Elevated plasma viscosity in extreme hemodilution increases perivascular nitric oxide concentration and microvascular perfusion

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 +/- 0.11 and 1.33 +/- 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.     

Improved oxygenation in ischemic hamster flap tissue is correlated with increasing hemodilution with Hb vesicles and their O2 affinity

The aim of this study was to test the influence of oxygen affinity of Hb vesicles (HbVs) and level of blood exchange on the oxygenation in collateralized, ischemic, and hypoxic hamster flap tissue during normovolemic hemodilution. Microhemodynamics were investigated with intravital microscopy. Tissue Po2 was measured with Clark-type microprobes. HbVs with a P50 of 15 mmHg (HbV15) and 30 mmHg (HbV30) were suspended in 6% Dextran 70 (Dx70). The Hb concentration of the solutions was 7.5 g/dl. A stepwise replacement of 15%, 30%, and 50% of total blood volume was performed, which resulted in a gradual decrease in total Hb concentration. In the ischemic tissue, hemodilution led to an increase in microvascular blood flow to maximally 141-166% of baseline in all groups (median; P < 0.01 vs. baseline, not significant between groups). Oxygen tension was transiently raised to 121 +/- 17% after the 30% blood exchange with Dx70 (P < 0.05), whereas it was increased after each step of hemodilution with HbV15-Dx70 and HbV30-Dx70, reaching 217 +/- 67% (P < 0.01) and 164 +/- 33% (P < 0.01 vs. baseline and other groups), respectively, after the 50% blood exchange. We conclude that despite a decrease in total Hb concentration, the oxygenation in the ischemic, hypoxic tissue could be improved with increasing blood exchange with HbV solutions. Furthermore, better oxygenation was obtained with the left-shifted HbVs.     

Effects of erythrocyte flexibility on microvascular perfusion and oxygenation during acute anemia

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-microm 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 +/- 0.07 of baseline) vs. GRBC (0.32 +/- 0.10 of baseline) and Dex70 (0.38 +/- 0.10 of baseline) groups. Microvascular tissue Po(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.     

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.     

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.     

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.     

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.     

Effect of plasma expander viscosity on the cell free layer

The effect of low and high viscosity hemodilution with plasma expanders on the extent of the cell free layer (CFL) width was analyzed in the microcirculation of the exteriorized cremaster muscle preparation of Sprague-Dawley male rats. Anesthetized animals were subjected to 40% hemodilution by blood volume, using 5% human serum albumin (HSA) or 6% Hetastarch (hydroxyethyl starch 670 kDa). Arterioles (n=5 for each treatment) were investigated. Mean arterial pressure, heart rate, vessel flow velocity and CFL width were measured at baseline and 5, 20 and 40 min post-exchange transfusion. Blood and plasma viscosity was determined from terminal blood collections. CFL width and pseudoshear rate, diameter and flow, normalized to baseline, were significantly elevated at all post-exchange assessments. Peripheral vascular resistance decreased. The increase of the CFL width was greater with HSA by comparison with Hetastarch hemodilution (p<0.05). Hetastarch blood and plasma viscosities increased significantly compared to those of HSA (p<0.05). This study shows that CFL widths are influenced by plasma expander viscosity, a phenomenon proportional to the increase in molecular weight of the colloids in solution.     

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.     

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.     

Cardiac mechanoenergetic cost of elevated plasma viscosity after moderate hemodilution

