Biphasic impairment of erythrocyte deformability in response to repeated, short duration exposures of supraphysiological, subhaemolytic shear stress

Red blood cell sublethal damage: haemocompatibility is not the absence of haemolysis

Blood is a complex fluid owing to its two-phase suspension of formed cellular elements within a protein-rich plasma. Vital to its role in distributing nutrients throughout the circulatory system, the mechanical properties of blood - and particularly red blood cells (RBC)-primarily determine bulk flow characteristics and microcirculatory flux. Various factors impair the physical properties of RBC, including cellular senescence, many diseases, and exposure to mechanical forces. Indeed, the latter is increasingly relevant following the advent of modern life support, such as mechanical circulatory support (MCS), which induce unique interactions between blood and artificial environments that leave blood cells with the signature of aging, albeit accelerated, and crucially underlie various serious complications, including death. Accumulating evidence indicates that these complications appear to be associated with mechanical shear forces present within MCS that are not extreme enough to overtly rupture cells, yet may still induce "sublethal" injury and "fatigue" to vital blood constituents. Impaired RBC physical properties following elevated shear exposure-a hallmark of sublethal injury to blood-are notable and may explain, at least in part, systemic complications and premature mortality associated with MCS. Design of optimal next-generation MCS devices thus requires consideration of biocompatibility and blood-device interactions to minimize potential blood complications and promote clinical success. Presented herein is a contemporary understanding of "blood damage," with emphasis on shear exposures that alter microrheological function but do not overtly destroy cells (ie, sublethal damage). Identification of key cellular factors perturbed by supraphysiological shear exposure are examined, offering potential pathways to enhance design of MCS and blood-contacting medical devices.

Bovine erythrocytes are poor surrogates for human when exposed to sublethal shear stress

Animal blood products are routinely used as surrogates for human tissue in haemocompatibility testing of rotary blood pumps. Bovine blood is particularly attractive due to the animal's large blood volume; however, bovine red blood cells (RBC) differ substantially from those of human, both in biophysical properties and molecular composition. We aimed to determine whether differences also exist in the sensitivity of bovine RBC to a standardised shear stress protocol. Fresh blood from healthy human and bovine donors was exposed to discrete combinations of shear stress using a Couette shearing system, prior to assessment of cellular deformability and mechanical sensitivity. Each sample was exposed to 25 sublethal shear stress combinations (ranging 60-100 Pa × 5-300 s). While bovine RBC exhibited decreased maximal elongation in the absence of conditioning shear, overall deformability at lower shears was ~1.8-fold greater than human. When exposed to any conditioning shear stresses >80 Pa (or 60-70 Pa beyond 5 s), human RBC were significantly rigidified, with greater magnitudes and prolonged exposure compounding this effect. Significantly larger shears were required to rigidify bovine RBC; the most extreme shear condition (100 Pa × 300 s) resulted in approximately three-times more rigidification of human RBC than bovine (137% and 47% respectively). Bovine RBC have superior resilience to mechanical stress when compared with human. Using bovine blood in ex vivo evaluation of rotary blood pumps may thus misrepresent and overestimate device-blood success, and may also have flow-on effects for eventual users. Fresh human blood during early-phase ex vivo testing is thus recommended, given shear-inducing blood pumps are designed for humans - not cattle.

Erythrocyte morphological symmetry analysis to detect sublethal trauma in shear flow

The viscoelastic properties of red blood cells (RBC) facilitate flexible shape change in response to extrinsic forces. Their viscoelasticity is intrinsically linked to physical properties of the cytosol, cytoskeleton, and membrane-all of which are highly sensitive to supraphysiological shear exposure. Given the need to minimise blood trauma within artificial organs, we observed RBC in supraphysiological shear through direct visualisation to gain understanding of processes leading to blood damage. Using a custom-built counter-rotating shear generator fit to a microscope, healthy red blood cells (RBC) were directly visualised during exposure to different levels of shear (10-60 Pa). To investigate RBC morphology in shear flow, we developed an image analysis method to quantify (a)symmetry of deforming ellipsoidal cells-following RBC identification and centroid detection, cell radius was determined for each angle around the circumference of the cell, and the resultant bimodal distribution (and thus RBC) was symmetrically compared. While traditional indices of RBC deformability (elongation index) remained unaltered in all shear conditions, following ~100 s of exposure to 60 Pa, the frequency of asymmetrical ellipses and RBC fragments/extracellular vesicles significantly increased. These findings indicate RBC structure is sensitive to shear history, where asymmetrical morphology may indicate sublethal blood damage in real-time shear flow.

