Pressure effects on the flow behavior of sickle (HbSS) red cells in isolated (ex-vivo) microvascular system

Shear dependent red blood cell adhesion in microscale flow

Non-adherence and deformability are the key intrinsic biomechanical features of the red blood cell (RBC), which allow it to tightly squeeze and pass through even the narrowest of microcirculatory networks. Blockage of microcirculatory flow, also known as vaso-occlusion, is a consequence of abnormal cellular adhesion to the vascular endothelium. In sickle cell disease (SCD), an inherited anaemia, even though RBCs have been shown to be heterogeneous in adhesiveness and deformability, this has not been studied in the context of physiologically relevant dynamic shear gradients at the microscale. We developed a microfluidic system that simulates physiologically relevant shear gradients of microcirculatory blood flow at a constant single volumetric flow rate. Using this system, shear dependent adhesion of RBCs from 28 subjects with SCD and from 11 healthy subjects was investigated using vascular endothelial protein functionalized microchannels. We defined a new term, RBC Shear Gradient Microfluidic Adhesion (SiGMA) index to assess shear dependent RBC adhesion in a subject-specific manner. We have shown for the first time that shear dependent adhesion of RBCs is heterogeneous in a microfluidic flow model, which correlates clinically with inflammatory markers and iron overload in subjects with SCD. This study reveals the complex dynamic interactions between RBC-mediated microcirculatory occlusion and clinical outcomes in SCD. These interactions may also be relevant to other microcirculatory disorders and microvascular diseases.

Conjunctival microvascular hemodynamics following vaso-occlusive crisis in sickle cell disease

Painful vaso-occlusive crisis (VOC) is the clinical hallmark of sickle cell disease (SCD). Microcirculatory hemodynamic changes following painful VOC may be indicative of future development of VOC events in subjects with SCD. The purpose of the present study was to determine alterations in conjunctival microvascular hemodynamics during non-crisis state in SCD subjects with a history of VOC. Conjunctival microcirculation imaging was performed to measure conjunctival diameter (D) and axial blood velocity (V) in 10 control and 30 SCD subjects. SCD subjects were categorized into two groups based on their history of VOC within a 2-year period before imaging (with or without VOC-H) and also based on whether there was progression in the rate of VOCs during a 2-year period following imaging as compared to before imaging (with or without VOC-P). Conjunctival V was significantly higher in SCD subjects with VOC-H than in both control subjects and SCD subjects without VOC-H (P≤0.03). Conjunctival V was also significantly higher in SCD subjects with VOC-P compared with control subjects and SCD subjects without VOC-P (P≤0.03). Assessment of the conjunctival microcirculation may be useful for understanding hemodynamic changes that lead to VOC events in SCD subjects.

Kinetics of sickle cell biorheology and implications for painful vasoocclusive crisis

We developed a microfluidics-based model to quantify cell-level processes modulating the pathophysiology of sickle cell disease (SCD). This in vitro model enabled quantitative investigations of the kinetics of cell sickling, unsickling, and cell rheology. We created short-term and long-term hypoxic conditions to simulate normal and retarded transit scenarios in microvasculature. Using blood samples from 25 SCD patients with sickle hemoglobin (HbS) levels varying from 64 to 90.1%, we investigated how cell biophysical alterations during blood flow correlated with hematological parameters, HbS level, and hydroxyurea (HU) therapy. From these measurements, we identified two severe cases of SCD that were also independently validated as severe from a genotype-based disease severity classification. These results point to the potential of this method as a diagnostic indicator of disease severity. In addition, we investigated the role of cell density in the kinetics of cell sickling. We observed an effect of HU therapy mainly in relatively dense cell populations, and that the sickled fraction increased with cell density. These results lend support to the possibility that the microfluidic platform developed here offers a unique and quantitative approach to assess the kinetic, rheological, and hematological factors involved in vasoocclusive events associated with SCD and to develop alternative diagnostic tools for disease severity to supplement other methods. Such insights may also lead to a better understanding of the pathogenic basis and mechanism of drug response in SCD.

Heterogeneous red blood cell adhesion and deformability in sickle cell disease

We present a microfluidic approach that allows simultaneous interrogation of RBC properties in physiological flow conditions at a single cell level. With this method, we studied healthy hemoglobin A (HbA) and homozygous sickle hemoglobin (HbS) containing RBCs using whole blood samples from twelve subjects. We report that HbS-containing RBCs are heterogeneous in terms of adhesion and deformability in flow.