The purpose of this study was to investigate how plasma viscosity affects cardiac and vascular function during moderate hemodilution. Twelve anesthetized hamsters were hemodiluted by 40% of blood volume with two different viscosity plasma expanders. Experimental groups were based on the plasma expander viscosity, namely: high viscosity plasma expander (HVPE, 6.3 mPa?·?s) and low viscosity plasma expander (LVPE, 2.2 mPa?·?s). Left ventricular (LV) function was intracardiacally measured with a high temporal resolution miniaturized conductance catheter and concurrent pressure-volume results were used to calculate different LV indices. Independently of the plasma expander, hemodilution decreased hematocrit to 28% in both groups. LVPE hemodilution reduced whole blood viscosity by 40% without changing plasma viscosity, while HVPE hemodilution reduced whole blood viscosity by 23% and almost doubled plasma viscosity relative to baseline. High viscosity plasma expander hemodilution significantly increased cardiac output, stroke volume and stroke work compared to baseline, whereas LVPE hemodilution did not. Furthermore, an increase in plasma viscosity during moderate hemodilution produced a higher energy transfer per unit volume of ejected blood. Systemic vascular resistance decreased after hemodilution in both groups. Counter-intuitively, HVPE hemodilution showed lower vascular resistance and vascular hindrance than LVPE hemodilution. This result suggests that geometrical changes in the circulatory system are induced by the increase in plasma viscosity. In conclusion, an increase in plasma viscosity after moderate hemodilution directly influenced cardiac and vascular function by maintaining hydraulic power and reducing systemic vascular resistance through vasodilation.     

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.     

PEG-albumin plasma expansion increases expression of MCP-1 evidencing increased circulatory wall shear stress: an experimental study

Treatment of blood loss with plasma expanders lowers blood viscosity, increasing cardiac output. However, increased flow velocity by conventional plasma expanders does not compensate for decreased viscosity in maintaining vessel wall shear stress (WSS), decreasing endothelial nitric oxide (NO) production. A new type of plasma expander using polyethylene glycol conjugate albumin (PEG-Alb) causes supra-perfusion when used in extreme hemodilution and is effective in treating hemorrhagic shock, although it is minimally viscogenic. An acute 40% hemodilution/exchange-transfusion protocol was used to compare 4% PEG-Alb to Ringer's lactate, Dextran 70 kDa and 6% Hetastarch (670 kDa) in unanesthetized CD-1 mice. Serum cytokine analysis showed that PEG-Alb elevates monocyte chemotactic protein-1 (MCP-1), a member of a small inducible gene family, as well as expression of MIP-1α, and MIP-2. MCP-1 is specific to increased WSS. Given the direct link between increased WSS and production of NO, the beneficial resuscitation effects due to PEG-Alb plasma expansion appear to be due to increased WSS through increased perfusion and blood flow rather than blood viscosity.     

Normovolemic hemodilution with Hb vesicle solution attenuates hypoxia in ischemic hamster flap tissue

The aim of this study was to test whether oxygenation in acutely ischemic, collateralized tissue may be improved by normovolemic hemodilution with a solution containing liposome-encapsulated human Hb (HbV). A skin flap model in anesthetized hamsters was used, which consisted of two parts receiving either anatomic or collateral perfusion. Microhemodynamics were investigated with intravital microscopy. Partial tissue oxygen tension was measured with a Clark-type microprobe. Hemodilution was obtained by exchanging 50% of the total blood volume with HbV suspended in 8% human serum albumin (HSA8) or 6% Dextran 70 (Dx70). The size of the vesicles was 276 nm, the P(50) was 22 mmHg, and the Hb concentration of the solutions was 7.5 g/dl. Colloid osmotic pressure and viscosity were 49.9 mmHg and 8.7 cP for HbV-Dx70 and 40.0 mmHg and 2.9 cP for HbV-HSA8, respectively. Hemodilution with HbV-Dx70 led to an increase in microvascular blood flow in the ischemic microvessels to maximally 158% (median, P < 0.01), whereas blood flow remained virtually unchanged after hemodilution with HbV-HSA8. In the ischemic tissue, oxygen tension was improved from 11.9 to 17.0 mmHg (P < 0.01) after hemodilution with HbV-Dx70 but remained virtually unchanged after hemodilution with HbV-HSA8. Our study suggests that the oxygenation in acutely ischemic, collateralized tissue may be improved by normovolemic hemodilution with HbV suspended in Dx70. The effect was achieved by an increase in microcirculatory blood flow related to the rheological properties of the suspending medium.     

