resuscitation with crystalloids/colloids clinically often appears as diffuse microvascular bleeding. Administration of fibrinogen reduces bleeding and increases maximum clot firmness, measured by thromboelastometry. Study objective was to implement a model where microvascular bleeding can be directly assessed by visualizing clot formation in microvessels, and correlations can be made to thromboelastometry. DESIGN: Randomized animal study. SETTING: University research laboratory. SUBJECTS: Male Syrian Golden hamsters. INTERVENTIONS: Microvessels of Syrian Golden hamsters fitted with a dorsal window chamber were studied using videomicroscopy. After 50% hemorrhage followed by 1 hour of hypovolemia resuscitation with 35% of blood volume using a high-molecular-weight hydroxyethyl starch solution (Hextend, Hospira, MW 670 kD) occurred. Animals were then treated with 250 mg/kg fibrinogen IV (Laboratoire français du Fractionnement et des Biotechnologies, Paris, France) or an equal volume of saline before venular vessel wall injuries was made by directed laser irradiation, and the ability of microthrombus formation was assessed. MEASUREMENTS AND MAIN RESULTS: Thromboelastometric measurements of maximum clot firmness were performed at the beginning and at the end of the experiment. Resuscitation with hydroxyethyl starch and sham treatment significantly decreased FIBTEM maximum clot firmness from 32 ± 9 mm at baseline versus 13 ± 5 mm after sham treatment (p < 0.001). Infusion of fibrinogen concentrate significantly increased maximum clot firmness, restoring baseline levels (baseline 32 ± 9 mm; after fibrinogen administration 29 ± 2 mm). In vivo microthrombus formation in laser-injured vessels significantly increased in fibrinogen-treated animals compared with sham (77% vs 18%). CONCLUSIONS: Fibrinogen treatment leads to increased clot firmness in dilutional coagulopathy as measured with thromboelastometry. At the microvascular level, this increased clot strength corresponds to an increased prevalence of thrombus formation in vessels injured by focused laser irradiation.

Plasma expander and blood storage effects on capillary perfusion in transfusion after hemorrhage.

BACKGROUND: Treating hemorrhage with blood transfusions in subjects previously hemodiluted with different colloidal plasma expanders, using fresh autologous blood or blood that has been stored for 2 weeks, allows identifying the interaction between type of plasma expander and differences in blood storage. STUDY DESIGN AND METHODS: Studies used the hamster window chamber model. Fresh autologous plasma, 130-kDa starch-based plasma expander (hydroxyethyl starch [HES]), or 4% polyethylene glycol-conjugated albumin (PEG-Alb) was used for 20% of blood volume (BV) hemodilution. Hemodilution was followed by a 55% by BV 40-minute hemorrhagic shock period, treated with transfusion of fresh or blood that was stored for 2 weeks. Outcome was evaluated 1 hour after blood transfusion in terms of microvascular and systemic variables. RESULTS: Results were principally dependent on the type of colloidal solution used during hemodilution, 4% PEG-Alb yielding the best microvascular recovery evaluated in terms of the functional capillary density. This result was consistent whether fresh blood or stored blood was used in treating the subsequent shock period. Fresh blood results were significantly better in systemic and microvascular terms relative to stored blood. HES and fresh plasma hemodilution yielded less favorable results, a difference that was enhanced when fresh versus stored blood was compared in their efficacy of correcting the subsequent hemorrhage. CONCLUSION: The type of plasma expander used for hemodilution influences the short-term outcome of subsequent volume resuscitation using blood transfusion, 4% PEG-Alb providing the most favorable outcome by comparison to HES or fresh plasma.

Prony Analysis of Left Ventricle Pressure and Volume.

