Amy G. Tsai

Microvascular perfusion upon exchange transfusion with stored red blood cells in normovolemic anemic conditions

BACKGROUND:  Transfusions are intended to augment oxygen‐carrying capacity. The ability of fresh and stored red blood cells (RBCs) to maintain microvascular perfusion and oxygen delivery to the tissue has not been directly measured.

Plasma viscosity regulates capillary perfusion during extreme hemodilution in hamster skinfold model

Effect of increasing blood viscosity during extreme hemodilution on capillary perfusion and tissue oxygenation was investigated in the awake hamster skinfold model. Two isovolemic hemodilution steps were performed with 6% Dextran 70 [molecular weight (MW) = 70,000] until systemic hematocrit (Hct) was reduced by 65%. A third step reduced Hct by 75% and was performed with the same solution [low viscosity (LV)] or a high-molecular-weight 6% Dextran 500 solution [MW = 500,000, high viscosity (HV)]. Final plasma viscosities were 1.4 and 2.2 cP (baseline of 1.2 cP). Hct was reduced to 11.2 ± 1.1% from 46.2 ± 1.5% for LV and to 11.9 ± 0.7% from 47.3 ± 2.1% for HV. HV produced a greater mean arterial blood pressure than LV. Functional capillary density (FCD) was substantially higher after HV (85 ± 12%) vs. LV (38 ± 30%) vs. baseline (100%).[Formula: see text] levels measured with Pd-porphyrin phosphorescence microscopy were not statistically changed from baseline until after the third hemodilution step. Wall shear rate (WSR) decreased in arterioles and venules after LV and only in arterioles after HV. Wall shear stress (WSR × plasma viscosity) was substantially higher after HV vs. LV. Increased mean arterial pressure and shear stress-dependent release of endothelium-derived relaxing factor are possible mechanisms that improved arteriolar and venular blood flow and FCD after HV vs. LV exchange protocols.

Resuscitation with polyethylene glycol-modified human hemoglobin improves microcirculatory blood flow and tissue oxygenation after hemorrhagic shock in awake hamsters

OBJECTIVE To determine whether resuscitation with polyethylene glycol-modified human hemoglobin (MalPEG-Hb), an oxygen-carrying blood replacement fluid with 4 g/dL Hb, viscosity of 2.5 cP, colloid osmotic pressure of 49 mm Hg, and p50 of 5.5 mm Hg, improves systemic and microvascular variables after hemorrhage compared with shed blood (SB) and 5% hydroxyethyl starch (HES). SETTING Laboratory. SUBJECTS Golden Syrian hamsters. DESIGN Prospective study. INTERVENTIONS Hamsters implemented with a skin fold chamber were hemorrhaged 50% of blood volume and resuscitated with 50% shed blood volume (SB, HES, or MalPEG-Hb). MEASUREMENTS AND MAIN RESULTS Shock and resuscitation were monitored for 1 hr each. Microvascular events were characterized in terms of vessel diameter, flow velocity, functional capillary density, and Po(2) in arterioles, venules, and extravascular tissue. Systemic variables include mean arterial pressure, heart rate, Po(2), Pco(2), pH, and base excess. MalPEG-Hb resuscitation increased functional capillary density to 64% vs. 44% for SB and 32% for HES relative to baseline before shock. Microvascular flow increased 16% for MalPEG-Hb relative to baseline and remained decreased by 44% for SB and 80% for HES. Hemoglobin concentration was 10.4 g/dL with SB, 7.5 (6.8 g/dL in red blood cells and 0.9 g/dL in plasma) with MalPEG-Hb, and 7.5 g/dL with HES, leading to tissue Po(2) of 19, 8, and 5 mm Hg respectively. Calculations of oxygen extraction show that 0.9 g/dL of MalPEG-Hb increased oxygen extraction per gram of red cell hemoglobin in the tissue analyzed compared with SB. These measurements correlate well with a systemic indicator of recovery, base excess, 5.4 +/- 4.7 (MalPEG-Hb), 1.7 +/- 3.8 (SB), and -0.3 +/- 5.7 (HES). CONCLUSION The presence of 0.9 g/dL of high oxygen affinity MalPEG-Hb improves microvascular blood flow and oxygen transport during shock to a significantly greater extent than that attainable with blood or HES.

