Search

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 [+ or -] 5.5 mmHg) when Hct decreased 8.4 [+ or -] 1.9% (P < 0.005). Cardiac output increased continuously with hemodilution. These conditions were reached at ~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. hemodilution; acute anemia; blood viscosity; nitric oxide doi: 10.1152/ajpheart.00496.2010.

Hemoglobin (Hb)-based [O.sub.2] carriers (HBOCs) constitute a class of therapeutic agents designed to correct the [O.sub.2] deficit under conditions of anemia and traumatic blood loss. The [O.sub.2] transport capacity of ultrahigh-molecular-weight bovine Hb polymers (PolybHb), polymerized in the tense (T) state and relaxed (R) state, were investigated in the hamster chamber window model using microvascular measurements to determine [O.sub.2] delivery during extreme anemia. The anemic state was induced by hemodilution with a plasma expander (70-kDa dextran). After an initial moderate hemodilution to 18% hematocrit, animals were randomly assigned to exchange transfusion groups based on the type of PolybHb solution used (namely, T-state PolybHb and R-state PolybHb groups). Measurements of systemic parameters, microvascular hemodynamics, capillary perfusion, and intravascular and tissue [O.sub.2] levels were performed at 11% hematocrit. Both PolybHbs were infused at 10 g/dl, and their viscosities were higher than nondiluted blood. Restitution of the [O.sub.2] carrying capacity with T-state PolybHb exhibited lower arterial pressure and higher functional capillary density compared with R-state PolybHb. Central arterial [O.sub.2] tensions increased significantly for R-state PolybHb compared with T-state PolybHb; conversely, microvascular [O.sub.2] tensions were higher for T-state PolybHb compared with R-state PolybHb. The increased tissue P[O.sub.2] attained with T-state PolybHb results from the larger amount of [O.sub.2] released from the PolybHb and maintenance of macrovascular and microvascular hemodynamics compared with R-state PolybHb. These results suggest that the extreme high [O.sub.2] affinity of R-state PolybHb prevented [O.sub.2] bound to PolybHb from been used by the tissues. The results presented here show that T-state PolybHb, a high-viscosity [O.sub.2] carrier, is a quintessential example of an appropriately engineered [O.sub.2] carrying solution, which preserves vascular mechanical stimuli (shear stress) lost during anemic conditions and reinstates oxygenation, without the hypertensive or vasoconstriction responses observed in previous generations of HBOCs. hemoglobin-based oxygen carrier; oxygen carrying capacity; blood substitute; polymerized hemoglobin; exchange transfusion; hemodilution; tissue oxygen; hemoglobin oxygen affinity; hypertension; vasoconstriction doi:10.1152/ajpheart.01022.2009

[O.sub.2]-carrying fluids based on hemoglobin (Hb) are in various stages of clinical trials to determine their suitability as [O.sub.2]-carrying plasma expanders. Polymerized Hb solutions are characterized by their vasoactivity, low [O.sub.2] affinity, oncotic effect, prolonged shelf life, and stability. Physiological responses to facilitated [O.sub.2] 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 02 levels was performed at 18% Hct. Restitution of [O.sub.2]-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 PBH 13. 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 [O.sub.2] tensions were improved with PBH4 and PBH8 by preventing [O.sub.2] precapillary release and increasing 02 reserve. Further studies to establish PBH optimal dosage, efficacy, safety, and its effect on outcome are indicated before Hb-based [O.sub.2]-carrying blood substitutes are implemented in routine practice. microcirculation; red blood cells; hemodilution; functional capillary density; hemoglobin oxygen affinity; tissue oxygen.

Responses to exchange transfusion using red blood cells (RBCs) with normal and reduced flexibility were studied in the hamster window chamber model during acute moderate isovolemic hemodilution to determine the role of RBC membrane stiffness in microvascular perfusion and tissue oxygenation. Erythrocyte stiffness was increased by 30-min incubation in 0.02% glutaraldehyde solution, and unreacted glutaraldehyde was completely removed. Filtration pressure through 5-[micro]m pore size filters was used to quantify stiffness of the RBCs. Anemic conditions were induced by two isovolemic hemodilution steps using 6% 70-kDa dextran to a hematocrit (Hct) of 18% (moderate hemodilution). The protocol continued with an exchange transfusion to reduce native RBCs to 75% of baseline (11% Hct) with either fresh RBCs (RBC group) or reduced-flexibility RBCs (GRBC group) suspended in 5% albumin at 18% Hct; a plasma expander (6% 70-kDa dextran; Dex70 group) was used as control. Systemic parameters, microvascular perfusion, capillary perfusion [functional capillary density (FCD)], and oxygen levels across the microvascular network were measured by noninvasive methods. RBC deformability for GRBCs was significantly decreased compared with RBCs and moderate hemodilution conditions. The GRBC group had a greater mean arterial blood pressure (MAP) than the RBC and Dex70 groups. FCD was substantially higher for RBC (0.81 [+ or -] 0.07 of baseline) vs. GRBC (0.32 [+ or -] 0.10 of baseline) and Dex70 (0.38 [+ or -] 0.10 of baseline) groups. Microvascular tissue P[O.sub.2] was significantly lower for Dex70 and GRBC vs. RBC groups and the moderate hemodilution condition. Results were attributed to decreased oxygen uploading in the lungs and obstruction of tissue capillaries by rigidified RBCs, indicating that the effects impairing RBC flexibility are magnified at the microvascular level, where perfusion and oxygenation may define transfusion outcome. microcirculation; red blood cell membrane; glutaraldehyde; functional capillary density; extreme hemodilution; plasma expander; transfusion; intravascular oxygen