Direct measurement of cardiac pressure-volume (PV) relationships is the gold standard for assessment of ventricular hemodynamics, but few innovations have been made to "multi-beat" PV analysis beyond traditional signal processing. The Prony method solves the signal recovery problem with a series of dampened exponentials or sinusoids. It achieves this by extracting the amplitude, frequency, dampening, and phase of each component. Since its inception, application of the Prony method to biologic and medical signal has demonstrated a relative degree of success, as a series of dampened complex sinusoids easily generalizes to multifaceted physiological processes. In cardiovascular physiology, the Prony analysis has been used to determine fatal arrythmia from electrocardiogram signals. However, application of the Prony method to simple left ventricular function based on pressure and volume analysis is absent. We have developed a new pipeline for analysis of pressure volume signals recorded from the left ventricle. We propose fitting pressure-volume data from cardiac catheterization to the Prony method for pole extraction and quantification of the transfer function. We implemented the Prony algorithm using open-source Python packages and analyzed the pressure and volume signals before and after severe hemorrhagic shock, and after resuscitation with stored blood. Each animal (n = 6 per group) underwent a 50% hemorrhage to induce hypovolemic shock, which was maintained for 30 min, and resuscitated with 3-week-old stored RBCs until 90% baseline blood pressure was achieved. Pressure-volume catheterization data used for Prony analysis were 1 s in length, sampled at 1000 Hz, and acquired at the time of hypovolemic shock, 15 and 30 min after induction of hypovolemic shock, and 10, 30, and 60 min after volume resuscitation. We next assessed the complex poles from both pressure and volume waveforms. To quantify deviation from the unit circle, which represents deviation from a Fourier series, we counted the number of poles at least 0.2 radial units away from it. We found a significant decrease in the number of poles after shock (p = 0.0072 vs. baseline) and after resuscitation (p = 0.0091 vs. baseline). No differences were observed in this metric pre and post volume resuscitation (p = 0.2956). We next found a composite transfer function using the Prony fits between the pressure and volume waveforms and found differences in both the magnitude and phase Bode plots at baseline, during shock, and after resuscitation. In summary, our implementation of the Prony analysis shows meaningful physiologic differences after shock and resuscitation and allows for future applications to broader physiological and pathophysiological conditions.

Autoregulation and mechanotransduction control the arteriolar response to small changes in hematocrit.

Here, we present an analytic model of arteriolar mechanics that accounts for key autoregulation mechanisms, including the myogenic response and the vasodilatory effects of nitric oxide (NO) in the vasculature. It couples the fluid mechanics of blood flow in arterioles with solid mechanics of the vessel wall and includes the effects of wall shear stress- and stretch-induced endothelial NO production. The model can be used to describe the regulation of blood flow and NO transport under small changes in hematocrit and to analyze the regulatory response of arterioles to small changes in hematocrit. Our analysis revealed that the experimentally observed paradoxical increase in cardiac output with small increases in hematocrit results from the combination of increased NO production and the effects of a strong myogenic response modulated by elevated levels of WSS. Our findings support the hypothesis that vascular resistance varies inversely with blood viscosity for small changes in hematocrit in a healthy circulation that responds to shear stress stimuli. They also suggest beneficial effects independent of changes in O(2) carrying capacity associated with the postsurgical transfusion of one or two units of blood.

PEG-albumin supraplasma expansion is due to increased vessel wall shear stress induced by blood viscosity shear thinning.

We studied the extreme hemodilution to a hematocrit of 11% induced by three plasma expanders: polyethylene glycol (PEG)-conjugated albumin (PEG-Alb), 6% 70-kDa dextran, and 6% 500-kDa dextran. The experimental component of our study relied on microelectrodes and cardiac output to measure both the rheological properties of plasma-expander blood mixtures and nitric oxide (NO) bioavailability in vessel walls. The modeling component consisted of an analysis of the distribution of wall shear stress (WSS) in the microvessels. Our experiments demonstrated that plasma expansion with PEG-Alb caused a state of supraperfusion with cardiac output 40% above baseline, significantly increased NO vessel wall bioavailability, and lowered peripheral vascular resistance. We attributed this behavior to the shear thinning nature of blood and PEG-Alb mixtures. To substantiate this hypothesis, we developed a mathematical model of non-Newtonian blood flow in a vessel. Our model used the Quemada rheological constitutive relationship to express blood viscosity in terms of both hematocrit and shear rate. The model revealed that the net effect of the hemodilution induced by relatively low-viscosity shear thinning PEG-Alb plasma expanders is to reduce overall blood viscosity and to increase the WSS, thus intensifying endothelial NO production. These changes act synergistically, significantly increasing cardiac output and perfusion due to lowered overall peripheral vascular resistance.

