Yield stress of normal human blood as a function of endogenous fibrinogen

Impact of anti-coagulant choice on blood elongational behavior

Blood is a highly complex fluid with rheological properties that have a significant impact on various flow phenomena. In particular, it exhibits a non-Newtonian elongational viscosity that is comparable to polymer solutions. In this study, we investigate the effect of three different anticoagulants, namely EDTA (ethylene diamine tetraacetic acid), heparin, and citrate, on the elongational properties of both human and swine blood. We observe a unique two stage thinning process and a strong dependency of the characteristic relaxation time on the chosen anticoagulant, with the longest relaxation time and thus the highest elongational viscosity being found for the case of citrate. Our findings for the latter are consistent with the physiological values obtained from a dripping droplet of human blood without any anticoagulant. Furthermore, our study resolves the discrepancy found in the literature regarding the reported range of characteristic relaxation times, confirming that the elongational viscosity must be taken into account for a full rheological characterization of blood. These results have important implications for understanding blood flow in various physiological, pathological and technological conditions.

Thrombotic Risk and Calculated Whole Blood Viscosity in a Cohort of Patients With New Diagnosis of Multiple Myeloma

The pathogenesis of venous thromboembolism in multiple myeloma is still poorly understood because multiple factors are involved. In particular, the increase in whole blood viscosity has a key role and, therefore, we performed an evaluation of some hemorheological determinants in multiple myeloma patients, putting them in relation to the thrombotic risk, with the aim to evaluate if an alteration of the hemorheological pattern was associated with a higher thrombotic risk. We performed an observational retrospective cohort study with data collected from January 2017 to September 2022. In a group of 190 patients with newly diagnosed multiple myeloma, we have examined the trend of calculated blood viscosity according to the Merrill formula, and we stratified the patients for the thrombotic risk in accordance with the IMWG/NCCN guidelines and with IMPEDE VTE score. Using the thrombotic risk stratification proposed by IMWG/NCCN any variation in calculated blood viscosity is evident, while, with the IMPEDE VTE score, we observed an increase in calculated blood viscosity in patients with "intermediate + high" risk. The calculated blood viscosity is higher in subjects presenting an "intermediate + high" thrombotic risk according to the IMPEDE VTE score. This association could therefore lay the groundwork for further research with the aim to confirm the role of hemorheological pattern in MM-related thrombotic risk.

Calculated Whole Blood Viscosity and Albumin/Fibrinogen Ratio in Patients with a New Diagnosis of Multiple Myeloma: Relationships with Some Prognostic Predictors

BACKGROUND

In this single center study, we retrospectively evaluated the calculated hemorheological profile in patients with a new diagnosis of multiple myeloma, with the aim to evaluate possible relationships with some prognostic predictors, such as ISS, albumin levels, beta2-microglobulin, red cell distribution width, and bone marrow plasma cell infiltration.

METHODS

In a cohort of 190 patients, we examined the calculated blood viscosity using the de Simone formula, and the albumin/fibrinogen ratio as a surrogate of erythrocyte aggregation, and then we related these parameters to prognostic factors, using the Kruskal-Wallis and the Mann-Whitney tests, respectively.

RESULTS

From our analysis, it emerged that the evaluated hemorheological pattern differed in the three isotypes of multiple myeloma, and the whole blood viscosity was higher in IgA and IgG isotypes with respect to the light chain multiple myeloma (p < 0.001). Moreover, we observed that, as the ISS stage progressed, the albumin/fibrinogen ratio was reduced, and the same hemorheological trend was traced in subgroups with lower albumin levels, higher beta2-microglobulin and red cell distribution width RDW values, and in the presence of a greater bone marrow plasma cell infiltrate.

CONCLUSIONS

Through the changes in blood viscosity in relation to different prognostic factors, this analysis might underline the role of the hemorheological pattern in multiple myeloma.

