Erythrocyte rheology

Technologies for measuring red blood cell deformability

Human red blood cells (RBCs) are approximately 8 μm in diameter, but must repeatedly deform through capillaries as small as 2 μm in order to deliver oxygen to all parts of the body. The loss of this capability is associated with the pathology of many diseases, and is therefore a potential biomarker for disease status and treatment efficacy. Measuring RBC deformability is a difficult problem because of the minute forces (∼pN) that must be exerted on these cells, as well as the requirements for throughput and multiplexing. The development of technologies for measuring RBC deformability date back to the 1960s with the development of micropipette aspiration, ektacytometry, and the cell transit analyzer. In the past 10 years, significant progress has been made using microfluidics by leveraging the ability to precisely control fluid flow through microstructures at the size scale of individual RBCs. These technologies have now surpassed traditional methods in terms of sensitivity, throughput, consistency, and ease of use. As a result, these efforts are beginning to move beyond feasibility studies and into applications to enable biomedical discoveries. In this review, we provide an overview of both traditional and microfluidic techniques for measuring RBC deformability. We discuss the capabilities of each technique and compare their sensitivity, throughput, and robustness in measuring bulk and single-cell RBC deformability. Finally, we discuss how these tools could be used to measure changes in RBC deformability in the context of various applications including pathologies caused by malaria and hemoglobinopathies, as well as degradation during storage in blood bags prior to blood transfusions.

Integrated automated particle tracking microfluidic enables high-throughput cell deformability cytometry for red cell disorders

Investigating individual red blood cells (RBCs) is critical to understanding hematologic diseases, as pathology often originates at the single-cell level. Many RBC disorders manifest in altered biophysical properties, such as deformability of RBCs. Due to limitations in current biophysical assays, there exists a need for high-throughput analysis of RBC deformability with single-cell resolution. To that end, we present a method that pairs a simple in vitro artificial microvasculature network system with an innovative MATLAB-based automated particle tracking program, allowing for high-throughput, single-cell deformability index (sDI) measurements of entire RBC populations. We apply our technology to quantify the sDI of RBCs from healthy volunteers, Sickle cell disease (SCD) patients, a transfusion-dependent beta thalassemia major patient, and in stored packed RBCs (pRBCs) that undergo storage lesion over 4 weeks. Moreover, our system can also measure cell size for each RBC, thereby enabling 2D analysis of cell deformability vs cell size with single cell resolution akin to flow cytometry. Our results demonstrate the clear existence of distinct biophysical RBC subpopulations with high interpatient variability in SCD as indicated by large magnitude skewness and kurtosis values of distribution, the "shifting" of sDI vs RBC size curves over transfusion cycles in beta thalassemia, and the appearance of low sDI RBC subpopulations within 4 days of pRBC storage. Overall, our system offers an inexpensive, convenient, and high-throughput method to gauge single RBC deformability and size for any RBC population and has the potential to aid in disease monitoring and transfusion guidelines for various RBC disorders.

Effects of aging and gender on micro-rheology of blood in 3 to 18 months old male and female Wistar (Crl:WI) rats

BACKGROUND

Age- and gender-related alterations of hemorheological parameters have not been completely elucidated to date. Experiments on older animals may give valuable information on this issue. However, the majority of rheological studies have been performed in young rodents.

OBJECTIVE

We aimed to investigate the influence of aging and gender on hemorheological parameters in rats.

METHODS

Coeval male (n=10) and female (n=10) Wistar (Crl:WI) rats were followed-up over 15 months. Blood samples were obtained from the lateral tail vein at 3, 4, 5, 9, 12, 15 and 18 months of age. Hematological parameters, red blood cell deformability (elongation under shear), osmotic gradient deformability and erythrocyte aggregation were tested. Body weight and the estrus cycle (in females) were also examined.

RESULTS

Erythrocyte aggregation showed age- and gender-related variations. Red blood cell deformability was greater in females and gradually decreased over the 15-month period in both genders. Erythrocyte aggregation was greater in male rats at most ages, but did not show consistent changes with age.

CONCLUSIONS

The micro-rheological parameters showed age-related alterations with gender differences. The effect of the estrous cycle cannot be excluded in female rats. The results provide reference data for studies of aging in rats and of the mechanism related to age and gender differences in hemorheology.

