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High-molecular-weight linear polymers (HMWLPs) have earned the name "drag-reducing polymers" because of their ability to reduce drag in turbulent flows. Recently, these polymers have become popular in bioengineering applications. This study investigated whether the addition of HMWLP in a venoarterial extracorporeal circulation (ECC) model could improve microvascular perfusion and oxygenation. Golden Syrian hamsters were instrumented with a dorsal skinfold window chamber and subjected to ECC using a circuit comprised of a peristaltic pump and a bubble trap. The circuit was primed with lactated Ringer solution (LR) containing either 5 ppm of polyethylene glycol (PEG) with a low molecular weight of 500 kDa (PEG500k) or 5 ppm of PEG with a high molecular weight of 3,500 kDa (PEG3500k). After 90 min of ECC at 15% of the animal's cardiac output, the results showed that the addition of PEG3500k to LR improved microvascular blood flow in arterioles and venules acutely (2 h after ECC), whereas functional capillary density showed improvement up to 24 h after ECC. Similarly, PEG3500k improved venular hemoglobin O2 saturation on the following day after ECC. The serum and various excised organs all displayed reduced inflammation with the addition of PEG3500k, and several of these organs also had a reduction in markers of damage with the HMWLPs compared to LR alone. These promising results suggest that the addition of small amounts of PEG3500k can help mitigate the loss of microcirculatory function and reduce the inflammatory response from ECC procedures. [ABSTRACT FROM AUTHOR]

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

The oxygen transport capacity of phospholipid vesicles encapsulating purified Hb (HbV) produced with a [Po.sub.2] at which Hb is 50% saturated ([P.sub.50]) of 8 (Hb[V.sub.8]) and 29 mmHg (Hb[V.sub.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.sub.50] was modified by coencapsulating pyridoxal 5'-phosphate. Final Hct was 11% for the HbV groups, with a plasma [Hb] of 2.1 [+ or -] 0.1 g/dl after exchange with Hb[V.sub.8] or Hb[V.sub.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 Hb[V.sub.8] group compared with the Hb[V.sub.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.sub.2] were statistically higher for the Hb[V.sub.8] vs. the Hb[V.sub.29] and rHSA groups (P < 0.05). Improved tissue [Po.sub.2] is obtained when red blood cells deliver oxygen in combination with a high- rather than low-affinity oxygen carrier. oxygen-carrying capacity; blood substitutes; tissue oxygen; hemoglobin oxygen affinity

We tested the hypothesis that high-viscosity (HV) plasma in extreme hemodilution causes wall shear stress to be greater than low-viscosity (LV) plasma, leading to enhanced production of nitric oxide (NO). The perivascular concentration of NO was measured in arterioles and venules and the tissue of the hamster chamber window model, subjected to acute extreme hemodilution, with a hematocrit (Hct) of 11% using Dextran 500 (n = 6) or Dextran 70 (n = 5) with final plasma viscosities of 1.99 [+ or -] 0.11 and 1.33 [+ or -] 0.04 cp, respectively. HV plasma significantly increased the periarteriolar, perivenular, and tissue NO concentration by 2.0, 1.9, and 1.4 times the control (n = 7). The NO concentration with LV plasma was not statistically different from control. Arteriolar shear stress was significantly increased in HV plasma relative to LV plasma in arterioles but not in venules. Aortic endothelial NO synthase (eNOS) protein expression was increased with HV plasma but not with LV plasma. There was a weak correlation between perivascular NO concentration and the locally calculated shear stress induced by the procedures, when blood viscosity was corrected according to Hct values previously determined in studies of microvascular Hct distribution. The finding that the periarteriolar and venular NO concentration in HV plasma was the same although arteriolar shear stress was significantly greater than venular shear stress maybe be due to differences in vessel wall metabolism between arterioles and venules and the presence of NO transport through the blood stream in the microcirculation. Results support the concept that in extreme hemodilution HV plasma maintains functional capillary density through a NO-mediated vasodilatation. functional capillary density; microvascular flow; shear stress; plasma expanders; mechanotransduction

