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]. P(O2) levels 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 P(O2) to 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.     

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.     

Validation of OPS imaging for microvascular measurements during isovolumic hemodilution and low hematocrits

Orthogonal polarization spectral (OPS) imaging is a new technique that can be used to visualize the microcirculation with reflected light. It uses hemoglobin absorption to visualize the red blood cells (RBCs). Thus the method could fail at low hematocrit (Hct). The aim of this study was to validate OPS imaging for quantitative measurements of diameter and functional capillary density (FCD) under conditions of hemodilution of varying degrees to achieve a wide range of Hcts. The validation was performed in the dorsal skinfold chamber of nine awake Syrian golden hamsters. Measurements of vessel diameter and FCD were performed off-line using Cap-Image on video sequences captured using OPS imaging and standard intravital fluorescence microscopy at baseline, 85, 70, 55, and 40% of the initial Hct. For hemodilution, isovolumic exchange of blood for 6% Dextran 60 was performed. Bland-Altman plots for the vessel diameter and FCD show good agreement between the two methods for both parameters at all studied Hcts. As expected, there was a systematic bias of approximately 4 microm in the diameter measurements since the RBC column was measured and not the intravascular diameter. In conclusion, OPS imaging can be used to measure diameter and FCD at a wide range of Hcts.     

Effect of hemodilution on RBC velocity, supply rate, and hematocrit in the cerebral capillary network.

The effect of isovolemic hemodilution on the circulation of red blood cells (RBCs) in the cerebrocortical capillary network was studied by intravital videomicroscopy with use of a closed-cranial-window technique in the rat. Velocity and supply rate of RBCs were measured by tracking the movement and counting the number of fluorescently labeled cells. Arterial blood was withdrawn in increments of 2 ml and replaced by serum albumin. Arterial blood pressure was maintained constant with an infusion of methoxamine. Both velocity and supply rate of RBCs increased, by approximately equal amounts, as arterial hematocrit was reduced from 44 to 15%. The maximum increase in RBC velocity was 4.6 and in RBC supply rate was 5.2 times the baseline value. Calculated lineal density of RBC, an index of capillary hematocrit, did not change with hemodilution. The results suggest that RBC flow and oxygen supply in the cerebral capillary network are maintained during isovolemic hemodilution. The "optimal hematocrit" is as low as 15%.     

Paradoxical hypotension following increased hematocrit and blood viscosity

Hematocrit (Hct) of awake hamsters and CD-1 mice was acutely increased by isovolemic exchange transfusion of packed red blood cells (RBCs) to assess the relation between Hct and blood pressure. Increasing Hct 7-13% of baseline decreased mean arterial blood pressure (MAP) by 13 mmHg. Increasing Hct above 19% reversed this trend and caused MAP to rise above baseline. This relationship is described by a parabolic function (R2 = 0.57 and P < 0.05). Hamsters pretreated with the nitric oxide (NO) synthase (NOS) inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) and endothelial NOS-deficient mice showed no change in MAP when Hct was increased by <19%. Nitrate/nitrite plasma levels of Hct-augmented hamsters increased relative to control and L-NAME treated animals. The blood pressure effect was stable 2 h after exchange transfusion. These findings suggest that increasing Hct increases blood viscosity, shear stress, and NO production, leading to vasodilation and mild hypotension. This was corroborated by measuring A1 arteriolar diameters (55.0 +/- 21.5 microm) and blood flow in the hamster window chamber preparation, which showed statistically significant increased vessel diameter (1.04 +/- 0.1 relative to baseline) and microcirculatory blood flow (1.39 +/- 0.68 relative to baseline) after exchange transfusion with packed RBCs. Larger increases of Hct (>19% of baseline) led blood viscosity to increase >50%, overwhelming the NO effect through a significant viscosity-dependent increase in vascular resistance, causing MAP to rise above baseline values.     