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.     

Oxygen release from arterioles with normal flow and no-flow conditions.

The rate of oxygen release from arterioles ( approximately 55 microm diameter) was measured in the hamster window chamber model during flow and no-flow conditions. Flow was stopped by microvascular transcutaneous occlusion using a glass pipette held by a manipulator. The reduction of the intra-arteriolar oxygen tension (Po2) was measured by the phosphorescence quenching of preinfused Pd-porphyrin, 100 microm downstream from the occlusion. Oxygen release from arterioles was found to be 53% greater during flow than no-flow conditions (2.6 vs. 1.7 x 10(-5) ml O2.cm(-2).s(-1), P < 0.05). Acute hemodilution with dextran 70 was used to reduce vessel oxygen content, significantly increase wall shear stress (14%, P < 0.05), reduce Hct to 28.4% (SD 1.0) [vs. 48.8% (SD 1.8) at baseline], lower oxygen supply by the arterioles (10%, P < 0.05), and increase oxygen release from the arterioles (39%, P < 0.05). Hemodilution also increased microcirculation oxygen extraction (33% greater than nonhemodilution, P < 0.05) and oxygen consumption by the vessel wall, as shown by an increase in vessel wall oxygen gradient [difference in Po2 between the blood and the tissue side of the arteriolar wall, nonhemodiluted 16.2 Torr (SD 1.0) vs. hemodiluted 18.3 Torr (SD 1.4), P < 0.05]. Oxygen released by the arterioles during flow vs. nonflow was increased significantly after hemodilution (3.6 vs. 1.8 x 10(-5) ml O2.cm(-2).s(-1), P < 0.05). The oxygen cost induced by wall shear stress, suggested by our findings, may be >15% of the total oxygen delivery to tissues by arterioles during flow in this preparation.     

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.     

Plasma viscosity and cerebral blood flow

We hypothesized that the response of cerebral blood flow (CBF) to changing viscosity would be dependent on "baseline" CBF, with a greater influence of viscosity during high-flow conditions. Plasma viscosity was adjusted to 1.0 or 3.0 cP in rats by exchange transfusion with red blood cells diluted in lactated Ringer solution or with dextran. Cortical CBF was measured by H(2) clearance. Two groups of animals remained normoxic and normocarbic and served as controls. Other groups were made anemic, hypercapnic, or hypoxic to increase CBF. Under baseline conditions before intervention, CBF did not differ between groups and averaged 49.4 +/- 10.2 ml. 100 g(-1). min(-1) (+/-SD). In control animals, changing plasma viscosity to 1. 0 or 3.0 cP resulted in CBF of 55.9 +/- 8.6 and 42.5 +/- 12.7 ml. 100 g(-1). min(-1), respectively (not significant). During hemodilution, hypercapnia, and hypoxia with a plasma viscosity of 1. 0 cP, CBF varied from 98 to 115 ml. 100 g(-1). min(-1). When plasma viscosity was 3.0 cP during hemodilution, hypercapnia, and hypoxia, CBF ranged from 56 to 58 ml. 100 g(-1). min(-1) and was significantly reduced in each case (P < 0.05). These results support the hypothesis that viscosity has a greater role in regulation of CBF when CBF is increased. In addition, because CBF more closely followed changes in plasma viscosity (rather than whole blood viscosity), we believe that plasma viscosity may be the more important factor in controlling CBF.     

Dissociation between skeletal muscle microvascular PO2 and hypoxia-induced microvascular inflammation.