The role of plasma in the yield stress of blood

BACKGROUND

Yielding and shear elasticity of blood are merely discussed within the context of hematocrit and erythrocyte aggregation. However, plasma might play a substantial role due its own viscoelasticity.

OBJECTIVE

If only erythrocyte aggregation and hematocrit would determine yielding, blood of different species with comparable values would present comparable yield stresses.

METHODS

rheometry (SAOS: amplitude and frequency sweep tests; flow curves) of hematocrit-matched samples at 37°C. Brillouin Light Scattering Spectroscopy at 38°C.

RESULTS

Yield stress for pig: 20mPa, rat: 18mPa, and human blood: 9mPa. Cow and sheep blood were not in quasi-stationary state supporting the role of erythrocyte aggregation for the development of elasticity and yielding. However, pig and human erythrocytes feature similar aggregability, but yield stress of porcine blood was double. Murine and ruminant erythrocytes both rarely aggregate, but their blood behavior was fundamentally different. Pig plasma was shear-thinning and murine plasma was platelet-enriched, supporting the role of plasma for triggering collective effects and gel-like properties.

CONCLUSIONS

Blood behavior near zero shear flow is not based solely on erythrocyte aggregation and hematocrit, but includes the hydrodynamic interaction with plasma. The shear stress required to break down elasticity is not the critical shear stress for dispersing erythrocyte aggregates, but the shear stress required to fracture the entire assembly of blood cells within their intimate embedding.

Numerical investigation of different viscosity models on pulsatile blood flow of thoracic aortic aneurysm (TAA) in a patient-specific model

Aortic aneurysm is one of the most common aortic diseases that can lead to unfortunate consequences. Numerical simulations have an important role in the prediction of the aftereffects of vascular diseases including aneurysm. In this research, numerical simulation of pulsatile blood flow is performed for a 3-dimensional patient-specific model of a thoracic aortic aneurysm (TAA). Since the choice of blood viscosity model may have a significant impact on the simulation results, the effects of four non-Newtonian models of blood viscosity namely Carreau, Casson, Herschel-Bulkley, power low, and the Newtonian model on the wall shear stress (WSS) distribution, shear rate, and oscillatory shear index (OSI) have been analyzed. Simulation results showed that all the non-Newtonian and Newtonian models generally, predict similar patterns for blood flow and shear rate. At high flow rates in the cardiac cycle, the WSS value for all the models are similar to each other except for the power-law model due to the shear thinning behavior. All models predict high values of OSI on the inner wall of the ascending aorta and broad areas of the inner wall of the aneurysm sac. However, the Newtonian model predicts the OSI less than the non-Newtonian models in some areas of the aneurysm sac. Results indicated that the Newtonian model generally can predict the hemodynamic parameters of the blood flow similar to the non-Newtonian but for more precise analysis and to predict the regions prone to rupture and atherosclerosis, choosing a proper non-Newtonian model is recommended.

Recent advances in blood rheology: a review

Due to the potential impact on the diagnosis and treatment of various cardiovascular diseases, work on the rheology of blood has significantly expanded in the last decade, both experimentally and theoretically. Experimentally, blood has been confirmed to demonstrate a variety of non-Newtonian rheological characteristics, including pseudoplasticity, viscoelasticity, and thixotropy. New rheological experiments and the development of more controlled experimental protocols on more extensive, broadly physiologically characterized, human blood samples demonstrate the sensitivity of aspects of hemorheology to several physiological factors. For example, at high shear rates the red blood cells elastically deform, imparting viscoelasticity, while at low shear rates, they form "rouleaux" structures that impart additional, thixotropic behavior. In addition to the advances in experimental methods and validated data sets, significant advances have also been made in both microscopic simulations and macroscopic, continuum, modeling, as well as novel, multiscale approaches. We outline and evaluate the most promising of these recent developments. Although we primarily focus on human blood rheology, we also discuss recent observations on variations observed across some animal species that provide some indication on evolutionary effects.