Erythrocyte-associated transients in capillary PO2: an isovolemic hemodilution study in the rat spinotrapezius muscle

Mathematical simulations of oxygen delivery to tissue from capillaries that take into account the particulate nature of blood flow predict the existence of oxygen tension (Po(2)) gradients between erythrocytes (RBCs). As RBCs and plasma alternately pass an observation point, these gradients are manifested as rapid fluctuations in Po(2), also known as erythrocyte-associated transients (EATs). The impact of hemodilution on EATs and oxygen delivery at the capillary level of the microcirculation has yet to be elucidated. Therefore, in the present study, phosphorescence quenching microscopy was used to measure EATs and Po(2) in capillaries of the rat spinotrapezius muscle at the following systemic hematocrits (Hct(sys)): normal (39%) and after moderate (HES1; 27%) or severe (HES2; 15%) isovolemic hemodilution using a 6% hetastarch solution. A 532-nm laser, generating 10-micros pulses concentrated onto a 0.9-microm spot, was used to obtain plasma Po(2) values 100 times/s at points along surface capillaries of the muscle. Mean capillary Po(2) (Pc(O(2)); means +/- SE) significantly decreased between conditions (normal: 56 +/- 2 mmHg, n = 45; HES1: 47 +/- 2 mmHg, n = 62; HES2: 27 +/- 2 mmHg, n = 52, where n = capillary number). In addition, the magnitude of Po(2) transients (DeltaPo(2)) significantly decreased with hemodilution (normal: 19 +/- 1 mmHg, n = 45; HES1: 11 +/- 1 mmHg, n = 62; HES2: 6 +/- 1 mmHg, n = 52). Results suggest that the decrease in Pc(O(2)) and DeltaPo(2) with hemodilution is primarily dependent on Hct(sys) and subsequent microvascular compensations.     

Pericardial delivery of omega-3 fatty acid: a novel approach to reducing myocardial infarct sizes and arrhythmias

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.     

Effects of extreme hemodilution with hemoglobin-based O2 carriers on microvascular pressure

A surface-modified polyethylene glycol-conjugated human hemoglobin (MP4) and alpha alpha-cross-linked human hemoglobin (alpha alpha Hb) were used to restore oxygen carrying capacity in conditions of extreme hemodilution (hematocrit 11%) in the hamster window model preparation. Changes in microvascular function were analyzed in terms of effects on capillary pressure and functional capillary density (FCD). MP4, at 1.0 +/- 0.2 g/dl blood concentration, significantly lowered mean arterial pressure (MAP) below baseline (99.6 +/- 7.6 mmHg) to 82.4 +/- 6.9 mmHg (P < 0.05) and decreased of FCD to 70 +/- 9%. alpha alpha Hb caused a greater recovery in MAP to 94.4 +/- 6.2 mmHg and lowered FCD to 62 +/- 8%. However, differences between alpha alpha Hb and MP4 in FCD were not statistically significant. Capillary pressures were in the ranges of 17-21 mmHg for MP4 and 15-19 mmHg for alpha alpha Hb, with both significantly lower than baseline (P < 0.05). Pressure in 80-microm-diameter arterioles was significantly increased with alpha alpha Hb relative to MP4 (P < 0.05). These results were compared with previous findings on the relation between capillary pressure and FCD; they supported the concept of a relationship between FCD and capillary pressure. Measurement of changes in arteriolar diameter, microvascular blood flow, and FCD show that there was no statistical difference between using alpha alpha Hb and MP4 in extreme hemodilution. Microvascular resistance in arterioles with a diameter range of 70-80 microm showed an increase relative to control with alpha alpha Hb, whereas MP4 caused a decrease.     

Oxygen delivery and consumption in the microcirculation after extreme hemodilution with perfluorocarbons

The oxygen transport capacity of fluorocarbons was investigated in the hamster chamber window model microcirculation to determine the rate at which oxygen is delivered to the tissue in conditions of extreme hemodilution [hematocrit (Hct) 11%]. Hydroxyethlyl starch (HES 200; 200 kDa molecular mass) was used as a plasma expander for two isovolemic hemodilutions performed with 10% HES 200 until a Hct of 65%. A third step reduced the Hct to 75% of baseline and was performed with either HES 200 or a 60% perfluorocarbon (PFC) emulsion. Comparisons of HES 200-only-hemodiluted animals versus 4.2 g/kg PFC emulsion-hemodiluted animals were made at 21% and 100% normobaric oxygen ventilation. It was found that systemic and microvascular oxygen delivery was 25% and 400% higher in the PFC animals compared with HES 200 animals, respectively, showing that PFCs deliver oxygen to the tissue when combined with hyperoxic ventilation in the present experiments, with no evidence of vasoconstriction or impaired microvascular function. Oxygen ventilation (100%) led to a positive base excess for the PFC group (5.5 +/- 2.5 mmol/l) versus a negative balance (-0.8 +/- 1.4 mmol/l) for the HES 200 group, suggesting that microvascular findings corresponded to systemic events.     