Impact of small fractions of abnormal erythrocytes on blood rheology

Red blood cell (RBC) populations are inherently heterogeneous, given mature RBC lack the transcriptional machinery to re-synthesize proteins affected during in vivo aging. Clearance of older, less functional cells thus aids in maintaining consistent hemorheological properties. Scenarios occur, however, where portions of mechanically impaired RBC are re-introduced into blood (e.g., damaged from circulatory support, blood transfusion) and may alter whole blood fluid behavior. Given such perturbations are associated with poor clinical outcomes, determining the tolerable level of abnormal RBC in blood is valuable. Thus, the current study aimed to define the critical threshold of blood fluid properties to re-infused physically-impaired RBC. Cell mechanics of RBC were impaired through membrane cross-linking (glutaraldehyde) or intracellular oxidation (phenazine methosulfate). Mechanically impaired RBC were progressively re-introduced into the native cell population. Negative alterations of cellular deformability and high shear blood viscosity were observed following additions of only 1-5% rigidified RBC. Low-shear blood viscosity was conversely decreased following addition of glutaraldehyde-treated cells; high-resolution microscopy of these mixed cell populations revealed decreased capacity to form reversible aggregates and decreased aggregate size. Mixed RBC populations, when exposed to supraphysiological shear, presented with compounded mechanical impairment. Collectively, key determinants of blood flow behavior are sensitive to mechanical perturbations in RBC, even when only 1-5% of the cell population is affected. Given this fraction is well-below the volume of rigidified RBC introduced during circulatory support or transfusion practice, it is plausible that some adverse events following surgery and/or transfusion may be related to impaired blood fluidity.

Extracorporeal Membrane Oxygenation-Induced Hemolysis: An In Vitro Study to Appraise Causative Factors

In vitro hemolysis testing is commonly used to determine hemocompatibility of ExtraCorporeal Membrane Oxygenation (ECMO). However, poor reproducibility remains a challenging problem, due to several unidentified influencing factors. The present study investigated potential factors, such as flow rates, the use of anticoagulants, and gender of blood donors, which could play a role in hemolysis. Fresh human whole blood was anticoagulated with either citrate (n = 6) or heparin (n = 12; 6 female and 6 male blood donors). Blood was then circulated for 360 min at 4 L/min or 1.5 L/min. Regardless of flow rate conditions, hemolysis remained unchanged over time in citrated blood, but significantly increased after 240 min circulation in heparinized blood (p ≤ 0.01). The ratio of the normalized index of hemolysis (NIH) of heparinized blood to citrated blood was 11.7-fold higher at 4 L/min and 16.5–fold higher at 1.5 L/min. The difference in hemolysis between 1.5 L/min and 4 L/min concurred with findings of previous literature. In addition, the ratio of NIH of male heparinized blood to female was 1.7-fold higher at 4 L/min and 2.2-fold higher at 1.5 L/min. Our preliminary results suggested that the choice of anticoagulant and blood donor gender could be critical factors in hemolysis studies, and should be taken into account to improve testing reliability during ECMO.

Ex vivo assessment of erythrocyte tolerance to the HeartWare ventricular assist device operated in three discrete configurations

Despite technological advances in ventricular assist devices (VADs) to treat end-stage heart failure, hemocompatibility remains a constant concern, with supraphysiological shear stresses an unavoidable reality with clinical use. Given that impeller rotational speed is related to the instantaneous shear within the pump housing, it is plausible that the modulation of pump speed may regulate peak mechanical shear stresses and thus ameliorate blood damage. The present study investigated the hemocompatibility of the HeartWare HVAD in three configurations typical of clinical applications: standard systemic support left VAD (LVAD), pediatric support LVAD, and pulmonary support right VAD (RVAD) conditions. Two ex vivo mock circulation blood loops were constructed using explanted HVADs, in which pump speed and external loop resistance were manipulated to reflect the flow rates and differential pressures reported in configurations for standard adult LVAD (at 3150 rev⸱min^-1 ), pediatric LVAD (at 2400 rev⸱min^-1 ), and adult RVAD (at 1900 rev⸱min^-1 ). Using bovine blood, the mock circulation blood loops were tested at 37°C over a period of 6 hours (consistent with ASTM F1841-97) and compared with static control. Hemocompatibility assessments were conducted for each test condition, examining hematology, hemolysis (absolute and normalized index), osmotic fragility, and blood viscosity. Regardless of configuration, continuous exposure of blood to the VAD over the 6-hour period significantly altered hematological and rheological blood parameters, and induced increased hemolysis when compared with a static control sample. Comparison of the three operational VAD configurations identified that the adult LVAD condition-associated with the highest pump speed, flow rate, and differential pressure across the pump-resulted in increased normalized hemolysis index (NIH; 0.07) when compared with the lower pump speed "off-label" counterparts (NIH of 0.04 in pediatric LVAD and 0.01 in adult RVAD configurations). After normalizing blood residence times between configurations, pump speed was identified as the primary determinant of accumulated blood damage; plausibly, blood damage could be limited by restricting pump speed to the minimum required to support matched cardiac output, but not beyond.