Comparison between whole blood viscosity measured and calculated in subjects with monoclonal gammopathy of undetermined significance and in patients with multiple myeloma: Re-evaluation of our survey

BACKGROUND

in this study, with a re-evaluation of the hemorheological determinants previously described in MGUS subjects and in MM patients, we have detected the calculated whole blood viscosity, according whether to the hematocrit and total plasma protein concentration (de Simone formula) or to the haematocrit and plasma fibrinogen level (Merrill formula), and a marker of the erythrocyte aggregation (albumin/fibrinogen level).

METHODS

data were expressed as means±standard deviation. Student's t test for unpaired data was used to compare MGUS subjects and MM patients. The correlation coefficient between mean erythrocyte aggregation (MEA) and hematocrit (Ht) was evaluated in MGUS, MM and MGUS + MM groups using the Spearman test.

RESULTS

the comparison between MGUS and MM shows that the measured blood viscosity and calculated blood viscosity based on hematocrit and total plasma protein, but not which estimated in relation to the hematocrit and plasma fibrinogen, differentiate the two groups. A difference between the two groups also regards the measured erythrocyte aggregation and its surrogate marker. In addition, the measured plasma viscosity at low shear rate (0.51 s-1) and, in particular, the ratio between plasma viscosity at low (0.51 s-1) and high (450 s-1) shear rates distinguish MGUS and MM.

CONCLUSIONS

calculated blood viscosity (de Simone formula and other formulas) and the surrogate marker of erythrocyte aggregation disclose an alike trend with the corresponding hemorheological determinants obtained by using their direct measurement.

Advanced Constitutive Modeling of the Thixotropic Elasto-Visco-Plastic Behavior of Blood: Steady-State Blood Flow in Microtubes

The present work focuses on the in-silico investigation of the steady-state blood flow in straight microtubes, incorporating advanced constitutive modeling for human blood and blood plasma. The blood constitutive model accounts for the interplay between thixotropy and elasto-visco-plasticity via a scalar variable that describes the level of the local blood structure at any instance. The constitutive model is enhanced by the non-Newtonian modeling of the plasma phase, which features bulk viscoelasticity. Incorporating microcirculation phenomena such as the cell-free layer (CFL) formation or the Fåhraeus and the Fåhraeus-Lindqvist effects is an indispensable part of the blood flow investigation. The coupling between them and the momentum balance is achieved through correlations based on experimental observations. Notably, we propose a new simplified form for the dependence of the apparent viscosity on the hematocrit that predicts the CFL thickness correctly. Our investigation focuses on the impact of the microtube diameter and the pressure-gradient on velocity profiles, normal and shear viscoelastic stresses, and thixotropic properties. We demonstrate the microstructural configuration of blood in steady-state conditions, revealing that blood is highly aggregated in narrow tubes, promoting a flat velocity profile. Additionally, the proper accounting of the CFL thickness shows that for narrow microtubes, the reduction of discharged hematocrit is significant, which in some cases is up to 70%. At high pressure-gradients, the plasmatic proteins in both regions are extended in the flow direction, developing large axial normal stresses, which are more significant in the core region. We also provide normal stress predictions at both the blood/plasma interface (INS) and the tube wall (WNS), which are difficult to measure experimentally. Both decrease with the tube radius; however, they exhibit significant differences in magnitude and type of variation. INS varies linearly from 4.5 to 2 Pa, while WNS exhibits an exponential decrease taking values from 50 mPa to zero.