Systemic hypoxia (SHx) produces microvascular inflammation in mesenteric, cremasteric, and pial microcirculations. In anesthetized rats, SHx lowers arterial blood pressure (MABP), which may alter microvascular blood flow and microvascular Po(2) (Pm(O(2))) and influence SHx-induced leukocyte-endothelial adherence (LEA). These experiments attempted to determine the individual contributions of the decreases in Pm(O(2)), venular blood flow and shear rate, and MABP to the hypoxia-induced increase in LEA. Cremaster microcirculation of anesthetized rats was visualized by intravital microscopy. Pm(O(2)) was measured by a phosphorescence-quenching method. SHx [inspired Po(2) of 70 Torr for 10 min, MABP of 65 +/- 3 mmHg, arterial Po(2) (Pa(O(2))) of 33 +/- 1 Torr] and cremaster ischemia (MABP of 111 +/- 7 mmHg, Pa(O(2)) of 86 +/- 3 Torr) produced similar Pm(O(2)): 7 +/- 2 and 6 +/- 2 Torr, respectively. However, LEA increased only in SHx (1.9 +/- 0.9 vs. 11.2 +/- 1.1 leukocytes/100 microm, control vs. SHx, P < 0.05). Phentolamine-induced hypotension (MABP of 55 +/- 4 mmHg) in normoxia lowered Pm(O(2)) to 26 +/- 6 Torr but did not increase LEA. Cremaster equilibration with 95% N(2)-5% CO(2) during air breathing (Pa(O(2)) of 80 +/- 1 Torr) lowered Pm(O(2)) to 6 +/- 1 Torr but did not increase LEA. On the other hand, when cremaster Pm(O(2)) was maintained at 60-70 Torr during SHx (Pa(O(2)) of 35 +/- 1 Torr), LEA increased from 2.1 +/- 1.1 to 11.1 +/- 1.5 leukocytes/100 microm (P < 0.05). The results show a dissociation between Pm(O(2)) and LEA and support the idea that SHx results in the release of a mediator responsible for the inflammatory response.     

Interaction of free fatty acids and epinephrine in regulating hepatic glucose production in conscious dogs

To determine the effects of an increase in lipolysis on the glycogenolytic effect of epinephrine (EPI), the catecholamine was infused portally into 18-h-fasted conscious dogs maintained on a pancreatic clamp in the presence [portal (Po)-EPI+FFA, n = 6] and absence (Po-EPI+SAL, n = 6) of peripheral Intralipid infusion. Control groups with high glucose (70% increase) and free fatty acid (FFA; 200% increase; HG+FFA, n = 6) and high glucose alone (HG+SAL, n = 6) were also included. Hepatic sinusoidal EPI levels were elevated (Delta 568 +/- 77 and Delta 527 +/- 37 pg/ml, respectively) in Po-EPI+SAL and EPI+FFA but remained basal in HG+FFA and HG+SAL. Arterial plasma FFA increased from 613 +/- 73 to 1,633 +/- 101 and 746 +/- 112 to 1,898 +/- 237 micromol/l in Po-EPI+FFA and HG+FFA but did not change in EPI+SAL or HG+SAL. Net hepatic glycogenolysis increased from 1.5 +/- 0.3 to 3.1 +/- 0.4 mg x kg(-1) x min(-1) (P < 0.05) by 30 min in response to portal EPI but did not rise (1.8 +/- 0.2 to 2.1 +/- 0.3 mg x kg(-1) x min(-1)) in response to Po-EPI+FFA. Net hepatic glycogenolysis decreased from 1.7 +/- 0.2 to 0.9 +/- 0.2 and 1.6 +/- 0.2 to 0.7 +/- 0.2 mg x kg(-1) x min(-1) by 30 min in HG+FFA and HG+SAL. Hepatic gluconeogenic flux to glucose 6-phosphate increased from 0.6 +/- 0.1 to 1.2 +/- 0.1 mg x kg(-1) x min(-1) (P < 0.05; by 3 h) and 0.7 +/- 0.1 to 1.6 +/- 0.1 mg x kg(-1) x min(-1) (P < 0.05; at 90 min) in HG+FFA and Po-EPI+FFA. The gluconeogenic parameters remained unchanged in the Po-EPI+SAL and HG+SAL groups. In conclusion, increased FFA markedly changed the mechanism by which EPI stimulated hepatic glucose production, suggesting that its overall lipolytic effect may be important in determining its effect on the liver.