Additive effect of red blood cell rigidity and adherence to endothelial cells in inducing vascular resistance

To explore the contribution of red blood cell (RBC) deformability and interaction with endothelial cells (ECs) to circulatory disorders, these RBC properties were modified by treatment with hydrogen peroxide (H2O2), and their effects on vascular resistance were monitored following their infusion into rat mesocecum vasculature. Treatment with 0.5 mM H2O2 increased RBC/EC adherence without significant alteration of RBC deformability. At 5.0 mM H2O2, RBC deformability was considerably reduced, inducing a threefold increase in the number of undeformable cells, whereas RBC/EC adherence was not further affected by the increased H2O2 concentration. This enabled the selective manipulation of RBC adherence and deformability and the testing of their differential effect on vascular resistance. Perfusion of RBCs with enhanced adherence and unchanged deformability (treatment with 0.5 mM H2O2) increased vascular resistance by about 35% compared with untreated control RBCs. Perfusion of 5.0 mM H2O2-treated RBCs, with reduced deformability (without additional increase of adherence), further increased vascular resistance by about 60% compared with untreated control RBCs. These results demonstrate the specific effects of elevated adherence and reduced deformability of oxidized RBCs on vascular resistance. These effects can be additive, depending on the oxidation conditions. The oxidation-induced changes applied in this study are moderate compared with those observed in RBCs in pathological states. Yet, they caused a considerable increase in vascular resistance, thus demonstrating the potency of RBC/EC adherence and RBC deformability in determining resistance to blood flow in vivo.

Renal vascular resistance in sickle cell painful crisis

Vaso-occlusive painful crisis is one of the characteristic manifestations of sickle cell disease (SCD). We aimed to investigate the state of renal vascular resistance by means of Doppler ultrasonography during vaso-occlusive painful crisis in patients with SCD. The 60 patients with homozygous SCD who entered the study were divided into 2 groups. Group 1 included 45 patients who were living in steady-state conditions and had no history of transfusions within the 3 months before the initiation of the study. Group 2 consisted of 15 patients with signs of painful vaso-occlusive crisis during evaluation. Group 2 patients had significant reductions in 3 measures of flow velocity in both main renal arteries, compared with group 1 patients (P < .04, P < .001, and P < .01). Mean and end-diastolic velocities in the segmental arteries (P < .01, and P < .001, respectively) and end-diastolic velocities in the interlobar arteries (P < .04) were lower in group 2 patients than in group 1 patients. Analysis of resistive (RI) and pulsatile (PI) indices in the investigated arteries demonstrated that the RI of the renal (P < .001; P < .0001), segmental (P < .002; P < .0001) and interlobar (P < .001; P < .0001) arteries of both kidneys in group 2 < .0001; P < .0001) for both kidneys were markedly higher in group 2 patients than in group 1 patients and the healthy subjects, respectively. Our preliminary results suggest a reduction of renal blood flow and an increase in renal vascular resistance during painful crisis compared with steady-state SCD.

cGMP and cAMP cause pulmonary vasoconstriction in the presence of hemolysate

We recently reported that addition of a small amount of hemolysate to the salt solution that perfused isolated rat lungs hypersensitized the vasculature to subsequent additions of ANG II or exposure to hypoxia, and addition of NO gas (. NO) to the perfusate that contained hemolysate caused a strong vasoconstrictor rather than a vasodilator response. In the present study, we demonstrate that CO and the secondary messengers cGMP and cAMP (usually associated with vasodilation) exert similar effects in hemolysate-perfused lungs. Analogs of the cyclic nucleotides cGMP or cAMP (8-bromo-cGMP and dibutyryl-cAMP, respectively) caused profound vasoconstriction in the isolated rat lung perfused with a salt solution that contained hemolysate. The cGMP- or cAMP-analog-induced vasoconstriction was inhibited by chemically dissimilar Ca2+ antagonists, by the protein phosphatase inhibitor okadaic acid, and, to a lesser degree, by protein kinase inhibitor H-7. Antiphosphothreonine immunoblotting demonstrated that lungs perfused with hemolysate exhibit increased phosphorylation of several proteins. These data indicate that, in the presence of hemolysate, pulmonary vasculature responds to nominally vasodilatory stimuli, including analogs of cGMP and cAMP, with vasoconstriction rather than vasodilation. The importance of our finding is the paradoxical nature of the response to (analogs of) cyclic nucleotides because, to our knowledge, cyclic nucleotide-induced vasoconstriction has not been previously reported.