Abnormal Microcirculation and Red Blood Cell Function as a Cardiovascular Risk Factor in Metabolic Syndrome

The metabolic syndrome is a highly prevalent condition associated with increased cardiovascular risk in the population. Microvasculature is the terminal part of the cardiovascular system which primarily reacts to the increased secretion of the pro inflammatory adipokines typical for the metabolic syndrome. Microcirculation and blood cell abnormalities are the leading mechanisms of cardiovascular events development in this condition. Prevalence of microcirculation abnormalities and red blood cell dysfunction in metabolic syndrome and their role in the increased blood viscosity and cardiovascular events development are covered by the paper. The microcirculation abnormalities with a special focus on red blood cell dysfunction (impaired aggregation, stiffness) seen in metabolic syndrome and associated conditions are discussed in detail. The impact of abnormal red blood cell deformability (secondary to cholesterol accumulation in the cellular membranes) on the hemorheological abnormalities is revealed. Abnormal red blood cell surface charge due to proinflammatory changes associated with insulin resistance in diabetes mellitus is highlighted. These abnormalities lead to increased red blood cell aggregation and plasma viscosity that are the essential components of cardiovascular events pathogenesis. Their timely diagnosis is crucial for effective cardiovascular prevention.

Assessment of red blood cell deformability in type 2 diabetes mellitus and diabetic retinopathy by dual optical tweezers stretching technique

A pilot cross sectional study was conducted to investigate the role of red blood cells (RBC) deformability in type 2 diabetes mellitus (T2DM) without and with diabetic retinopathy (DR) using a dual optical tweezers stretching technique. A dual optical tweezers was made by splitting and recombining a single Nd:YAG laser beam. RBCs were trapped directly (i.e., without microbead handles) in the dual optical tweezers where they were observed to adopt a "side-on" orientation. RBC initial and final lengths after stretching were measured by digital video microscopy, and a Deformability index (DI) calculated. Blood from 8 healthy controls, 5 T2DM and 7 DR patients with respective mean age of 52.4 yrs, 51.6 yrs and 52 yrs was analysed. Initial average length of RBCs for control group was 8.45 ± 0.25 μm, 8.68 ± 0.49 μm for DM RBCs and 8.82 ± 0.32 μm for DR RBCs (p < 0.001). The DI for control group was 0.0698 ± 0.0224, and that for DM RBCs was 0.0645 ± 0.03 and 0.0635 ± 0.028 (p < 0.001) for DR group. DI was inversely related to basal length of RBCs (p =  .02). DI of RBC from DM and DR patients was significantly lower in comparison with normal healthy controls. A dual optical tweezers method can hence be reliably used to assess RBC deformability.

Deformability based sorting of red blood cells improves diagnostic sensitivity for malaria caused by Plasmodium falciparum

The loss of red blood cell (RBC) deformability is part of the pathology of many diseases. In malaria caused by Plasmodium falciparum infection, metabolism of hemoglobin by the parasite results in progressive reduction in RBC deformability that is directly correlated with the growth and development of the parasite. The ability to sort RBCs based on deformability therefore provides a means to isolate pathological cells and to study biochemical events associated with disease progression. Existing methods have not been able to sort RBCs based on deformability or to effectively enrich for P. falciparum infected RBCs at clinically relevant concentrations. Here, we develop a method to sort RBCs based on deformability and demonstrate the ability to enrich the concentration of ring-stage P. falciparum infected RBCs (Pf-iRBCs) by >100× from clinically relevant parasitemia (<0.01%). Deformability based sorting of RBCs is accomplished using ratchet transport through asymmetrical constrictions using oscillatory flow. This mechanism provides dramatically improved selectivity over previous biophysical methods by preventing the accumulation of cells in the filter microstructure to ensure that consistent filtration forces are applied to each cell. We show that our approach dramatically improves the sensitivity of malaria diagnosis performed using both microscopy and rapid diagnostic test by converting samples with difficult-to-detect parasitemia (<0.01%) into samples with easily detectable parasitemia (>0.1%).

Deformability measurement of red blood cells using a microfluidic channel array and an air cavity in a driving syringe with high throughput and precise detection of subpopulations

Red blood cell (RBC) deformability has been considered a potential biomarker for monitoring pathological disorders. High throughput and detection of subpopulations in RBCs are essential in the measurement of RBC deformability. In this paper, we propose a new method to measure RBC deformability by evaluating temporal variations in the average velocity of blood flow and image intensity of successively clogged RBCs in the microfluidic channel array for specific time durations. In addition, to effectively detect differences in subpopulations of RBCs, an air compliance effect is employed by adding an air cavity into a disposable syringe. The syringe was equally filled with a blood sample (V(blood) = 0.3 mL, hematocrit = 50%) and air (V(air) = 0.3 mL). Owing to the air compliance effect, blood flow in the microfluidic device behaved transiently depending on the fluidic resistance in the microfluidic device. Based on the transient behaviors of blood flows, the deformability of RBCs is quantified by evaluating three representative parameters, namely, minimum value of the average velocity of blood flow, clogging index, and delivered blood volume. The proposed method was applied to measure the deformability of blood samples consisting of homogeneous RBCs fixed with four different concentrations of glutaraldehyde solution (0%-0.23%). The proposed method was also employed to evaluate the deformability of blood samples partially mixed with normal RBCs and hardened RBCs. Thereafter, the deformability of RBCs infected by human malaria parasite Plasmodium falciparum was measured. As a result, the three parameters significantly varied, depending on the degree of deformability. In addition, the deformability measurement of blood samples was successfully completed in a short time (∼10 min). Therefore, the proposed method has significant potential in deformability measurement of blood samples containing hematological diseases with high throughput and precise detection of subpopulations in RBCs.