Microvascular pressure and functional capillary density in extreme hemodilution with low- and high-viscosity dextran and a low-viscosity Hb-based O2 carrier

Blood losses are usually corrected initially by the restitution of volume with plasma expanders and subsequently by the restoration of oxygen-carrying capacity using either a blood transfusion or possibly, in the near future, oxygen-carrying plasma expanders. The present study was carried out to test the hypothesis that high-plasma viscosity hemodilution maintains perfused functional capillary density (FCD) by preserving capillary pressure. Microvascular pressure responses to extreme hemodilution with low- (LV) and high-viscosity (HV) plasma expanders and an exchange transfusion with a polymerized bovine cell-free Hb (PBH) solution were analyzed in the awake hamster window chamber model (n = 26). Systemic hematocrit was reduced from 50% to 11%. PBH produced a greater mean arterial blood pressure than the nonoxygen carriers. FCD was higher after a HV plasma expander (70 +/- 15%) vs. PBH (47 +/- 12%). Microvascular pressure spanning the capillary network was higher after a HV plasma expander (16-19 mmHg) compared with PBH (12-16 mmHg) and a LV plasma expander (11-14 mmHg) but lower than control (22-26 mmHg). FCD was found to be directly proportional to capillary pressure. The use of a HV plasma expander in extreme hemodilution maintained the number of perfused capillaries and tissue perfusion by comparison with a LV plasma expander due to increased mean arterial blood pressure and capillary pressure. The use of PBH increased mean arterial pressure but reduced capillary pressure due to vasoconstriction and did not maintain FCD.     

Microvascular and capillary perfusion following glycocalyx degradation.

Systemic parameters and microvascular and capillary hemodynamics were studied in the hamster window chamber model before and after hyaluronan degradation by intravenous injection of Streptomyces hyaluronidase (100 units, 40-50 U/ml plasma). Glycocalyx permeation was estimated using fluorescent markers of different molecular size (40, 70, and 2,000 kDa), and electrical charge. Systemic parameters (blood pressure, heart rate, blood gases) and microhemodynamics (vascular tone, velocity, and blood flow) remained statistically unchanged after injection of hyaluronidase, compared with inactivated hyaluronidase. Conversely, capillary hemodynamics were drastically affected. Functional capillary density, the capillaries perfused with red blood cells (RBCs), decreased by 35%, capillary Hct of the remaining functional capillaries increased from 16 to 27%, and penetration of 70-kDa fluorescent marker increased. Furthermore, plasma-only perfused capillaries statistically increased 30 min after hyaluronidase. The decrease in functional capillary density accounted for an increased RBC flux in the remainder of the capillaries, since the same number of RBCs had to traverse a reduced number of capillaries. Flux balances showed a reduction from baseline of 11% for the RBC flux and 20% for the plasma flux after treatment. These discrepancies are within the margin of error of the techniques used and could be explained by accounting for RBC over-velocity compared with plasma. These findings suggest that the decrease in the glycocalyx leads to capillary perfusion impairments.     

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

The O2-carrying blood substitute based on polymerized bovine hemoglobin (PBH) was used to determine efficacy in maintaining tissue Po2 after an 80% isovolemic blood exchange leading to a hematocrit of 19% [5.4 g Hb/dl from red blood cells (RBCs) and 6.3 g Hb/dl from PBH]. Effects were studied in terms of O2 delivery, O2 extraction, and tissue Po2 at the microcirculatory level at 1, 12, and 24 h after exchange transfusion in awake hamsters prepared with a window chamber model. At 1 h after exchange, arteriolar and venular diameters were decreased compared with baseline. Arteriolar diameter did not fully recover at 12 h after exchange, but venular diameter returned to normal. At 24 h after exchange, arteriolar and venular diameters were not different from baseline. Combining diameter and flow velocity data allowed us to calculate arteriolar and venular flows. At 1 h after exchange, arteriolar and venular flow was reduced compared with baseline. Arteriolar flow was lower at 12 h after exchange and recovered after 24 h. The number of capillaries with RBC passage [functional capillary density (FCD)] at 1 h after exchange with PBH was significantly lower than baseline. FCD remained decreased at 12 h; at 24 h after exchange transfusion, FCD was fully recovered. Tissue Po2 was maximal at 1 h after exchange and decreased progressively at 12 and 24 h after exchange. O2 release to the tissue was minimal at 1 h and increased at 12 and 24 h after exchange. These results suggest the impairment of tissue O2 metabolism after introduction of PBH into the circulation, which is mitigated as PBH concentration declines.     