Sublethal Supraphysiological Shear Stress Alters Erythrocyte Dynamics in Subsequent Low-Shear Flows

Blood is a non-Newtonian, shear-thinning fluid owing to the physical properties and behaviors of red blood cells (RBCs). Under increased shear flow, pre-existing clusters of cells disaggregate, orientate with flow, and deform. These essential processes enhance fluidity of blood, although accumulating evidence suggests that sublethal blood trauma-induced by supraphysiological shear exposure-paradoxically increases the deformability of RBCs when examined under low-shear conditions, despite obvious decrement of cellular deformation at moderate-to-higher shear stresses. Some propose that rather than actual enhancement of cell mechanics, these observations are "pseudoimprovements" and possibly reflect altered flow and/or cell orientation, leading to methodological artifacts, although direct evidence is lacking. This study thus sought to explore RBC mechanical responses in shear flow using purpose-built laser diffractometry in tandem with direct optical visualization to address this problem. Freshly collected RBCs were exposed to a mechanical stimulus known to drastically alter cell deformability (i.e., prior shear exposure (PSE) to 100 Pa × 300 s). Samples were subsequently transferred to a custom-built slit-flow chamber that combined laser diffractometry with direct cell visualization. Cell suspensions were sheared in a stepwise manner (between 0.3 and 5.0 Pa), with each step being maintained for 15 s. Deformability and cell orientation indices were recorded for small-scatter Fraunhofer diffraction patterns and also visualized RBCs. PSE RBCs had significantly decreased visualized and laser-derived deformability at any given shear stress ≥1 Pa. Novel, to our knowledge, observations demonstrated that PSE RBCs had increased heterogeneity of direct visualized orientation with flow vector at any shear, which may be due to greater vorticity and thus instability in 5-Pa flow compared with unsheared control. These findings indicate that shear exposure and stress-strain history can alter subsequent RBC behavior in physiologically relevant low-shear flows. These findings may yield insight into microvascular disorders in recipients of mechanical circulatory support and individuals with hematological diseases that alter physical properties of blood.

Mechanical sensitivity of red blood cells improves in individuals with hemochromatosis following venesection therapy

BACKGROUND

Individuals with hereditary hemochromatosis (HH) receive frequent blood withdrawals (ie, venesections) as part of their primary treatment to assist in normalizing blood iron levels. It remains unclear whether this source of blood is suitable for use in blood product development, as current data indicate that red blood cell (RBC) deformability, both before and after shear stress exposure, is impaired in individuals with HH, relative to healthy controls. Given that venesection therapy is known to significantly reduce circulating iron levels in individuals with HH, the current study examined whether venesection therapy is effective at improving RBC mechanical properties, both before and after shear stress exposure, in individuals with HH.

STUDY DESIGN AND METHODS

Blood samples were initially collected from untreated HH patients (age, 61 ± 9 years; 14% female) undergoing their first venesection, and then again during their second (approx. 9 weeks later) and third (approx. 16 weeks later) venesections. RBC deformability was measured at each time point with a commercial ektacytometer. Moreover, to determine cell responses to mechanical stimuli, the mechanical sensitivity of blood samples was determined at each time point.

RESULTS

The salient findings indicate that venesection therapy used for managing plasma ferritin concentration significantly improves the cellular deformability of RBC in individuals with HH. Further, the sensitivity of RBC to supraphysiological mechanical stress is decreased (ie, improved) in a dose-response fashion with routine venesection.

CONCLUSION

While cellular mechanics of RBC from individuals with HH are impaired when untreated, venesection therapy significantly improves cellular properties of RBC, supporting the use of venesections in blood product development from individuals with well-managed HH.