Systemic and microcirculatory effects of autologous whole blood resuscitation in severe hemorrhagic shock

Systemic and microcirculatory effects of autologous whole blood resuscitation after 4-h hemorrhagic shock with a mean arterial pressure (MAP) level of 40 mmHg were investigated in 63 conscious Syrian golden hamsters. Microcirculation of skeletal skin muscle and subcutaneous connective tissue was visualized in a dorsal skinfold. Shed blood was retransfused within 30 min after 4 h. Animals were grouped into survivors in good (SG) and poor condition (SP) and nonsurvivors (NS) according to 24-h outcome after resuscitation and studied before shock, during shock (60, 120, and 240 min), and 30 min and 24 h after resuscitation. Microvascular and interstitial PO2 values were determined by phosphorescence decay. Shock caused a significant increase of arterial PO2 and decrease of PCO2, pH, and base excess. In the microcirculation, there was a significant decrease in blood flow (QB), functional capillary density (FCD; capillaries with red blood cell flow), and interstitial PO2 [1.8 +/- 0.8 mmHg (SG), 1.3 +/- 1.3 mmHg (SP), and 0.9 +/- 1.1 mmHg (NS) vs. 23.0 +/- 6.1 mmHg at control]. Blood resuscitation caused immediate MAP recompensation in all animals, whereas metabolic acidosis, hyperventilation, and a significant interstitial PO2 decrease (40-60% of control) persisted. In NS (44.4% of the animals), systemic and microcirculatory alterations were significantly more severe both in shock and after resuscitation than in survivors. Whereas in SG (31.8% of the animals) there was only a slight (15-30%) but still significant impairment of microscopic tissue perfusion (QB, FCD) and oxygenation at 24 h, SP (23.8% of the animals) showed severe metabolic acidosis and substantial decreases (>/=50%) of FCD and interstitial PO2. FCD, interstitial PO2, and metabolic state were the main determinants of shock outcome.Systemic and microcirculatory effects of autologous whole blood resuscitation after 4-h hemorrhagic shock with a mean arterial pressure (MAP) level of 40 mmHg were investigated in 63 conscious Syrian golden hamsters. Microcirculation of skeletal skin muscle and subcutaneous connective tissue was visualized in a dorsal skinfold. Shed blood was retransfused within 30 min after 4 h. Animals were grouped into survivors in good (SG) and poor condition (SP) and nonsurvivors (NS) according to 24-h outcome after resuscitation and studied before shock, during shock (60, 120, and 240 min), and 30 min and 24 h after resuscitation. Microvascular and interstitial[Formula: see text] values were determined by phosphorescence decay. Shock caused a significant increase of arterial[Formula: see text] and decrease of[Formula: see text], pH, and base excess. In the microcirculation, there was a significant decrease in blood flow (Q˙B), functional capillary density (FCD; capillaries with red blood cell flow), and interstitial [Formula: see text][1.8 ± 0.8 mmHg (SG), 1.3 ± 1.3 mmHg (SP), and 0.9 ± 1.1 mmHg (NS) vs. 23.0 ± 6.1 mmHg at control]. Blood resuscitation caused immediate MAP recompensation in all animals, whereas metabolic acidosis, hyperventilation, and a significant interstitial [Formula: see text] decrease (40-60% of control) persisted. In NS (44.4% of the animals), systemic and microcirculatory alterations were significantly more severe both in shock and after resuscitation than in survivors. Whereas in SG (31.8% of the animals) there was only a slight (15-30%) but still significant impairment of microscopic tissue perfusion (Q˙B, FCD) and oxygenation at 24 h, SP (23.8% of the animals) showed severe metabolic acidosis and substantial decreases (≥50%) of FCD and interstitial[Formula: see text]. FCD, interstitial[Formula: see text], and metabolic state were the main determinants of shock outcome.