Oxygen transport by low and normal oxygen affinity hemoglobin vesicles in extreme hemodilution

The oxygen transport capacity of phospholipid vesicles encapsulating purified Hb (HbV) produced with a Po(2) at which Hb is 50% saturated (P 50 ) of 8 (HbV(8)) and 29 mmHg (HbV(29)) was investigated in the hamster chamber window model by using microvascular measurements to determine oxygen delivery during extreme hemodilution. Two isovolemic hemodilution steps were performed with 5% recombinant albumin (rHSA) until Hct was 35% of baseline. Isovolemic exchange was continued using HbV suspended in rHSA solution to a total [Hb] of 5.7 g/dl in blood. P(50) was modified by coencapsulating pyridoxal 5'-phosphate. Final Hct was 11% for the HbV groups, with a plasma [Hb] of 2.1 +/- 0.1 g/dl after exchange with HbV(8) or HbV(29). A reference group was hemodiluted to Hct 11% with only rHSA. All groups showed stable blood pressure and heart rate. Arterial oxygen tensions were significantly higher than baseline for the HbV groups and the rHSA group and significantly lower for the HbV groups compared with the rHSA group. Blood pressure was significantly higher for the HbV(8) group compared with the HbV(29) group. Arteriolar and venular blood flows were significantly higher than baseline for the HbV groups. Microvascular oxygen delivery and extraction were similar for the HbV groups but lower for the rHSA group (P < 0.05). Venular and tissue Po(2) were statistically higher for the HbV(8) vs. the HbV(29) and rHSA groups (P < 0.05). Improved tissue Po(2) is obtained when red blood cells deliver oxygen in combination with a high- rather than low-affinity oxygen carrier.     

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.     

Reduced erythrocyte deformability alters pulmonary hemodynamics

Isolated rat lungs were perfused with suspensions containing normal and stiffened erythrocytes (RBCs) to assess the effect of altered RBC deformability on pulmonary hemodynamics. RBC suspensions were prepared using cells previously incubated in isosmolar phosphate-buffered saline with or without 0.0125 or 0.01875% glutaraldehyde. Washed RBCs were resuspended in isosmolar 4% albumin saline solution. Isolated rat lungs were perfused with control and stiffened cells by the use of a perfusion system that allowed rapid switching between suspensions. Pressure-flow (P/Q) curves were constructed by measuring pulmonary arterial pressure (Ppa) over a range of flow rates. In a second set of experiments, P/Q curves were generated for perfusion with control and stiffened cells (0.0125% glutaraldehyde) before and after vasoconstriction with a synthetic prostaglandin analogue (U 46619). RBC deformability was quantified in all experiments by determination of filtration time of a dilute cell suspension through a 4.7 microns Nuclepore filter. Incubation with 0.0125 or 0.01875% glutaraldehyde produced a 6 or 21% decrease in RBC deformability, respectively. These decreases in deformability were associated with significant increases in Ppa at each flow rate. The increases in Ppa correlated significantly with the degree of RBC stiffening. With 0.0125% glutaraldehyde, the P/Q curve was shifted upward without a change in slope, whereas incubation with 0.01875% glutaraldehyde resulted in a significant increase in slope. Vasoconstriction and perfusion with stiffened RBCs had additive effects on Ppa. These findings suggest that decreases in RBC deformability cause physiologically significant elevations in hemodynamic resistance in the pulmonary circuit independent of vasoactivity.     

Normovolemic hemodilution improves oxygen extraction capabilities in endotoxic shock.