Diminution of Oxidative Damage to Human Erythrocytes and Lymphocytes by Creatine: Possible Role of Creatine in Blood

Creatine (Cr) is naturally produced in the body and stored in muscles where it is involved in energy generation. It is widely used, especially by athletes, as a staple supplement for improving physical performance. Recent reports have shown that Cr displays antioxidant activity which could explain its beneficial cellular effects. We have evaluated the ability of Cr to protect human erythrocytes and lymphocytes against oxidative damage. Erythrocytes were challenged with model oxidants, 2, 2'-azobis(2-amidinopropane) dihydrochloride (AAPH) and hydrogen peroxide (H₂O₂) in the presence and absence of Cr. Incubation of erythrocytes with oxidant alone increased hemolysis, methemoglobin levels, lipid peroxidation and protein carbonyl content. This was accompanied by decrease in glutathione levels. Antioxidant enzymes and antioxidant power of the cell were compromised while the activity of membrane bound enzyme was lowered. This suggests induction of oxidative stress in erythrocytes by AAPH and H₂O₂. However, Cr protected the erythrocytes by ameliorating the AAPH and H₂O₂ induced changes in these parameters. This protective effect was confirmed by electron microscopic analysis which showed that oxidant-induced cell damage was attenuated by Cr. No cellular alterations were induced by Cr alone even at 20 mM, the highest concentration used. Creatinine, a by-product of Cr metabolism, was also shown to exert protective effects, although it was slightly less effective than Cr. Human lymphocytes were similarly treated with H₂O₂ in absence and presence of different concentrations of Cr. Lymphocytes incubated with oxidant alone had alterations in various biochemical and antioxidant parameters including decrease in cell viability and induction of DNA damage. The presence of Cr attenuated all these H₂O₂-induced changes in lymphocytes. Thus, Cr can function as a blood antioxidant, protecting cells from oxidative damage, genotoxicity and can potentially increase their lifespan.

The relationship between red blood cell deformability metrics and perfusion of an artificial microvascular network

The ability of red blood cells (RBC) to undergo a wide range of deformations while traversing the microvasculature is crucial for adequate perfusion. Interpretation of RBC deformability measurements performed in vitro in the context of microvascular perfusion has been notoriously difficult. This study compares the measurements of RBC deformability performed using micropore filtration and ektacytometry with the RBC ability to perfuse an artificial microvascular network (AMVN). Human RBCs were collected from healthy consenting volunteers, leukoreduced, washed and exposed to graded concentrations (0-0.08%) of glutaraldehyde (a non-specific protein cross-linker) and diamide (a spectrin-specific protein cross-linker) to impair the deformability of RBCs. Samples comprising cells with two different levels of deformability were created by adding non-deformable RBCs (hardened by exposure to 0.08% glutaraldehyde) to the sample of normal healthy RBCs. Ektacytometry indicated a nearly linear decline in RBC deformability with increasing glutaraldehyde concentration. Micropore filtration showed a significant reduction only for concentrations of glutaraldehyde higher than 0.04%. Neither micropore filtration nor ektacytometry measurements could accurately predict the AMVN perfusion. Treatment with diamide reduced RBC deformability as indicated by ektacytometry, but had no significant effect on either micropore filtration or the AMVN perfusion. Both micropore filtration and ektacytometry showed a linear decline in effective RBC deformability with increasing fraction of non-deformable RBCs in the sample. The corresponding decline in the AMVN perfusion plateaued above 50%, reflecting the innate ability of blood flow in the microvasculature to bypass occluded capillaries. Our results suggest that in vitro measurements of RBC deformability performed using either micropore filtration or ektacytometry may not represent the ability of same RBCs to perfuse microvascular networks. Further development of biomimetic tools for measuring RBC deformability (e.g. the AMVN) could enable a more functionally relevant testing of RBC mechanical properties.

Intravenous solutions in the care of patients with volume depletion and electrolyte abnormalities

Infusion fluids are often given to restore blood pressure (volume resuscitation), but may also be administered to replace ongoing losses, match insensible losses, correct electrolyte or acid-base disorders, or provide glucose. The development of new infusion fluids has provided clinicians with a wide range of products. Although the choice for a certain infusion fluid is often driven more by habit than by careful consideration, we believe it is useful to approach infusion fluids as drugs and consider their pharmacokinetic and pharmacodynamic characteristics. This approach not only explains why infusion fluids may cause electrolyte and acid-base disturbances, but also why they may compromise kidney function or coagulation. In this teaching case, we present a 19-year-old patient in whom severe hypernatremia developed as a result of normal saline solution infusion and explore the pharmacokinetic and pharmacodynamic effects of frequently used infusion fluids. We review clinical evidence to guide the selection of the optimal infusion fluid.