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.

Improved resuscitation from hemorrhagic shock with Ringer's lactate with increased viscosity in the hamster window chamber model.

BACKGROUND: Infusion of large volume of fluid is practiced in the treatment of hemorrhagic shock although resuscitation with small fluid volumes reduces the risks associated with fluid overload. We explored the hypothesis that reduced Ringer's lactate (RL) volume restoration in hemorrhage is significantly improved by increasing its viscosity, leading to improved microvascular conditions. METHODS: Awake hamsters were subjected to a hemorrhage of 50% of blood volume followed by a shock period of 1 hour. They were resuscitated with conventional RL (n = 6) or with RL whose viscosity was increased by the addition of 0.3% alginate (RL-HV) (n = 6). In both cases, the volume infused was 200% of shed blood. RESULTS: After resuscitation, blood and plasma viscosities were 1.9 cp ± 0.18 cp and 1.0 cp ± 0.03 cp in RL and 2.5 cp ± 0.34 cp and 1.6 cp ± 0.05 cp in RL-HV. Mean arterial pressure was lower than baseline in RL. Arteriolar diameter and arteriolar and venular flow were significantly higher in RL-HV. Functional capillary density was significantly higher in RL-HV than RL. After 90 minutes of resuscitation, functional capillary density was lower than baseline in RL, whereas it was maintained in RL-HV. Arteriolar PO2 was higher in RL-HV than RL. Microcirculation O2 delivery and tissue PO2 were significantly higher in RL-HV. CONCLUSIONS: Increasing blood and plasma viscosities in resuscitation from hemorrhagic shock with increased viscosity RL improves microvascular hemodynamics and oxygenation parameters.

A model of anemic tissue perfusion after blood transfusion shows critical role of endothelial response to shear stress stimuli.

Although some of the cardiovascular responses to changes in hematocrit (Hct) are not fully quantified experimentally, available information is sufficient to build a mathematical model of the consequences of treating anemia by introducing RBCs into the circulation via blood transfusion. We present such a model, which describes how the treatment of normovolemic anemia with blood transfusion impacts oxygen (O2) delivery (DO2, the product of blood O2 content and arterial blood flow) by the microcirculation. Our analysis accounts for the differential response of the endothelium to the wall shear stress (WSS) stimulus, changes in nitric oxide (NO) production due to modification of blood viscosity caused by alterations of both hematocrit (Hct) and cell free layer thickness, as well as for their combined effects on microvascular blood flow and DO2. Our model shows that transfusions of 1- and 2-unit of blood have a minimal effect on DO2 if the microcirculation is unresponsive to the WSS stimulus for NO production that causes vasodilatation increasing blood flow and DO2. Conversely, in a fully WSS responsive organism, blood transfusion significantly enhances blood flow and DO2, because increased viscosity stimulates endothelial NO production causing vasodilatation. This finding suggests that evaluation of a patients' pretransfusion endothelial WSS responsiveness should be beneficial in determining the optimal transfusion requirements for treating patients with anemia.NEW & NOTEWORTHY Transfusion of 1 or 2 units of blood accounts for about 3/4 of the world blood consumption of 119 million units per year, whereas a current world demand deficit is on the order of 100 million units. Therefore, factors supporting the practice of transfusing 1 unit instead of 2 are of interest, given their potential to expand the number of interventions without increasing blood availability. Our mathematical model provides a physiological support for this practice.

The variability of blood pressure due to small changes of hematocrit.