Calcium dynamically alters erythrocyte mechanical response to shear

Red blood cells (RBC) are constantly exposed to varying mechanical forces while traversing the cardiovascular system. Upon exposure to mechanical stimuli (e.g., shear stress), calcium enters the cell and prompts potassium-efflux. Efflux of potassium is accompanied by a loss of intracellular fluid; thus, the volume of RBC decreases proportionately (i.e., 'Gárdos effect'). The mechanical properties of the cell are subsequently impacted due to complex interactions between cytosolic viscosity (dependent on cell hydration), the surface-area-to-volume ratio, and other molecular processes. The dynamic effects of calcium on RBC mechanics are yet to be elucidated, although accumulating evidence suggests a vital role. The present study thus examined the effects of calcium on contemporary biomechanical properties of RBC in conjunction with high-precision geometrical analyses with exposure to shear. Mechanical stimulation of RBC was performed using a co-axial Couette shearing system to deform the cell membrane; intracellular signaling events were observed via fluorescent imaging. Calcium was introduced into RBC using ionophore A23187. Increased intracellular calcium significantly impaired RBC deformability; these impairments were mediated by a calcium-induced reduction of cell volume through the Gárdos channel. Extracellular calcium in the absence of the ionophore only had an effect under shear, not at stasis. Under low shear, the presence of extracellular calcium induced progressive lysis of a sub-population of RBC; all remaining RBC exhibited exceptional capacity to deform, implying preferential removal of potentially aged cells. Collectively, we provide evidence of the mechanism by which calcium acutely regulates RBC mechanical properties.

Mixed hemodiafiltration reduces erythropoiesis stimulating agents requirement in dialysis patients: a prospective randomized study

BACKGROUND

Improved responsiveness to erythropoiesis stimulating agents (ESAs) in patients on on-line post-dilution hemodiafiltration (Post-HDF) compared with conventional hemodialysis (HD) was reported by some authors but challenged by others. This prospective, cross-over randomized study tested the hypothesis that an alternative infusion modality of HDF, mixed-dilution HDF (Mixed HDF), could further reduce ESAs requirement in dialysis patients compared to the traditional Post-HDF.

METHODS

One-hundred-twenty prevalent patients from 6 Dialysis Centers were randomly assigned to two six-months treatment sequences: A-B and B-A (A, Mixed HDF; B, Post-HDF). Primary outcome was comparative evaluation of ESA (darbepoetin alfa) requirement and ESA resistance. Treatments efficiency, iron and vitamins status, inflammation and nutrition parameters were monitored.

RESULTS

In sequence A, darbepoetin requirement decreased during Mixed HDF from 29.5 to 23.7 µg/month and increased significantly during Post-HDF (32.3 µg/month at 6th month) while, in sequence B, it increased during Post-HDF from 38.2 to 43.7 µg/month and decreased during Mixed HDF (23.9 µg/month at 6th month). Overall, EPO doses at 6 months on Mixed and Post-HDF were 23.8 and 38.4 µg/month, respectively, P < 0.01. A multiple linear model confirmed that Mixed HDF vs Post-HDF reduced significantly ESA requirement and ESA resistance (P < 0.0001), by a mean of 29% (CI 23-35%) in the last three months of the observation periods.

CONCLUSIONS

Mixed HDF decreased darbepoetin-alfa requirement in dialysis patients. This might help preventing the untoward side effects of high ESA doses, besides having a remarkable economic impact. Additional evidence is needed to confirm this potential benefit of Mixed-HDF.

The effects of stenting on hemorheological parameters: An in vitro investigation under various blood flow conditions

 Despite their wide clinical usage, stent functionality may be compromised by complications at the site of implantation, including early/late stent thrombosis and occlusion. Although several studies have described the effect of fluid-structure interaction on local haemodynamics, there is yet limited information on the effect of the stent presence on specific hemorheological parameters. The current work investigates the red blood cell (RBC) mechanical behavior and physiological changes as a result of flow through stented vessels. Blood samples from healthy volunteers were prepared as RBC suspensions in plasma and in phosphate buffer saline at 45% haematocrit. Self-expanding nitinol stents were inserted in clear perfluoroalkoxy alkane tubing which was connected to a syringe, and integrated in a syringe pump. The samples were tested at flow rates of 17.5, 35 and 70 ml/min, and control tests were performed in non-stented vessels. For each flow rate, the sample viscosity, RBC aggregation and deformability, and RBC lysis were estimated. The results indicate that the presence of a stent in a vessel has an influence on the hemorheological characteristics of blood. The viscosity of all samples increases slightly with the increase of the flow rate and exposure. RBC aggregation and elongation index (EI) decrease as the flow rate and exposure increases. RBC lysis for the extreme cases is evident. The results indicate that the stresses developed in the stent area for the extreme conditions could be sufficiently high to influence the integrity of the RBC membrane.