Multiplexed fluidic plunger mechanism for the measurement of red blood cell deformability

The extraordinary deformability of red blood cells gives them the ability to repeatedly transit through the microvasculature of the human body. The loss of this capability is part of the pathology of a wide range of diseases including malaria, hemoglobinopathies, and micronutrient deficiencies. We report on a technique for multiplexed measurements of the pressure required to deform individual red blood cell through micrometer-scale constrictions. This measurement is performed by first infusing single red blood cells into a parallel array of ~1.7 μm funnel-shaped constrictions. Next, a saw-tooth pressure waveform is applied across the constrictions to squeeze each cell through its constriction. The threshold deformation pressure is then determined by relating the pressure-time data with the video of the deformation process. Our key innovation is a self-compensating fluidic network that ensures identical pressures are applied to each cell regardless of its position, as well as the presence of cells in neighboring constrictions. These characteristics ensure the consistency of the measurement process and robustness against blockages of the constrictions by rigid cells and debris. We evaluate this technique using in vitro cultures of RBCs infected with P. falciparum, the parasite that causes malaria, to demonstrate the ability to profile the deformability signature of a heterogeneous sample.

Emerging Role for Use of Liposomes in the Biopreservation of Red Blood Cells

SUMMARY: Biopreservation is the process of maintaining the integrity and functionality of cells held outside the native environment for extended storage times. The development of red blood cell (RBC) biopreservation techniques that maintain in vitro RBC viability and function represents the foundation of modern blood banking. The biopreservation of RBCs for clinical use can be categorized based on the techniques used to achieve biologic stability, including hypothermic storage and cryopreservation. This review will examine the emerging role of liposomes in the RBC biopreservation, including the incorporation of liposomes into RBC membranes as an effective approach for minimizing RBC hypothermic storage membrane lesion and use of liposomes as a permeabilization strategy for the intracellular accumulation of novel intracellular cryoprotectants. Integration of current biopreservation research with blood banking practices offers enormous potential for future improvements of safety and efficacy of RBC transfusion.

Red blood cell damage after trauma-hemorrhage is modulated by gender

BACKGROUND

Previous studies have shown that trauma-hemorrhagic shock (T/HS) causes significant alterations in red blood cell (RBC) deformability and shape. Gender is becoming well recognized as a modulating factor in the pathophysiologic response to trauma. We hypothesize that female subjects are more resistant to adverse effects of T/HS on RBC deformability and shape than male subjects because of estrogen protection.

METHODS

Elongation index, a measure of RBC deformability, was examined in six male rats and four groups of female rats at different stages of the estrous cycle (six animals per group) before and 6 hours after T/HS by laser ektacytometry. RBC shape was determined by scanning electron microscopy. Lipid peroxidation was evaluated by measurement of malonyldialdehyde in plasma 6 hours after T/HS.

RESULTS

Male rats had a significant decrease in RBC elongation index after T/HS as compared with preshock values. RBC deformability of the proestrous (PES) and estrous female rats did not decrease after T/HS. Postshock RBC deformability values in metestrous and diestrous female rats did not differ from male rats. Male rats and metestrous and diestrous females had a significant increase in the percentage of abnormally shaped RBCs immediately after shock and during the postresuscitation period. In PES and ES female rats, RBC shape distribution did not significantly change after T/HS. Plasma malonyldialdehyde levels at 6 hours post-T/HS were higher in the male rats than in PES female rats.

CONCLUSION

These data show that high levels of estrogen in female rats are protective against shock-induced RBC damage and suggest that this effect is associated with a decrease in lipid peroxidation.

A microfluidic model for single-cell capillary obstruction by Plasmodium falciparum-infected erythrocytes

Severe malaria by Plasmodium falciparum is a potentially fatal disease, frequently unresponsive to even the most aggressive treatments. Host organ failure is associated with acquired rigidity of infected red blood cells and capillary blockage. In vitro techniques have played an important role in modeling cell deformability. Although, historically they have either been applied to bulk cell populations or to measure single physical parameters of individual cells. In this article, we demonstrate the unique abilities and benefits of elastomeric microchannels to characterize complex behaviors of single cells, under flow, in multicellular capillary blockages. Channels of 8-, 6-, 4-, and 2-microm widths were readily traversed by the 8 microm-wide, highly elastic, uninfected red blood cells, as well as by infected cells in the early ring stages. Trophozoite stages failed to freely traverse 2- to 4-microm channels; some that passed through the 4-microm channels emerged from constricted space with deformations whose shape-recovery could be observed in real time. In 2-microm channels, trophozoites mimicked "pitting," a normal process in the body where spleen beds remove parasites without destroying the red cell. Schizont forms failed to traverse even 6-microm channels and rapidly formed a capillary blockage. Interestingly, individual uninfected red blood cells readily squeezed through the blockages formed by immobile schizonts in a 6-microm capillary. The last observation can explain the high parasitemia in a growing capillary blockage and the well known benefits of early blood transfusion in severe malaria.