Spectral pulsed Doppler sonography of renal vascular resistance in sickle cell disease: clinical implications

The major complications in sickle cell disease (SCD) are microcirculation lesions and impairment of renal function. The aim of this study was the evaluation of renal vascular resistance by means of spectral pulsed Doppler sonography and its relationships with haematological and clinical features in patients with SCD. 40 patients with SCD (mean age 22.4 +/- 7.0) and 14 age and sex matched healthy subjects (mean age 25.7 +/- 9.5) were included into the study. Spectral Doppler sonography of main renal, segmental and interlobar arteries were performed on both kidneys in all patients and controls. Peak systolic, end diastolic and mean velocities through the entire cardiac cycle were obtained, with calculation of the resistive (RI) and pulsatility (PI) indices. All the patients were investigated in stable state conditions and had not been transfused within a month before investigation. Patients were followed for up to 6 months. Patients with SCD had higher values of RIs and PIs than control subjects (p < 0.0001, p < 0.0001, respectively). Patients with high value of RIs (RI > 0.70) had more pronounced disturbances of blood parameters (all p < 0.05), than patients with normal RIs (RI < 0.70). Significant positive correlation existed between RIs and ISC number, MCHC level (r = 0.52, p < 0.001 and r = 0.42, p < 0.01), while RBC count and Hb level correlated inversely with RIs (r = -0.39, p < 0.01 and r = -0.42, p < 0.01). During follow-up, nine patients (33.3%) with high RIs and only one patient (5.5%) with normal RI developed painful crises. In conclusion, renal vascular resistance, assessed by Doppler sonography was raised in SCD patients as compared with age matched apparently healthy persons. These changes were more pronounced in those with more severe manifestations of disease and correlated with haematological and clinical features of sickle cell disease.

Sickle cell vasoocclusion: many issues and some answers

The pathophysiology of sickle (SS) cell vasoocclusion is derived from the presence of hemoglobin S (HbS) which forms polymeric fibers in the deoxygenated state. Nevertheless, phenotypic expression of sickle cell disease (i.e., clinical severity) shows marked individual variations and is influenced by genetic modifiers such as epistatic effects of linked and unlinked genes. Furthermore, the polymerization of HbS is central but not the only event, and is more likely a consequence of disruptions of the steady state of flow. The available evidence indicates that the vasoocclusive crisis is a microcirculatory event in which multiple factors could be involved. We present a model of vasoocclusion as a two step process in which adhesion of deformable cells occurs first, followed by obstruction induced by less deformable SS cells. This review discusses, in addition, rheologic and microcirculatory behavior of SS erythrocytes and the interacting role of vascular factors, red cell heterogeneity, deoxygenation rates, and red cell-endothelial interactions in the pathophysiology of SS cell vasoocclusion.

Rheological and adherence properties of sickle cells. Potential contribution to hematologic manifestations of the disease

Hematologic manifestations of sickle cell disease are varied and complex, and there is also a great variation in these manifestations among different individuals with the disease. While substantial efforts have been invested in defining the cellular basis for these clinical manifestations--including the evaluation of the potential contributions of intracellular polymer content, kinetics of hemoglobin polymerization, rheological abnormalities, oxidant membrane damage, and adherence of sickle cells to vascular endothelial cells--we are still far from understanding the relative contributions of each of these factors to varied manifestations of the disease. While the data discussed in this paper raise interesting issues regarding the potential contribution of rheological and adherence properties of sickle cells to altered flow dynamics in the microvasculature, they fall short of defining the direct contributions of these factors to various clinical manifestations. Further detailed characterization of various cellular abnormalities of sickle cells and how each of these factors acting alone or in combination with other cellular and extracellular factors such as microvasculature changes contribute to different clinical manifestations will be needed to further our understanding of the pathophysiology of this complex disorder.