pO2 measurements by phosphorescence quenching: characteristics and applications of an automated system

An automated system for pO(2) analysis based upon phosphorescence quenching was tested. The system was calibrated in vitro with capillary samples of saline and blood. Results were compared to a conventional measuring procedure wherein pO(2) was calculated off-line by computer fitting of phosphorescence decay signals. PO(2) measurements obtained by the automated system were correlated (r(2) = 0.98) with readings simultaneously generated by a blood gas analyzer, accuracy being highest in the low (0-20 mm Hg) and medium pO(2) ranges (21-70 mm Hg). Measurements in in vivo studies in the hamster skin-fold preparation were similar to previously reported results. The automated system fits the phosphorescence decay data to a single exponential and allows repeated pO(2) measurements in rapid sequence.

Influence of cell-free Hb on local tissue perfusion and oxygenation in acute anemia after isovolemic hemodilution

BACKGROUND: Oxygen‐carrying solutions are intended to eliminate the blood transfusion trigger. Their ability to maintain microvascular perfusion and to deliver oxygen to tissue when they replace the RBCs as oxygen carriers has not been directly measured.

Subcutaneous microvascular responses to hemodilution with a red cell substitute consisting of polyethyleneglycol‐modified vesicles encapsulating hemoglobin

Phospholipid vesicles encapsulating purified hemoglobin [Hb vesicles (HbV); diameter 259 +/- 82 mm; oxygen affinity 31 mm Hg; [Hb] 5 and 10 g/dL] were developed to provide oxygen-carrying capacity to plasma expanders. Their function as a blood replacement was tested in the subcutaneous microvasculature of awake hamsters during severe hemodilution in which 80% of the red blood cell mass was substituted with suspensions of the vesicles in 5% human serum albumin (HSA) solution. Vesicles were tested with membranes that were unmodified (HbV/HSA) or conjugated with polyethyleneglycol (PEG) on the vesicular surface (PEG-HbV/HSA). The viscosity of 10 g/dL HbV/HSA was 8 cP at 358 s-1 owing to the intervesicular aggregation, while that of 10 g/dL PEG-HbV/HSA was 3.5 cP, since PEG chains inhibit aggregation. Both materials yielded normal mean arterial pressure, heart rate, and blood gas parameters at all levels of exchange, which could not be achieved with HSA alone. Subcutaneous microvascular studies showed that PEG-HbV/HSA significantly improved microhemodynamic conditions (flow rate, functional capillary density, vessel diameter, and oxygen tension) relative to unmodified HbV/HSA. Even though the enhancement of PEG modification did not achieve the functional characteristics of the blood-perfused microcirculation, PEG reduced vesicular aggregation and viscosity, improving microvascular perfusion relative to the unmodified type. These results highlight the significance of microvascular analysis in the design of red cell substitutes and the necessity of surface modification of HbV to prevent aggregation.

Is resuscitation from hemorrhagic shock limited by blood oxygen-carrying capacity or blood viscosity?