Hyperaggregating effect of hydroxyethyl starch components and University of Wisconsin solution on human red blood cells: a risk of impaired graft perfusion in organ procurement?

BACKGROUND

The standard preservation solution used during organ procurement and preservation of most organs is the University of Wisconsin (UW) solution. Despite its superiority over other cold storage solutions, the inclusion of hydroxyethyl starch (HES) as one of the components of the UW solution has been both advocated and denied. This study determined whether HES had any effect on red blood cell (RBC) aggregability and correlated aggregation parameters with HES molecular weight.

METHODS

Human RBC aggregability and deformability were investigated in vitro, at 4 degrees C, with a laser-assisted optical rotation cell analyzer. The study of RBC aggregation in a binary HES-HES system gave an indication about the nature of HES-RBCs interactions. Bright field microscopy and atomic force microscopy were used to morphologically characterize the aggregates size and form.

RESULTS

High molecular weight HES and UW solution had a potent hyperaggregating effect; low molecular weight HES had a hypoaggregating effect on RBC. RBC aggregates were of large size and their resistance to dissociation by flow-induced shear stress was high.

CONCLUSION

The authors' in vitro experiments conclusively showed that the physiologic function of RBCs to form aggregates is significantly affected in the presence of HES. The use of high molecular weight HES in UW solution accounts for extended and accelerated aggregation of erythrocytes that may result in stasis of blood and incomplete washout of donor organs before transplantation.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Disorders of blood viscosity

In clinical situations associated with disturbed blood flow, the primary focus is usually on improving cardiovascular performance. However, during recent decades, both basic science and clinical literature reports have presented evidence that the flow properties of blood must also be considered in these situations. Thus, the relatively new fields of haemorheology and clinical haemorheology have evolved; the former deals with the flow and deformation behaviour of blood, plasma and the formed elements of blood, whereas the latter relates to alterations of their behaviour in various pathophysiologic states. This review therefore summarizes some of the salient aspects of clinical haemorheology and of the determinants of blood flow properties (flow rate, haematocrit, plasma viscosity, red cell aggregation, red cell deformability). In addition, it briefly describes several clinical disorders associated with abnormal blood, plasma or cell rheology ('hyperviscosity syndromes' occurring in polycythaemia, leukaemia, sickle cell disease, paraproteinaemias).

Effects of electroacupuncture of "zusanli" acupoint on high blood pressure and blood hyperviscosity in stress rats

Elevation of blood pressure (BP) and blood viscosity (BV) was induced in unanesthetized Wistar rats by fixing and hanging. Electroacupuncture of "Zusanli" acupoint or microinjection of GABA (60 micrograms/10 microliters) into the IV ventricle of the brain could lower the high BP and BV induced by fixed-hanging, which could be blocked by a microinjection of GABAA receptor antagonist bicuculline (60 micrograms/10 microliters). The results showed that the depressant effect of electroacupuncture of "Zusanli" acupoint on high BP and blood hyperviscosity induced by fixed-hanging might be mediated by the activation of GABAA receptors in the brain.

The influence of iron deficiency on erythrocyte deformability

The influence of iron deficiency on erythrocyte deformability is controversial. The present study was designed to analyse cell deformability in 14 patients with iron deficiency and controls in a comprehensive way by three different methods, namely erythrocyte filtration, erythrocyte elongation, and measurement of membrane elasticity. Suspensions of washed erythrocytes (haematocrit 0.10) free of leucocytes were used. Erythrocyte deformability measured by filtration was increased by iron deficiency: The relative filtration resistance of a cell in a 3 microns pore was 26.5 +/- 6.9 and 75.8 +/- 23.8 in iron deficiency and controls, respectively (P less than 0.0001). In 5 microns pores the values were 2.80 +/- 1.23 and 3.46 +/- 0.51 (not significant); when the red cell number/volume was adjusted to that in control samples, the value for iron deficiency became significantly lower than in controls (2.32 +/- 0.60, P less than 0.0001). Erythrocyte elongation by centrifugation was unaffected (ratio length/width 1.66 +/- 0.11 and 1.60 +/- 0.10 in iron deficiency and controls, respectively). Membrane elasticity, as assessed by a filter aspiration technique, was also unchanged (membrane elastic modulus 3.94 +/- 0.31 and 3.94 +/- 0.37 x 10(-3) dyn/cm, respectively). It is concluded that iron deficiency does not affect erythrocyte membrane elasticity and that the deformability of whole cells is not impaired, but improved under certain conditions such as the passage of 3 microns pores because of microcytosis with preserved surface/volume ratio. These results are in contrast to earlier studies and they have pathophysiological and clinical implications.