The hematocrit (Hct) of awake hamsters was lowered to 90% of baseline by isovolemic hemodilution using hamster plasma to determine the acute effect of small changes in Hct and blood viscosity on systemic hemodynamics. Mean arterial blood pressure increased, reaching a maximum of about 10% above baseline (8.6 +/- 5.5 mmHg) when Hct decreased 8.4 +/- 1.9% (P < 0.005). Cardiac output increased continuously with hemodilution. These conditions were reached at approximately 60 min after exchange transfusion and remained stationary for 1 h. Peripheral vascular resistance was approximately constant up to a decrease of Hct of about 10% and then fell continuously with lowering Hct. Vascular hindrance or vascular resistance independent of blood viscosity increased by about 20% and remained at this level up to an Hct decrease of 20%, indicating that the vasculature constricted with the lowered Hct. The results for the initial 2-h period are opposite but continuous with previous findings with small increases in Hct. In conclusion, limited acute anemic conditions increase mean arterial blood pressure during the initial period of 2 h, an effect that is quantitatively similar but opposite to the acute increase of Hct during the same period.

Perfusion vs. oxygen delivery in transfusion with "fresh" and "old" red blood cells: the experimental evidence.

We review the experimental evidence showing systemic and microvascular effects of blood transfusions instituted to support the organism in extreme hemodilution and hemorrhagic shock, focusing on the use of fresh vs. stored blood as a variable. The question: "What does a blood transfusion remedy?" was analyzed in experimental models addressing systemic and microvascular effects showing that oxygen delivery is not the only function that must be addressed. In extreme hemodilution and hemorrhagic shock blood transfusions simultaneously restore blood viscosity and oxygen carrying capacity, the former being critically needed for re-establishing a functional mechanical environment of the microcirculation, necessary for obtaining adequate capillary blood perfusion. Increased oxygen affinity due to 2,3 DPG depletion is shown to have either no effect or a positive oxygenation effect, when the transfused red blood cells (RBCs) do not cause additional flow impairment due to structural malfunctions including increased rigidity and release of hemoglobin. It is concluded that fresh RBCs are shown to be superior to stored RBCs in transfusion, however increased oxygen affinity may be a positive factor in hemorrhagic shock resuscitation. Although experimental studies seldom reproduce emergency and clinical conditions, nonetheless they serve to explore fundamental physiological mechanisms in the microcirculation that cannot be directly studied in humans.

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.

A Mechanistic Analysis of Possible Blood Transfusion Failure to Increase Circulatory Oxygen Delivery in Anemic Patients.

The effects of changing hematocrit (Hct) on the rate of circulatory oxygen ([Formula: see text]) delivery were modeled analytically to describe transfusion of 0.5-3.0 units of packed red blood cells (pRBC, 300 mL/unit, 60% Hct) to anemic patients. In our model, Hct affects [Formula: see text] delivery to the microcirculation by changing blood [Formula: see text] carrying capacity and blood viscosity, which in turn affects blood flow velocity and, therefore, [Formula: see text] delivery. Changing blood velocity impacts the [Formula: see text] delivery by affecting the oxygen diffusive losses as blood transits through the arteriolar vasculature. An increase in Hct has two opposite effects: it increases the blood [Formula: see text] carrying capacity and decreases the flow velocity. This suggests the existence of an optimal Hct that maximizes [Formula: see text] delivery. Our results show that maximal [Formula: see text] delivery occurs in the anemic range, where [Formula: see text]%. Optimal blood management is associated with transfusing enough units up to reaching maximal [Formula: see text] delivery. Although somewhat complex to implement, this practice would result in both substantial blood savings and improved [Formula: see text] delivery.

Modulation of perfusion and oxygenation by red blood cell oxygen affinity during acute anemia.