Microvascular sites and characteristics of sickle cell adhesion to vascular endothelium in shear flow conditions: pathophysiological implications

To understand the role of sickle cell adherence to the vascular endothelium in the pathophysiology of sickle cell anemia (SS) vasoocclusion, we have carried out a microcirculatory study utilizing the ex vivo mesocecum vasculature of the rat. A single bolus of washed oxy-normal (AA) erythrocytes or oxy-SS cells (unseparated or density-defined SS cell classes) was infused. Hemodynamic monitoring and intravital microscopic observations of the microvascular flow revealed higher peripheral resistance for SS erythrocytes and adherence of these cells exclusively to the venular endothelium but rare or no adherence of AA cells. The extent of adhesion was inversely correlated with venular diameters (r = -0.812; P less than 0.00001). The adhesion of SS erythrocytes is density-class dependent: reticulocytes and young discocytes (SS1) greater than discocytes (SS2) greater than irreversible sickle cells and unsicklable dense discocytes (SS4). Selective secondary trapping of SS4 (dense cells) is found in postcapillary venules where deformable SS cells are preferentially adhered. We conclude that in the oxygenated condition, vasoocclusion can be induced by two events: (i) random precapillary obstruction by a small number of SS4 cells; (ii) increased adhesion of SS1 and SS2 cells in the immediate postcapillary venules. A combination of precapillary obstruction, adhesion in postcapillary venules, and secondary trapping of dense cells may induce local hypoxia, increased polymerization of hemoglobin S, and rigidity of SS erythrocytes, thereby extending obstruction to nearby vessels.

Oxygen transport studies of normal and sickle red cell suspensions in artificial capillaries

Oxygen transport from normal and sickle red cells was studied under known and carefully controlled conditions simulating the microcirculation. Oxygenated red cell suspensions became deoxygenated as they traversed silicone rubber artificial capillaries of 27 microns diameter. Oxygen saturation values of the flowing red cell suspensions were measured at several axial positions along the artificial capillary by use of a microspectrophotometric technique. Oxygen saturation decreased with increasing distance from the entrance of the artificial capillary and was influenced strongly by the flow rate. Under the same hematocrit and flow conditions, the rate of oxygen saturation decrease was significantly higher for the sickle red cells than that for the normal red cells. Similar results were obtained by use of a mathematical simulation of oxygen transport in the microcirculation for both normal and sickle red cells. Sickle red cells would be expected to have a higher diffusional resistance to oxygen transport than would normal red cells. However, the higher diffusional resistance is more than offset by the lower oxygen affinity of the sickle cells. The difference in oxygen affinity appears to account for the difference in oxygen transport rates between normal and sickle red cells.

Flow dynamics of human sickle erythrocytes in the mesenteric microcirculation of the exchange-transfused rat

To analyze the microvascular rheology of sickle cells in an intact animal model, rats were isovolemically exchange transfused with human normal (hemoglobin AA) or sickle (hemoglobin SS) erythrocytes (blood group O) or autologous red cells under ambient conditions, and the effects of the heterologous or autologous cells on (a) hemodynamics and respiration, (b) blood gases, and (c) acid-base status of the recipients were determined. Exchange transfusion of rats with autologous red cells or hemoglobin AA or hemoglobin SS erythrocytes was associated with stable mean arterial blood pressure, central venous pressure, respiration rate, blood pH, pCO2, and pO2 during the experimental period, except for tachycardia among the group of rats that received HbSS cells. Arteriovenous oxygen content varied among the three groups of animals, but, nonetheless, suggested adequate tissue oxygen supply under the conditions of the study. Acid-base status also was similar in the three groups of rats. The exchange-transfused rats were utilized to investigate the flow dynamics of red cells in the mesenteric microcirculation by applying intravital microscopy. Time-averaged velocities of the autologous red cells in 16- to 30-microns (id) vessels ranged from 1.07 to 1.25 mm/sec in single unbranched arterioles with varying flux and wall shear rates. Time-averaged velocities of the HbAA cells in single 15- to 35-microns arterioles ranged from 1.16 to 1.24 mm/sec with wall shear rates similar to the estimates for the autologous cells. For both rat and human HbAA RBCs, the flow dynamics were indicative of normal shear-dependent and deformability characteristics of the cells under the flow conditions. Sickle cells exhibited time-averaged velocities of 0.384 to 0.452 mm/sec, lower wall shear rates in 10- to 35-microns single unbranched arterioles, and three times less volumetric flux. In some arterioles, sickle cells with high axial ratio and low deformability showed definite adhesion to the endothelial surface, residing at such sites for several seconds until dislodged by the force of flow. Within single unbranched vessels or at microvascular bifurcations, sickle elliptocytes and sickle echinocytes with low deformability and high axial ratio obstructed flow and exhibited residence times of 2 to 88 sec, thereby causing stasis. These data illustrate the microvascular flow behavior of sickle cells and demonstrate the rheological disequilibrium state that can result as sickle cells course through successive segments of the microcirculation.