The clinical importance of erythrocyte deformability, a hemorrheological parameter

Hemorheology, the science of the flow behavior of blood, has become increasingly important in clinical situations. The rheology of blood is dependent on its viscosity, which in turn is influenced by plasma viscosity, hematocrit, erythrocyte aggregation, and erythrocyte deformability. In recent years it has become apparent that the shape and elasticity of erythrocytes may be important in explaining the etiology of certain pathological situations. Thus, clinicians have become increasingly interested in hemorheology in general and erythrocyte deformability in particular. In the course of time, many clinical studies have been performed, but no concise review has thus far been published. This article encompasses a review of the clinically based literature on this subject.

Red cell filterability determined using the cell transit time analyzer (CTTA): effects of ATP depletion and changes in calcium concentration

Cell transit time analysis (CTTA) is a new filtrometric technique for assessing red blood cell deformability by measuring the conductivity change caused by passage of erythrocytes through a polycarbonate filter. Most reported studies to date using CTTA have focused on the transit time (TT), the duration of passage of an individual red cell through a micropore. Bulk flow rate has not been previously measured via CTTA. The use of new enzyme based cleaning solutions make it possible to reduce clogging in micropore filters. Therefore, valid measures of the number of red cell transits per unit time (counts/s: C/S) can now be obtained. We evaluated both parameters, TT and C/S, as indicators of red cell filterability. Our goal was to evaluate the effect of metabolic changes shown by alternative techniques to affect red cell deformability. The two best established factors are changes in intracellular [ATP] and [Ca2+]. ATP depletion produces a very small increase in TT but a very marked decrease in C/S. In contrast, the addition of low concentrations of calcium produces an increase in TT with minimal decrease in C/S. The effects of calcium appear to be complex. The substantial changes in intracellular calcium induced by the ionophore A23187 result in a curvilinear pattern of increase in transit times and reduction in counts per s. Lanthanum, which inhibits egress of intracellular calcium, causes an increase in TT with a drop in C/S. We conclude that CTTA demonstrates the same changes in red cell deformability measurable by alternative filtrometric techniques; however, CTTA furnishes two separate and independent parameters which may be used to evaluate red cell deformability.

Erythrocyte deformability in diabetes and erythrocyte membrane lipid composition

Erythrocyte deformability was assessed in 40 diabetic patients, 24 insulin-dependent (IDD) and 16 non-insulin-dependent (NIDD), by measuring the initial filtration flow rate of whole blood, isolated red blood cells (RBC), and isolated RBC membranes with the Hanss hemorheometer, and its relationship to the plasma and ghost membrane lipid composition was investigated. RBC deformability was significantly reduced, whereas the deformability of the isolated RBC membranes did not differ significantly from the controls. In the plasma, the triglycerides were high, the high-density lipoprotein (HDL) cholesterol was reduced, and the ratio of total cholesterol over HDL cholesterol was high as compared with the controls. The RBC lipid composition expressed in mumol lipids/10(10) RBC showed significantly lower levels of free cholesterol, sphingomyelines, and phosphatidylcholine, which are the lipids principally located on the outer layer of the RBC membranes. These data suggest that in both IDD and NIDD patients, there may be a relation between these modifications in the RBC lipid composition and rheological impairment of the RBC.

Rheological studies on patients with posterior subretinal neovascularization and exudative age-related macular degeneration

To investigate the potential influence of a decreased perfusion rate of the choriocapillaris on the development of age-related macular degeneration (ARMD) with subretinal neovascularization (SRNV) apparently caused by disturbed flow properties of blood, we compared the hemorheological parameters of blood from 35 patients suffering from ARMD with SRNV with those from the 35 healthy patients of the same age. In both groups hematocrit, plasma viscosity, erythrocyte filtrability, aggregation, aggregating proteins, leukocyte and thrombocyte count, differentiation of leukocytes, thrombelastography, PTT, Quick test, and rheological profiles were comparable. The differences were not significant (P greater than 0.05). These results refute the hypothesis that changed flow properties of blood are the primary cause of the pathogenesis of ARMD with SRNV.

Effect of oxpentifylline on blood viscosity and cerebral blood flow in man

The effects of intravenous oxpentifylline on blood viscosity and cerebral blood flow were studied in eight patients with cerebrovascular disease using a double-blind placebo controlled design. A single dose of 200 mg oxpentifylline in 10 ml saline was given by intravenous injection over 10 min and compared with 10 ml saline alone. Whole blood and plasma viscosity were measured in a Contraves LS 30 coaxial viscometer at shear rates of 0.7 and 94.5 s-1. Cerebral blood flow was measured by the non-invasive intravenous xenon133 clearance method. The measurements were made before and then 30 min after the start of the injection of drug or saline alone. Oxpentifylline was found to have no significant effect on blood viscosity or cerebral blood flow.