Extreme hemodilution was performed in the hamster chamber window model using 6% Dextran 70, lowering systemic hematocrit by 60%. Animals were subsequently divided into three groups and hemodiluted to a hematocrit of 11% using 6% Dextran 70, 6% Dextran 500, and a 4% Dextran 70 + 0.7% alginate solution (n = 6 each group). Final plasma viscosities were 1.4 [+ or -] 0.2, 2.2 [+ or -] 0.1, and 2.7 [+ or -] 0.2 cp, respectively, (P < 0.05, high viscosity vs. low viscosity). Blood viscosities were 2.1 [+ or -] 0.2, 2.9 [+ or -] 0.4, and 3.9 [+ or -] 0.3 cp, respectively. The lowest blood and plasma viscosity group had a significantly lower functional capillary density, 37 [+ or -] 16%, whereas the two high-viscosity solutions were 71 [+ or -] 15% and 76 [+ or -] 12% (P < 0.05, high viscosity vs. low viscosity), respectively. Arteriolar and venular flow in the Dextran 500 and alginate groups was higher than baseline (i.e., normal nontreated animals), whereas the low-viscosity group showed a reduction in flow. These microvascular changes were paralleled by changes in base excess, which was negative for the Dextran 70 group and positive for the other groups. However, tissue Po2 was uniformly low for all groups (average of 1.4 mmHg). Calculation of tissue oxygen consumption in the window chamber based on the microvascular data, flow, and intravascular [Po.sub.2] showed that only the alginate + Dextran 70 solution-exchanged animals returned to baseline oxygen consumption, whereas the other groups were lower than baseline (P < 0.05). These results show that hemodilution performed with high-viscosity plasma expanders yields systemic arterial pressures and functional capillary densities that are significantly higher (P < 0.05) than those obtained with 6% Dextran 70, a fluid whose viscosity is similar to that of plasma. A condition for obtaining these results is that the oncotic pressure of the plasma expander be titrated to near normal, so that autotransfusion of fluid from the tissue into the vascular compartment does not reduce the effects of increasing plasma viscosity and increased shear stress on the microvascular wall. shear stress; blood pressure; functional capillary density

The microvascular distribution of oxygen was studied in the arterioles and venules of the awake hamster window chamber preparation to determine the contribution of vascular smooth muscle relaxation to oxygen consumption of the microvascular wall during verapamil-induced vasodilatation. Verapamil HCl delivered in a 0.1 mg/kg bolus injection followed by a continuous infusion of 0.01 mg x [kg.sup.-1] x [min.sup.-1] caused significant arteriolar dilatation, increased microvascular flow and functional capillary density, and decreased arteriolar vessel wall transmural [Po.sub.2] difference. Verapamil caused tissue [Po.sub.2] to increase from 25.5 [+ or -] 4.1 mmHg under control condition to 32.0 [+ or -] 3.7 mmHg during verapamil treatment. Total oxygen released by the microcirculation to the tissue remained the same as at baseline. Maintenance of the same level of oxygen release to the tissue, increased tissue [Po.sub.2], and decreased wall oxygen concentration gradient are compatible if vasodilatation significantly lowers vessel wall oxygen consumption, which in this model appears to constitute an important oxygen-consuming compartment. These findings show that treatment with verapamil, which increases oxygen supply through vasodilatation. may further improve tissue oxygenation by lowering oxygen consumption of the microcirculation. vasoactivity: oxygen gradients; tissue oxygenation; microvessel metabolism

The [O.sub.2]-carrying blood substitute based on polymerized bovine hemoglobin (PBH) was used to determine efficacy in maintaining tissue [Po.sub.2] 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 [O.sub.2] delivery, [O.sub.2] extraction, and tissue [Po.sub.2] 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 alter 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 [Po.sub.2] was maximal at 1 h after exchange and decreased progressively at 12 and 24 h after exchange. [O.sub.2] 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 [O.sub.2] metabolism after introduction of PBH into the circulation, which is mitigated as PBH concentration declines. blood substitutes; methemoglobin; functional capillary density; microcirculation