Systemic and microvascular hemodynamic responses to volume restoration from hemorrhagic shock were studied in the hamster window chamber model to determine the significance of blood rheological and oxygen transport properties. Moderated hemorrhage was induced by means of arterial controlled bleeding of 50% of the blood volume. The hypovolemic shock state was maintained for 1 h before resuscitation. The animals were resuscitated by infusion of 25% of blood volume using either fresh plasma or blood and were studied for 90 min. Transfusion was performed with either oxygen-carrying fresh red blood cells (RBCs) or non-oxygen-carrying RBCs whose hemoglobin was converted to methemoglobin (MetHb). Systemic parameters, including cardiac output, vital organ blood flow distribution, microvascular hemodynamics, and capillary perfusion (functional capillary density [FCD]), were measured during the resuscitation period. Fluorescent-labeled microspheres were used to measure organ blood flow (brain, heart, kidney, liver, lung, spleen, and window chamber). The blood viscosities at the end of the 90-min period were 2.4 cP after resuscitation with plasma, and 2.9 to 3.0 cP after blood transfusion (baseline, 4.2 cP). Resuscitation with RBCs with or without oxygen-carrying capacity had greater mean arterial pressure than did the plasma resuscitation group. The FCD was substantially higher for RBC transfusions (0.56% ± 7% of baseline) compared with plasma (46% ± 7% of baseline), and the presence of MetHb in the fresh RBC did not change the FCD or the microvascular hemodynamics. Oxygen delivery and extraction levels were significantly lower for resuscitation with plasma and MetHb-loaded RBCs compared with oxygen-carrying RBCs. The curtailed recovery of systemic and microvascular conditions after volume restitution with plasma seems to be due to the decrease in blood viscosity. Conversely, the restoration of blood rheological properties improves resuscitation independently of the restitution of oxygen-carrying capacity.

Endoglin (CD105) Up-regulation in Pulmonary Microvasculature of Ventilated Preterm Infants

RATIONALE Preterm infants exposed to mechanical ventilation and oxygen are at risk for bronchopulmonary dysplasia (BPD), a multifactorial chronic lung disorder characterized by arrested alveolar development. Studies have described disruption of microvascular development in BPD, characterized by primitive angioarchitectural patterns reminiscent of the canalicular/saccular stages of lung development. The molecular regulation of this BPD-associated dysangiogenesis remains undetermined. OBJECTIVES Endoglin (CD105), a hypoxia-inducible transforming growth factor-beta coreceptor, has been implicated as an important regulator of angiogenesis in various neoplastic and nonneoplastic conditions. The aim of this study was to investigate the expression of endoglin and other angiogenesis-related factors in ventilated preterm human lungs. METHODS We have studied endoglin protein and mRNA expression in postmortem lungs of short-term and long-term ventilated preterm infants. Control subjects were age-matched infants who had lived for less than 1 hour. MEASUREMENTS AND MAIN RESULTS Lungs of short-term ventilated preterm infants showed significant upregulation of endoglin mRNA and protein levels, immunolocalized to the microvasculature. Similar but more variable endoglin upregulation was noted in lungs of long-term ventilated infants with BPD. The mRNA levels of vascular endothelial growth factor, angiopoietin-1, and their respective receptors were significantly lower in ventilated lungs than in age-matched nonventilated control lungs. CONCLUSIONS BPD is associated with a shift from traditional angiogenic growth factors (vascular endothelial growth factor, angiopoietin-1) to alternative regulators such as endoglin, which may contribute to BPD-associated microvascular dysangiogenesis.

Cardiovascular benefits in moderate increases of blood and plasma viscosity surpass those associated with lowering viscosity: Experimental and clinical evidence

Decreasing blood viscosity has been proposed since the advent of hemodilution as a means for increasing perfusion in many pathological conditions, and increased plasma viscosity is associated with the presence of pathological conditions. However, experimental studies show that microvascular functions as represented by functional capillary density in conditions of significantly decreased viscosity is impaired, a problem corrected by increasing plasma and blood viscosity. Blood viscosity, primarily dependent on hematocrit (Hct) is a determinant of peripheral vascular resistance, and therefore blood pressure. In the healthy population Hct presents a variability, which is not reflected by the variability of blood pressure. This is due to a regulatory process at the level of the endothelium, whereby the increase of Hct (and therefore blood viscosity) leads to increased shear stress and the production of the vasodilator nitric oxide (NO), a finding supported by experimental studies showing that the acute increase of Hct lowers blood pressure. Studies that in the healthy population show that blood pressure and Hct have a weak positive correlation. However, when the effect of blood viscosity is factored out, blood pressure and Hct are negatively and significantly correlated, indicating that as blood viscosity increases, the circulation dilates. Conversely, lower Hct and blood viscosity conditions lead to a constricted circulation, associated with a condition of decreased NO bioavailability, and therefore a pro-inflammatory condition.