Erythrocyte Aging, Protection via Vesiculation: An Analysis Methodology via Oscillatory Flow

We demonstrate that erythrocyte deformations, specifically of a type as occur in splenic flow (Zhu et al., 2017), and of the type that promote vesiculation can be caused by simple, yet tailored, oscillatory shear flow. We show that such oscillatory shear flow provides an ideal environment to explore a wide variety of metabolic and biochemical effects that promote erythrocyte vesiculation. Deformation details, typical of splenic flow, such as in-folding and implications for membrane/skeleton interaction are demonstrated and quantitatively analyzed. We introduce a theoretical, essentially analytical, vesiculation model that directly couples to our more complex numerical, multilevel, model that clearly delineates various fundamental elements, i.e., sub-processes, that are involved and mediate the vesiculation process. This analytical model highlights particulary important vesiculation precursors such as areas of membrane/skeleton disruptions that trigger the vesiculation process. We demonstrate, using flow cytometry, that the deformations we experimentally induce on cells, and numerically simulate, do not induce lethal forms of cell damage but do induce vesiculation as theoretically forecasted. This, we demonstrate, provides a direct link to cell membrane/skeletal damage such as is associated with metabolic and aging damage. An additional noteworthy feature of this approach is the avoidance of artificial devices, e.g., micro-fluidic chambers, in which deformations and their time scales are often unrepresentative of physiological processes such as splenic flow.

Susceptibility of density-fractionated erythrocytes to subhaemolytic mechanical shear stress

INTRODUCTION:

Accumulating evidence demonstrates that subhaemolytic mechanical stresses, typical of circulatory support, induce physical and biochemical changes to red blood cells. It remains unclear, however, whether cell age affects susceptibility to these mechanical forces. This study thus examined the sensitivity of density-fractionated red blood cells to sublethal mechanical stresses.

METHODS:

Red blood cells were isolated and washed twice, with the least and most dense fractions being obtained following centrifugation (1500 g × 5 min). Red blood cell deformability was determined across an osmotic gradient and a range of shear stresses (0.3-50 Pa). Cell deformability was also quantified before and after 300 s exposure to shear stresses known to decrease (64 Pa) or increase (10 Pa) red blood cell deformability. The time course of accumulated sublethal damage that occurred during exposure to 64 Pa was also examined.

RESULTS:

Dense red blood cells exhibited decreased capacity to deform when compared with less dense cells. Cellular response to mechanical stimuli was similar in trend for all red blood cells, independent of density; however, the magnitude of impairment in cell deformability was exacerbated in dense cells. Moreover, the rate of impairment in cellular deformability, induced by 64 Pa, was more rapid for dense cells. Relative improvement in red blood cell deformability, due to low-shear conditioning (10 Pa), was consistent for both cell populations.

CONCLUSION:

Red blood cell populations respond differently to mechanical stimuli: older (more dense) cells are highly susceptible to sublethal mechanical trauma, while cell age (density) does not appear to alter the magnitude of improved cell deformability following low-shear conditioning.

Experimental measurement and numerical modelling of dye washout for investigation of blood residence time in ventricular assist devices

Ventricular assist devices have become the standard therapy for end-stage heart failure. However, their use is still associated with severe adverse events related to the damage done to the blood by fluid dynamic stresses. This damage relates to both the stress magnitude and the length of time the blood is exposed to that stress. We created a dye washout technique which combines experimental and numerical approaches to measure the washout times of ventricular assist devices. The technique was used to investigate washout characteristics of three commercially available and clinically used ventricular assist devices: the CentriMag, HVAD and HeartMate II. The time taken to reach 5% dye concentration at the outlet (T05) was used as an indicator of the total residence time. At a typical level of cardiac support, 5 L/min and 100 mmHg, T05 was 0.93, 0.28 and 0.16 s for CentriMag, HVAD and HeartMate II, respectively, and increased to 5.06, 1.64 and 0.96 s for reduced cardiac support of 1 L/min. Regional variations in washout characteristics are described in this article. While the volume of the flow domain plays a large role in the differences in T05 between the ventricular assist devices, after standardising for ventricular assist device volume, the secondary flow path was found to increase T05 by 35%. The results explain quantitatively, for the first time, why the CentriMag, which exerts low shear stress magnitude, has still been found to cause acquired von Willebrand Syndrome in patients.