Microvascular [Po.sub.2] during extreme hemodilution with hemoglobin site specifically PEGylated at Cys-93([beta]) in hamster window chamber. Am J Physiol Heart Circ Physiol 287: H1609-H1617, 2004. First published June 10, 2004: 10.1152/ajpheart.00146.2004.--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.sub.50] = 8.6, and P10K2, [P.sub.50] = 8.3; [P.sub.50] is the hemoglobin [Po.sub.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.sub.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 [+ or -] 2.5 mmHg) and P10K2 (6.3 [+ or -] 2.3 mmHg) versus MP4 (1.7 [+ or -] (0.5 mmHg) or the nonoxygen carrier Dextran 70 (1.3 = [+ or -] 1.2 mmHg). Microvascular oxygen delivery was higher after P5K2 and P10K2 (2.22 and 2.34 mi [O.sub.2]/dl blood) compared with MP4 (1.41 ml [O.sub.2]/dl blood) or Dextran 70 (0.90 ml [O.sub.2]/dl blood): however, all these values were lower than control (7.42 ml [O.sub.2]/dl blood). The total hemoglobin in blood was similar in all cases: therefore, the improvement in tissue [Po.sub.2] and oxygen delivery appears to be due to the increased cooperativity of the new molecules. surface-modified hemoglobin: functional capillary density; oxygen-carrying capacity: blood substitutes: tissue oxygen delivery; hemoglobin cooperativity

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 [+ or -] 15%) vs. PBH (47 [+ or -] 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. shear stress; perfusion; plasma expander; blood pressure

The oxygen transport capacity of fluorocarbons was investigated in the hamster chamber window mode 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 [+ or -] 2.5 retool/l) versus a negative balance (-0.8 [+ or -] 1.4 mmol/l) for the HES 200 group, suggesting that microvascular findings corresponded to systemic events. hyperoxia; functional capillary density; oxygen-carrying capacity blood substitutes; tissue oxygen delivery

In this study, we assessed the magnitude of the erratic deviations in the radial position of red blood cells (RBCs) in the laminar flow regime of arterioles in a hamster window preparation and the intraluminal [Po.sub.2] profile to determine whether this variability affects the intraluminal distribution of oxygen in conditions of normal hematocrit and hemodilution. A gated image intensifier was used to visualize fluorescently labeled RBCs in tracer quantities and obtain multiple measurements of RBC radial and longitudinal positions at time intervals on the order of 5 ms within single arterioles (diameter range 40-95 [micro]m). RBCs in the velocity range of 0.3-14 mm/s exhibit a mean coefficient of variation of velocity of 16.9 [+ or -] 10.5% and a SD of the radial position of 1.98 [+ or -] 0.98 [micro]m. Both quantities were inversely related to hematocrit, and the former was significantly lowered by hemodilution. Our experimental results presented very similar values and shape compared with the intraluminal oxygen profile derived theoretically for normal hematocrit, suggesting that shear-augmented diffusion due to the measured radial displacement of RBCs did not significantly affect oxygen diffusion from blood into the arteriolar vessel wall. [Po.sub.2] profiles in the arterioles assumed an increasingly parabolic configuration with increasing levels of hemodilution. arterioles; red blood cell trajectories; shear-induced particle diffusion; intraluminal oxygen profiles