Responses to exchange transfusion using red blood cells (RBCs) with modified hemoglobin (Hb) oxygen (O(2)) affinity were studied in the hamster window chamber model during acute anemia to determine its role on microvascular perfusion and tissue oxygenation. Allosteric effectors were introduced in the RBCs by electroporation. Inositol hexaphosphate (IHP) and 5-hydroxymethyl-2-furfural (5HMF) were used to decrease and increase Hb-O(2) affinity. In vitro P50s (partial pressure of O(2) at 50% Hb saturation) were modified to 10, 25, 45, and 50 mm Hg (normal P50 is 32 mm Hg). Allosteric effectors also decreased the Hill coefficient. Anemic condition was induced by isovolemic hemodilution exchanges using 6% dextran 70 kD to 18% hematocrit (Hct). Modified RBCs (at 18% Hct in 5% albumin solution) were infused by exchange transfusion of 35% of blood volume. Systemic parameters, microvascular perfusion, capillary perfusion (functional capillary density, FCD), and microvascular Po(2) levels were measured. RBcs with P50 of 45 mm Hg increased tissue Po(2) and decreased O(2) delivery (Do(2)) and extraction (Vo(2)) and RBCs with P50 of 60 mmHg reduced FCD, microvascular flow, tissue Po(2), Do(2) and Vo(2). Erythrocytes with increased Hb-O(2) affinity maintained hemodynamic conditions, Do(2) and decreased tissue Po(2). This study shows that in an anemic condition, maximal tissue Po(2) does not correspond to maximal Do(2) and Vo(2).

Lowering of blood pressure by increasing hematocrit with non nitric oxide scavenging red blood cells.

Isovolemic exchange transfusion of 40% of the blood volume in awake hamsters was used to replace native red blood cells (RBCs) with RBCs whose hemoglobin (Hb) was oxidized to methemoglobin (MetHb), MetRBCs. The exchange maintained constant blood volume and produced different final hematocrits (Hcts), varying from 48 to 62% Hct. Mean arterial pressure (MAP) did not change after exchange transfusion, in which 40% of the native RBCs were replaced with MetRBCs, without increasing Hct. Increasing Hct with MetRBCs lowered MAP by 12 mm Hg when Hct was increased 12% above baseline. Further increases of Hct with MetRBCs progressively returned MAP to baseline, which occurred at 62% Hct, a 30% increase in Hct from baseline. These observations show a parabolic "U" shaped distribution of MAP against the change in Hct. Cardiac index, cardiac output divided by body weight, increased between 2 and 17% above baseline for the range of Hcts tested. Peripheral vascular resistance (VR) was decreased 18% from baseline when Hct was increased 12% from baseline. VR and MAP were above baseline for increases in Hct higher than 30%. However, vascular hindrance, VR normalized by blood viscosity (which reflects the contribution of vascular geometry), was lower than baseline for all the increases in Hct tested with MetRBC, indicating prevalence of vasodilation. These suggest that acute increases in Hct with MetRBCs increase endothelium shear stress and stimulate the production of vasoactive factors (e.g., nitric oxide [NO]). When MetRBCs were compared with functional RBCs, vasodilation was augmented for MetRBCs probably due to the lower NO scavenging of MetHb. Consequently, MetRBCs increased the viscosity related hypotension range compared with functional RBCs as NO shear stress vasodilation mediated responses are greater.

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.

Volume resuscitation from hemorrhagic shock with albumin and hexaPEGylated human serum albumin.

The effect of restoring intravascular volume with polyethylene glycol (PEG) conjugated to human serum albumin (PEG-Alb) on systemic parameters and microvascular hemodynamics after hemorrhagic shock resuscitation was studied in the hamster window chamber model. Moderate hemorrhagic shock was induced by controlled arterial bleeding of 50% of blood volume, and hypovolemia was maintained for 1h. Fluid resuscitation was accomplished by infusion of 25% of blood volume and recovery was followed over 90 min. The PEG-Alb (six chains of maleimide phenyl PEG conjugated human serum albumin at 4%) resuscitation group was compared human serum albumin (HSA) at 5% (HSA5) and 10% (HSA10) protein concentrations. Systemic parameters, microvascular perfusion and capillary perfusion (functional capillary density, FCD) were measured by noninvasive methods. Hyperoncotic solutions provided rapid restoration of blood pressure, blood gas parameters and microvascular perfusion. Systemic and microvascular recovery was best and most rapid with PEG-Alb and followed by HSA10 and HSA5. Only recovery with PEG-Alb was sustained beyond 90 min. Hemodynamic functional benefits of PEG-Alb and the potential disadvantages associated with HSA, suggest PEG-Alb as better resuscitation solution.