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 stressmediated 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. microcirculation; extreme hemodilution; plasma expander; organ flow distribution; intravascular oxygen; functional capillary density

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]. [Po.sub.2] 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 [Po.sub.2] 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. microcirculation; extreme hemodilution; plasma expander; intravascular oxygen; methemoglobin; methylene blue; functional capillary density doi:10.1152/ajpheart.01279.2005

The effect of small hematocrit (Hct) increases on cardiac index (cardiac output/body wt) and oxygen release to the microcirculation was investigated in the awake hamster window chamber model by means of exchange transfusions of homologous packed red blood cells. Increasing Hct between 8 and 13% from baseline increased cardiac index by 5-31% from baseline (P < 0.05) and significantly lowered systemic blood pressure (P < 0.05). The relationship between Hct and cardiac index is described by a second-order polynomial ([R.sup.2] = 0.84; P < 0.05) showing that Hct increases up to 20% from baseline increase cardiac index, whereas increases over 20% from baseline decrease cardiac index. Combining this data with measurements of blood pressure allowed to determine total peripheral vascular resistance, which was a minimum at 8-13% Hct increase and was described by a second-order polynomial ([R.sup.2] = 0.83; P < 0.05). Oxygen measurements in arterioles, venules, and the tissue at 8-13% Hct increase were identical to control; thus, as a consequence of increased flow and oxygen-carrying capacity, oxygen delivery and extraction increased, but the change was not statistically significant. Previous results with the same model showed that the observed effects are related to shear stress-mediated release of nitric oxide, an effect that should be also present in the heart microcirculation, leading to increased blood flow, myocardial oxygen consumption, and contractility. We conclude that a minimum viscosity level is necessary for generating the shear stress required for maintaining normal cardiovascular function. shear stress; blood viscosity; hematocrit; blood pressure; cardiovascular function

Isovolemic hemodilution to 11% systemic hematocfit 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 [Po.sub.2] 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 [Po.sub.2] between groups. Total Hb in the MP4 group was 4.8 [+ or -] 0.4 g/all, 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. microcirculation; functional capillary density; cardiac output; oxygen release; blood substitutes; plasma expanders; polyethylene glycolalbumin; polyethylene glycol-hemoglobin

A phospholipid vesicle encapsulating Hb [Hb vesicle (HbV)] has been developed as a transfusion alternative. One characteristic of HbV is that the [O.sub.2] affinity [P[O.sub.2] at which Hb is 50% saturated ([P.sub.50])] of Hb can be easily regulated by the amount of the coencapsulated allosteric effector pyridoxal 5'-phosphate. In this study, we prepared two HbVs with different [P.sub.50S] (8 and 29 mmHg, termed Hb[V.sub.8] and Hb[V.sub.29], respectively) and observed their [O.sub.2]-releasing behavior from an occluded arteriole in a hamster skinfold window model. Conscious hamsters received Hb[V.sub.8] or Hb[V.sub.29] at a dose rate of 7 ml/kg. In the microscopic view, an arteriole (diameter: 53.0 [+ or -] 6.6 [micro]m) was occluded transcutaneously by a glass pipette on a manipulator, and the reduction of the intra-arteriolar P[O.sub.2] 100 [micro]m down from the occlusion was measured by the phosphorescence quenching of preinfused Pd-porphyrin. The baseline arteriolar P[O.sub.2] (50-52 mmHg) decreased to about 5 mmHg for all the groups. Occlusion after Hb[V.sub.8] infusion showed a slightly slower rate of P[O.sub.2] reduction compared with that after Hb[V.sub.29] infusion. The arteriolar [O.sub.2] content was calculated at each reducing P[o.sub.2] in combination with the [O.sub.2] equilibrium curves of Hb[V.sub.8], and it was clarified that Hb[V.sub.8] showed a significantly slower rate of [O.sub.2] release compared with Hb[V.sub.29] and was a primary source of [O.sub.2] (maximum fraction, 0.55) overwhelming red blood cells when the P[O.sub.2] was reduced (e.g., < 10 mmHg) despite a small dosage of HbV. This result supports the possible utilization of Hb-based [O.sub.2] carriers with lower [P.sub.50] for oxygenation of ischemic tissues. blood substitutes; artificial red blood cells; occlusion; microhemodynamics; liposome

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 [+ or -] 0.2 g/dl blood concentration, significantly lowered mean arterial pressure (MAP) below baseline (99.6 [+ or -] 7.6 mmHg) to 82.4 [+ or -] 6.9 mmHg (P < 0.05) and decreased of FCD to 70 [+ or -] 9%. [alpha][alpha]Hb caused a greater recovery in MAP to 94.4 [+ or -] 6.2 mmHg and lowered FCD to 62 [+ or -] 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-[micro]m-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 [micro]m showed an increase relative to control with [alpha][alpha]Hb, whereas MP4 caused a decrease. shear stress; perfusion; plasma expander; blood pressure; blood substitutes