Oxygen phosphorescence quenching was used to measure tissue P[O.sub.2] of lymphatic vessels of 43.6 [+ or -] 23.1 [micro]m (mean [+ or -] SD) diameter in tissue locations of the rat mesentery classified according to anatomic location. Lymph and adipose tissue P[O.sub.2] were 20.6 [+ or -] 9.1 and 34.1 [+ or -] 7.8 mmHg, respectively, with the difference being statistically significant. Rare microlymphatic vessels in connective tissue not surrounded by microvessels had a P[O.sub.2] of 0.8 [+ or -] 0.2 mmHg, whereas the surrounding tissue P[O.sub.2] was 3.0 [+ or -] 3.2 mmHg, with both values being significantly lower than those of adipose tissue. Lower of lymph fluid P[O.sub.2] relative to the surrounding tissue was also evident in paired measurements of P[O.sub.2] in the lymphatic vessels and perilymphatic adipose tissue, which was significantly lower than the P[O.sub.2] in paired adipose tissue. The P[O.sub.2] of the lymphatic fluid of the mesenteric microlymphatics is consistently lower than that of the surrounding adipose tissue by ~11 mmHg; therefore, lymph fluid has the lowest P[O.sub.2] of this tissue. The disparity between lymph and tissue P[O.sub.2] is attributed to the microlymphatic vessel wall and lymphocyte oxygen consumption. lymph oxygen; lymphatic wall; oxygen consumption

Saldivar, Enrique, Pedro Cabrales, Amy G. Tsai, and Marcos Intaglietta, Microcirculatory changes during chronic adaptation to hypoxia. Am J Physiol Heart Circ Physiol 285: H2064-H2071, 2003; 10.1152/ajpheart.00349.2003.--Microcirculatory changes in the window chamber preparation in Syrian golden hamsters, secondary to chronic hypoxia adaptation, are presented herein, Adaptation was attained by keeping animals in a 10% oxygen environment for 1 wk and 5% the following week. The following groups were studied: group 1, adapted to chronic hypoxia and kept in a 5% oxygen environment throughout the experiment; group 2, adapted to chronic hypoxia and kept in a 21% oxygen environment 24 h before and during the experiment; and group 3, control. Adaptation caused venule enlargement and hematocrit increase (68.6 [+ or -] 2.44 in group 1, 70 [+ or -] 2.66 in group 2, and 43.27 [+ or -] 2.30 in group 3; P < 0.05). Whereas heart rate decreased in adapted animals, blood pressure remained constant. Group 1 presented alkalosis, hypocapnia, and hypoxemia. The adapted groups had decreased blood flow velocity in arterioles and veins. We found no difference in microvasculature oxygen tension between groups 2 and 3; however, the number of capillaries with flow was markedly reduced in group 1 but significantly increased in group 2. Our findings suggest that, as an adaptation to hypoxia, erythropoiesis may prove beneficial by increasing blood viscosity and shear stress, leading to vasodilatation, in addition to the increase in oxygen-carrying capacity. Calculations show that oxygen extraction in the tissue of the window chamber model was significantly lowered in adapted animals breathing 5% oxygen, but was unchanged from the control when breathing 21% oxygen, even though blood hemoglobin content was increased from 14.5 [+ or -] 0.07 g/dl at control to 21.04 [+ or -] 1.24 g/dl in the adapted animals (P < 0.05). microcirculation; adaptation; hematocrit

The microvascular effects and hemodynamic events following exposure to normobaric hyperoxia (because of inspiration of 100% [O.sub.2]) were studied in the awake hamster window chamber model and compared with normoxia. Hyperoxia increased arterial blood P[O.sub.2] to 477.9 [+ or -] 19.9 from 60.0 [+ or -] 1.2 mmHg (P < 0.05). Heart rate and blood pressure were unaltered, whereas cardiac index was reduced from 196 [+ or -] 13 to 144 [+ or -] 31 ml * [min.sup.-1] * [kg.sup.-1] (P < 0.05)in hyperoxia. Direct measurements in the microcirculation showed there was arteriolar vasoconstriction, reduction of microvascular flow (83% of control, P < 0.05), and functional capillary density (FCD, 74 [+ or -] 16% of control), the latter change being significant (P < 0.05). Calculations of oxygen delivery and oxygen consumption based on the measured changes in microvascular blood flow velocity and diameter and estimates of oxygen saturation corrected for the Bohr effect due to the lowered pH and increased PC[O.sub.2] showed that oxygen transport in the microvascular network did not change between normal and hyperoxic condition. The congruence of systemic and microvascular hemodynamics events found with hyperoxia suggests that the microvascular findings are common to most tissues in the organism, and that hyperoxia, due to vasoconstriction and the decrease of FCD, causes a maldistribution of perfusion in the microcirculation. functional capillary density; oxygen consumption; microvascular regulation