We studied the effects of normovolemic hemodilution on tissue oxygen extraction capabilities in a canine model of endotoxic shock. Eighteen anesthetized and mechanically ventilated dogs underwent normovolemic hemodilution with 6% hydroxyethyl starch solution to reach hematocrit (Hct) levels around 40, 30, or 20% before the administration of 2 mg/kg of Escherichia coli endotoxin. Cardiac tamponade was then induced by repeated injections of normal saline into the pericardial sac to reduce cardiac output and study whole body oxygen extraction capabilities. Whole body critical oxygen delivery was lower in the Hct 20% and 30% groups (8.4 +/- 0.4 and 10.4 +/- 0.7 ml. kg(-1). min(-1), respectively) than in the Hct 40% group (12.8 +/- 0.8 ml. kg(-1). min(-1)) (both P < 0.005). The whole body critical oxygen extraction ratio was higher in the Hct 30% and 20% groups (49.1 +/- 8.2 and 55.2 +/- 4.6%, respectively) than in the Hct 40% group (37.1 +/- 4.4 %) (both P < 0.05). Liver critical oxygen extraction ratio was also higher in the Hct 30% and 20% groups than in the Hct 40% group. The arterial lactate concentrations and the gradient between ileum mucosal PCO(2) and arterial PCO(2) were lower in the Hct 20% and 30% groups than in the Hct 40% group. We conclude that, during an acute reduction in blood flow during endotoxic shock in dogs, normovolemic hemodilution is associated with improved tissue perfusion and increased oxygen extraction capabilities.     

Microvascular PO2 during extreme hemodilution with hemoglobin site specifically PEGylated at Cys-93(beta) in hamster window chamber

The oxygen transport capacity of nonhypertensive polyethylene glycol (PEG)-conjugated hemoglobin solutions were investigated in the hamster chamber window model. Microvascular measurements were made to determine oxygen delivery in conditions of extreme hemodilution [hematocrit (Hct) 11%]. Two isovolemic hemodilution steps were performed with a 6% Dextran 70 (70-kDa molecular mass) plasma expander until Hct was 35% of control. Isovolemic blood volume exchange was continued using two surface-modified PEGylated hemoglobins (P5K2, P(50) = 8.6, and P10K2, P(50) = 8.3; P(50) is the hemoglobin Po(2) corresponding to its 50% oxygen saturation) until Hct was 11%. P5K2 and P10K2 are PEG-conjugated hemoglobins that maintain most of the hemoglobin allosteric properties and have a cooperativity index of n = 2.2. The effects of these molecular solutions were compared with those obtained in a previous study using MP4, a PEG-modified hemoglobin whose P(50) was 5.4 and cooperativity was 1.2 (Tsai et al., Am J Physiol Heart Circ Physiol 285: H1411-H1419, 2003). Tissue oxygen levels were higher after P5K2 (7.0 +/- 2.5 mmHg) and P10K2 (6.3 +/- 2.3 mmHg) versus MP4 (1.7 +/- 0.5 mmHg) or the nonoxygen carrier Dextran 70 (1.3 +/- 1.2 mmHg). Microvascular oxygen delivery was higher after P5K2 and P10K2 (2.22 and 2.34 ml O(2)/dl blood) compared with MP4 (1.41 ml O(2)/dl blood) or Dextran 70 (0.90 ml O(2)/dl blood); however, all these values were lower than control (7.42 ml O(2)/dl blood). The total hemoglobin in blood was similar in all cases; therefore, the improvement in tissue Po(2) and oxygen delivery appears to be due to the increased cooperativity of the new molecules.     

Plasma expander viscosity effects on red cell-free layer thickness after moderate hemodilution