Hemodynamic and hemorheological profiles in women with proteinuric hypertension of pregnancy and in pregnant controls

We obtained blood samples from 52 patients with pre-eclampsia and from 40 pregnant controls for measurement of plasma urate levels, hematocrit, white cell count and various hemorheological parameters. We also used impedance cardiography to measure cardiac output in both groups and from the results derived values for total peripheral resistance and oxygen transport. Central venous pressure was measured with a superior vena cava catheter in patients with pre-eclampsia but not in controls. Women with pre-eclampsia had significantly lower cardiac output and central venous pressure when compared with a control group. A modest correlation was observed between central venous pressure and cardiac output. The majority of pre-eclamptic patients had significantly raised hematocrit, leucocyte count, uric acid and red cell aggregation. Red cell deformability was significantly decreased in patients with pre-eclampsia. Most patients with severe pre-eclampsia (BP diast. greater than 100 mmHg) had a low Antithrombin III and colloid osmotic pressure level. The leucocyte count was raised when compared with the women with moderate pre-eclampsia. Oxygen delivery was reduced in patients with pre-eclampsia because of impaired rheological properties of their blood.

Interaction between erythrocytes and a perfluorochemical blood substitute

Perfluorochemicals (PFCs) are being widely developed for use as erythrocyte substitutes to carry oxygen to tissue in acute crises. Oxypherol, a commercially available PFC preparation commonly used for animal tests, reduces erythrocyte deformability in the presence of plasma. This undesirable effect further complicates oxygen delivery by erythrocytes. Our experiments indicate that one or more plasma proteins must be present to observe Oxypherol-induced reduction in erythrocyte deformability, but the latter is not protein specific. Neither platelet activation nor enhanced protein adsorption could account for the mechanism of altered erythrocyte flexibility. Using fluorine-19 NMR, it was found that a small amount of Oxypherol droplets is adsorbed on the surface of the erythrocytes. The amount of adsorbed droplets increases when Oxypherol is incubated with the erythrocytes in the presence of plasma, in parallel with a decrease in erythrocyte deformability. Therefore, it seems likely that the loss of deformability of the erythrocytes is caused by the adsorption of small Oxypherol droplets on the erythrocytes.

Rheology of the sickle cell disorders

The sickling process causes secondary changes in cell shape, size, cation and water content, and membrane structure that contribute to the impairment of intrinsic cell deformability (Figure 2). This rheological defect is partially compensated by a low haematocrit, which moderates the rise in whole-blood viscosity, and by a rise in cardiac output which increases capillary flow velocity (Berger and King, 1982). A delicate balance exists between these mechanisms and any local disturbance of this balance by pathological changes in factors extrinsic to the sickle cell (Figure 2) can precipitate vaso-occlusion. There is still considerable controversy over the site (arteriolar, capillary, or venular) of vaso-occlusion, the type of sickle cell (reversibly sickled or irreversibly sickled) that is primarily involved, and the relative importance of extra-erythrocytic precipitating factors such as stasis, hypoxia, hyperosmolality, acidosis, alteration in temperature, acute-phase rise in plasma proteins and leukocytes, prothrombotic changes in coagulation factors and platelets, and adhesion of blood cells to vascular endothelium (Figure 2). A low-grade hypercoagulable state has been described in patients with SS (Leichtman and Brewer, 1978; Richardson et al, 1979) which may be related to the procoagulant effect of the shift of phosphatidyl serine to the outer lipid bilayer of the sickle cell (Chiu et al, 1981; Franck et al, 1985). Platelets appear to accumulate at sites of vaso-occlusion (Siegel et al, 1985) and their migration to the vessel wall may be enhanced by the presence of poorly deformable erythrocytes (Aarts et al, 1984). Endothelial cell damage in the arterial or venous circulation may also contribute (Klug et al, 1982). Thus vaso-occlusion appears to result from a complex interaction between blood cells, plasma proteins and endothelium and any one of several precipitating factors may disturb the fragile steady state and cause a painful crisis. The study of sickle cells by rheological methods has considerable potential for investigating the pathophysiology of vaso-occlusive episodes in the SCD and for monitoring, both in vitro and ex vivo, the efficacy of antisickling compounds. Because of the multiple intrinsic and extrinsic factors that contribute to the rheological defect, it is not yet known which of these should be the primary target for an antisickling agent. In-vitro rheological studies in which different metabolic stresses can be applied to intact sickle cells in the presence of a putative antisickling drug should help to answer this question.(ABSTRACT TRUNCATED AT 400 WORDS)