Ischemia-reperfusion injury increased vascular permeability, resulting in fluid extravasation from the intravascular compartment into the tissue space. Fluid and small protein extravasation lead to increased interstitial fluid pressure and capillary collapse, impairing capillary exchange. Polymerized human serum albumin (PolyHSA) has an increased molecular weight (MW) compared with unpolymerized human serum albumin (HSA) and can improve intravascular fluid retention and recovery from ischemia-reperfusion injury. To test the hypothesis that polymerization of HSA can improve recovery from ischemia-reperfusion injury, we studied how exchange transfusion of 20% of the blood volume with HSA or PolyHSA immediately before reperfusion can affect local ischemic tissue microhemodynamics, vascular integrity, and tissue viability in a hamster dorsal window chamber model. Microvascular flow and functional capillary density were maintained in animals exchanged with PolyHSA compared with HSA. Likewise, exchange transfusion with PolyHSA preserved vascular permeability measured with extravasation of fluorescently labeled dextran. The intravascular retention time of the exchanged PolyHSA was significantly longer compared with the intravascular retention time of HSA. Lastly, the viability of tissue subjected to ischemia-reperfusion injury increased in animals exchanged with PolyHSA compared with HSA. Therefore maintenance of microvascular perfusion, improvement in vascular integrity, and reduction in tissue damage resulting from reperfusion with PolyHSA suggest that PolyHSA is a promising fluid therapy to improve outcomes of ischemia-reperfusion injury. [ABSTRACT FROM AUTHOR]

Adaptation to hypoxia requires compensatory mechanisms that affect O2 transport and utilization. Decreased hemoglobin (Hb) O2 affinity is considered part of the physiological adaptive process to chronic hypoxia. However, this study explores the hypothesis that increased Hb O2 affinity can complement acute physiological responses to hypoxia by increasing O2 uptake and delivery compared with normal Hb O2 affinity during acute severe hypoxia. To test this hypothesis, Hb O2 affinity in mice was increased by oral administration of 2-hydroxy-6-{[(2S)-1-(pyridine-3-carbonyl)piperidin-2yl] methoxy}benzaldehyde (GBT1118; 70 or 140 mg/kg). Systemic and microcirculatory hemodynamics and oxygenation parameters were studied during hypoxia in awake-instrumented mice. GBT1118 increased Hb O2 affinity and decreased the PO2 at which 50% of Hb is saturated with O 2(P50) from 43 ±1.1 to 18.3 ±0.9 mmHg (70 mg/kg) and 7.7 ±0.2 mmHg (140 mg/kg). In a dose-dependent fashion, GBT1118 increased arterial O2 saturation by 16% (70 mg/kg) and 40% (140 mg/kg) relative to the control group during 5% O2 hypoxia. In addition, a GBT1118-induced increase in Hb O2 affinity reduced hypoxia-induced hypotension compared with the control group. Moreover, microvascular blood flow was higher during hypoxia in GBT1118-treated groups than the control group. The increased O2 saturation and improved blood flow in GBT1118-treated groups preserved higher interstitial tissue PO2 than in the control group during 5% O2 hypoxia. In conclusion, increased Hb O2affinity enhanced physiological tolerance to hypoxia, as evidenced by improved hemodynamics and tissue oxygenation. Therefore, pharmacologically induced increases in Hb O2affinity become a potential therapeutic approach to improve tissue oxygenation in pulmonary diseases characterized by severe hypoxemia. NEW & NOTEWORTHY This study establishes that pharmacological modification of hemoglobin O2 affinity can be a promising and novel therapeutic strategy for the treatment of hypoxic hypoxia and paves the way for the clinical development of molecules that prevent hypoxemia. [ABSTRACT FROM AUTHOR]