The objective of the study was to investigate the effects of plasma viscosity after hemodilution on the thickness of the erythrocyte cell free layer (CFL) and on the interface between the flowing column of erythrocytes and the vascular endothelium. The erythrocyte CFL thickness was measured in the rat cremaster muscle preparation. Plasma viscosity was modified in an isovolemic hemodilution, in which the systemic hematocrit (Hctsys) was lowered to 30%. The plasma expanders (PE) of similar nature and different viscosities were generated by glutaraldehyde polymerization of human serum albumin (HSA) at various molar ratios glutaraldehyde to HSA: (i) unpolymerized HSA; (ii) PolyHSA24:1, molar ratio = 24 and (iii) PolyHSA60:1, molar ratio = 60. The HSA viscosities determined at 200 s(-1) were 1.1, 4.2 and 6.0 dyn x cm(-2), respectively. CFL thickness, vessel diameter and blood flow velocity were measured, while volumetric flow, shear rate and stress were calculated. Hemodilution with PolyHSA60:1 increased plasma viscosity and the blood showed marked shear thinning behavior. CFL thickness decreased as plasma viscosity increased after hemodilution; thus the CFL thickness with HSA and PolyHSA24:1 increased compared to baseline. Conversely, the CFL thickness of PolyHSA60:1 was not different from baseline. Blood flow increased with both PolyHSA's compared to baseline. Wall shear rate and shear stress increased for PolyHSA60:1 compared to HSA and PolyHSA24:1, respectively. In conclusion, PE viscosity determined plasma viscosity after hemodilution and affected erythrocyte column hydrodynamics, changing the velocity profile, CFL thickness, and wall shear stress. This study relates the perfusion caused by PolyHSA60:1 to hemodynamic changes induced by the rheological properties of blood diluted with PolyHSA60:1.     

Effect of hematocrit on cerebral blood flow with induced polycythemia

Cerebral blood flow (CBF) is lowered during polycythemia. Whether this fall is due to an increase in red blood cell concentration (Hct) or to an increase in arterial O2 content (Cao2) is controversial. We examined the independent effects of Hct and Cao2 on CBF as Hct was raised from 30 to 55% in anesthetized 1- to 7-day-old sheep. CBF was measured by the radiolabeled microsphere technique before and after isovolemic exchange transfusion with either oxyhemoglobin-containing erythrocytes (in 5 control animals) or with methemoglobin-containing erythrocytes (in 9 experimental animals). Following exchange transfusion in the control animals, Hct rose (30 +/- 1 vs. 55 +/- 1%, mean +/- SE), Cao2 increased (15.1 +/- 0.8 vs. 26.7 +/- 0.9 vol%), and CBF fell (66 +/- 9 vs. 35 +/- 5 ml X min-1 X 100 g-1). Because the fall in CBF was proportionate to the rise in Cao2, cerebral O2 transport (CBF X Cao2) was unchanged. Following exchange transfusion in the experimental animals, Hct rose (32 +/- 1 vs. 55 +/- 1%) but Cao2 did not change. Nevertheless, CBF still fell (73 +/- 4 vs. 48 +/- 2 ml X min-1 X 100 g-1) and, as a result, cerebral O2 transport also fell. The latter cannot be attributed to a fall in cerebral O2 uptake, as cerebral O2 uptake was unaffected during each of these conditions. Comparison of the two groups of animals showed that approximately 60% of the fall in CBF may be attributed to the increase in red cell concentration alone. It is probable that this effect is due largely to changes in blood viscosity.     

High viscosity plasma expanders: Volume restitution fluids for lowering the transfusion trigger

Hemorheological studies lead to the axiom that high plasma viscosity is detrimental and that it is beneficial to lower blood viscosity, a precept embodied in the practice of hemodilution, where improved perfusion is attributed to the lowering of blood viscosity. Hemodilution is limited by the transfusion trigger, hemoglobin content of blood of about 7-8 g/dl, which indicates when further volume replacements must restore oxygen carrying capacity with red blood cells (RBC). However, oxygen consumption and delivery are not compromised upon passing this landmark. The reduced blood viscosity does not transmit adequate pressure to the capillaries, causing functional capillary density (FCD) to decrease, jeopardizing organ function through the inadequate extraction of products of metabolism from the tissue by the capillaries. Studies in hemorrhagic shock show that survival is primarily determined by the maintenance of FCD and secondarily by tissue oxygenation. FCD is maintained as hematocrit is reduced beyond the transfusion trigger by increasing plasma viscosity, which transmits systemic pressure to the capillaries and induces vasodilatation through the increased shear stress dependent release of vasodilators. Consequently the transfusion trigger is also a "viscosity trigger" indicating when blood and plasma viscosity are too low. In this condition increasing plasma viscosity is beneficial and extends the transfusion trigger reducing the use of blood transfusions.