Intravital microscopy of capillary hemodynamics in sickle cell disease

Direct intravital microscopic examinations were made in nailfold capillaries in subjects with homozygous sickle cell disease (HbSS red cells). In the resting state, capillary red cell (rbc) flux exhibited greater intermittence compared with normal subjects, which increased with painful crisis. In crisis-free HbSS subjects, capillary occlusion and red cell sequestration occurred in only 8.2% of all capillaries and diminished to 5.8% during crisis, possibly due to sequestration of less deformable rbcs in other organs. Velocities of rbc's (Vrbc) were measured by video techniques under resting conditions and during postocclusive reactive hyperemia (PORH) induced by a pressure cuff around the finger. Resting Vrbc was normal in crisis-free HbSS subjects, averaging 0.7 mm/s. In contrast, Vrbc was significantly elevated during crisis, to 0.98 mm/s, apparently due to compensatory arteriolar dilation. Crisis subjects exhibited a significantly depressed PORH with the ratio of peak red cell velocity to resting values reduced by 15% due to a loss of vasodilatory reserve, whereas crisis-free subjects exhibited a normal response. A 55% increase in the time to attain peak Vrbc was attributed to resistance increases, possibly resulting from red cell and leukocyte-to-endothelium adhesion during the induced ischemia.

The development of a filtration system for evaluating flow characteristics of erythrocytes

A complete description of the pathophysiology of sickle cell disease requires a physiologically meaningful measurement of red cell deformability. We have designed and built a system which allows one to determine filtration characteristics of erythrocytes. A dilute red cell suspension is forced through a 3.0-micron polycarbonate Nuclepore membrane with a constant positive pressure of 20 mm Hg. Under these conditions blockage of the pores in the polycarbonate membrane is insignificant and flow is linear. We use the relative number of cells filtered through the membrane as a means of approximating the means deformability of cells in the suspension. Using this system we have compared erythrocytes from various mammals and shown that our technique is sensitive in detecting not only differences in cell deformabilities between mammalian species but also changes in cell deformability of human red cells due to exchange transfusion and application of drugs. There was a positive correlation between cell filtrability and percentage cell recovery (coefficient of correlation, 0.65) and a negative correlation between cell size and filtrability (coefficient of correlation, -0.61). The filtrabilities of normal volunteers and sickle cell disease patients were found to be 71.8 +/- 6.6 and 53.6 +/- 5.0%, respectively. This system is sensitive and reliable, and should be useful in evaluating both the contribution of filtrability to the viability of red cells in vivo and potential therapeutic agents for sickle cell disease.

Oxygen transport studies of normal and sickle erythrocyte suspensions in artificial capillaries

Oxygen transport from normal and sickle erythrocytes was studied under known and carefully controlled conditions simulating the microcirculation. Oxygenated erythrocyte suspensions became deoxygenated as they traversed silicone rubber capillaries of 27 microns diameter. Oxygen saturation values of the flowing erythrocyte suspensions were measured at several axial positions along the capillary by use of a microspectrophotometric technique. Oxygen saturation decreased with increasing distance from the entrance of the capillary and was strongly influenced by the flow rate. Under the same hematocrit and flow conditions, the rate of oxygen saturation decrease was significantly higher for the sickle cells than for normal cells. Sickle cells would be expected to have a higher diffusional resistance to oxygen transport than that of normal cells. However, the lower oxygen affinity of the sickle cells tends to increase the oxygen delivery rate. The difference in oxygen affinity appears to account for the difference in oxygen delivery rates between normal and sickle cells.