Blood rheology in vitro and in vivo

Blood rheology tests are traditionally used for detection of organic disease and for monitoring disease activity. More recently they have been used for prediction of blood flow in vivo, not only in overt hyperviscosity syndromes but also in the covert hyperviscosity of low-flow states. The traditional ESR test result increases with red cell aggregation induced by increases in large, asymmetrical plasma globulins. However, small increases in haematocrit and large increases in plasma viscosity each decrease the ESR, reducing both its diagnostic utility and its ability to predict blood flow in vivo. The ESR should be corrected to a standard haematocrit, or else replaced by the ZSR or plasma viscosity, which are more rapid, simple, sensitive and independent of haematocrit. For prediction of blood flow in vivo, these tests can be supplemented by measurement of whole-blood viscosity, which can be performed simply and cheaply in capillary viscometers at high shear rates. Whole-blood viscosity is determined by plasma viscosity, haematocrit and red cell deformability at high shear rates. Its measurement is useful in overt hyperviscosity syndromes, particularly in estimating the effect of red cell transfusion in anaemic patients with plasma hyperviscosity, hyperleukocytic leukaemias or sickling disorders. Blood viscosity should be related to the haematocrit or haemoglobin concentration in order to estimate oxygen delivery to tissues. Changes in blood viscosity can be compensated readily in the normal circulation but not in the compromised, low-flow circulation. In these circumstances, systemic increases in plasma viscosity, haematocrit, whole-blood viscosity, red cell aggregation and in the numbers of circulating rigid red or white blood cells can perpetuate low-flow states and ischaemia. Red cell deformability in narrow vessels is best measured by micropore filtration systems, in which the effect of white cells has been eliminated. Red cell deformability is reduced by change in shape, decrease in the ratio of surface area to volume, decreased membrane flexibility and increased internal viscosity (MCHC and inclusions). White cells have negligible effects on bulk-blood viscosity but have important effects on blood flow in narrow vessels, due to their high internal viscosity and their adhesiveness when activated. White cell filterability is lowest for monocytes and for activated granulocytes and these adhesive and rigid cells may have important effects on microcirculatory blood flow in low-flow states.

Laser diffraction ellipsometry of erythrocytes under controlled shear stress using a rotational viscosimeter

An ektacytometric extension for the Contraves LS-30 viscosimeter is described, as well as the procedure to measure erythrocyte deformability and rouleau formation with this combination. The method error (coefficient of variation) for the measurements of the erythrocyte elongation index appeared to be less than 1%, while both intra- and interindividual variation were around 2%. Preliminary clinical experiments performed on blood from different patient groups (i.e., diabetes, uraemia, sickle cell anaemia) clearly demonstrated more rigid erythrocytes than normal. It can be concluded that it is possible now to analyse with 1 instrument: plasma viscosity, whole blood viscosity at shear rates from 0.01 to 236 s-1, erythrocyte deformability and rouleau formation.

Painful sickle cell crises precipitated by stopping prophylactic exchange transfusions

A patient with homozygous sickle cell disease showed a reduced incidence of painful crises as a result of regular exchange transfusion, but on three occasions when transfusion treatment was interrupted, a painful crisis occurred. Onset of painful crisis was associated with raised packed cell volume (PCV) or percentage of haemoglobin S (HbS%), or both. Measurement of whole blood viscosity using in vitro mixtures of blood group compatible normal (AA) and sickle (SS) cells showed that above an HbS of 25% any increase in PCV caused a disproportionate increase in whole blood viscosity. These clinical observations and laboratory data suggest that when regular exchange transfusions are terminated both HbS% and PCV should be carefully monitored. Prophylactic venesection should be considered for patients who maintain their PCV after transfusion as HbS% rises.

Fluid trapping of erythrocytes under hypoosmolar conditions

125I albumin was used to assess the amount of trapped fluid after microhematocrit centrifugation of erythrocytes suspended in buffers of different osmolality. Surprisingly the total amount of trapped fluid per volume unit of packed erythrocytes decreased with decreasing osmolality of the suspending buffer despite erythrocyte swelling. However, if the contribution of the individual erythrocyte to the trapped fluid was calculated, the trapped fluid per erythrocyte did not change between 311 mosm/kg and 256 mosm/kg. For osmolalities below 256 mosm/kg a significant increase of trapped fluid was obtained. It is concluded that the packing ability of erythrocytes is not impaired in suspending fluid of moderate to severe infraphysiological tonicity. The daily clinical experience that considerable degrees of plasma hypoosmolality are tolerated in vivo without hemolysis or impairment of oxygen transport by erythrocytes may be explained by the excellent ability of shape adaptation of erythrocytes to each other and to other surfaces such as vascular endothelia. The method of trapped fluid determination might be of potential value as a complementary method in the evaluation of erythrocyte rheology if the amount of trapped fluid is related to the individual erythrocyte.

Filterability of ACD-stored red cells

The deformability of stored erythrocytes was studied using the Myrenne filtrometer MF4. Blood was stored in ACD under usual blood banking conditions and filterability studies were done at various intervals during a 3-week period. Marked reductions in stored erythrocyte filterability were noted as early as day 5 of storage. These results show that ACD is not a good "rheological' storage medium and indicate that further studies are required, using different storage media to assess the importance and significance of this rheological lesion.