Advanced Constitutive Modeling of the Thixotropic Elasto-Visco-Plastic Behavior of Blood: Description of the Model and Rheological Predictions

This work focuses on the advanced modeling of the thixotropic nature of blood, coupled with an elasto-visco-plastic formulation by invoking a consistent and validated model for TEVP materials. The proposed model has been verified for the adequate description of the rheological behavior of suspensions, introducing a scalar variable that describes dynamically the level of internal microstructure of rouleaux at any instance, capturing accurately the aggregation and disaggregation mechanisms of the RBCs. Also, a non-linear fitting is adopted for the definition of the model's parameters on limited available experimental data of steady and transient rheometric flows of blood samples. We present the predictability of the new model in various steady and transient rheometric flows, including startup shear, rectangular shear steps, shear cessation, triangular shear steps and LAOS tests. Our model provides predictions for the elasto-thixotropic mechanism in startup shear flows, demonstrating a non-monotonic relationship of the thixotropic index on the shear-rate. The intermittent shear step test reveals the dynamics of the structural reconstruction, which in turn is associated with the aggregation process. Moreover, our model offers robust predictions for less examined tests such as uniaxial elongation, in which normal stress was found to have considerable contribution. Apart from the integrated modeling of blood rheological complexity, our implementation is adequate for multi-dimensional simulations due to its tensorial formalism accomplished with a single time scale for the thixotropic effects, resulting in a low computational cost compared to other TEVP models.

Absence of association between whole blood viscosity and delirium after cardiac surgery: a case-controlled study

Background

Delirium after cardiothoracic surgery is common and associated with impaired outcomes. Although several mechanisms have been proposed (including changes in cerebral perfusion), the pathophysiology of postoperative delirium remains unclear. Blood viscosity is related to cerebral perfusion and thereby might contribute to the development of delirium after cardiothoracic surgery. The aim of this study was to investigate whether whole blood viscosity differs between cardiothoracic surgery patients with and without delirium.

Methods

In this observational study postoperative whole blood viscosity of patients that developed delirium (cases) were compared with non-delirious cardiothoracic surgery patients (controls). Cases were matched with the controls, yielding a 1:4 case–control study. Serial hematocrit, fibrinogen, and whole blood viscosity were determined pre-operatively and at each postoperative day. Delirium was assessed using the validated Confusion Assessment Method for the Intensive Care Unit or Delirium Screening Observation scale.

Results

In total 80 cardiothoracic surgery patients were screened of whom 12 delirious and 48 matched non-delirious patients were included. No significant difference was found between both groups in fibrinogen (p = 0.36), hematocrit (p = 0.23) and the area under curve of the whole blood viscosity between shear rates 0.02 and 50 s^-1 (p = 0.80) or between shear rates 0.02 and 5 s^-1 (p = 0.78).

Conclusion

In this case control study in cardiothoracic surgery patients changes in whole blood viscosity were not associated with the development of delirium.

Role of red blood cell flow behavior in hemodynamics and hemostasis

The primary role of red blood cells (RBCs) is to transport oxygen to the tissues, which is performed predominantly in the blood capillaries. However, RBCs have unique flow-affecting properties that play a key role in blood flow in all blood vessel types and sizes. While RBCs as oxygen carriers have been studied extensively, their hemodynamic function has been examined less comprehensively. This review aims to bridge this gap, focusing on the role of RBC flow properties in hemodynamics, hemostasis and thrombosis.

Blood viscosity, lipid profile, and lipid peroxidation in type-1 diabetic patients with good and poor glycemic control

Background:

In diabetic patients, persistent hyperglycemia and poor glycemic control cause disturbances of lipid profiles, especially an increased production of oxygen free radicals. Lipid peroxidation has been considered to be a pathogenic factor of diabetic complications in Type-1 Diabetes mellitus.

Aims:

The aim of the present study was to investigate the effect of glycemic control on blood viscosity, lipid profile, and lipid peroxidation in Type-1 Diabetic subjects.

Materials and Methods:

The study included three groups; Group-I (age-matched healthy control subjects, n = 50), Group-II (Type-1 Diabetics with good glycemic control, n = 10), and Group-III (Type-1 Diabetics with poor glycemic control, n = 15). The Type 1 diabetic patients with duration of diabetes for more than 5 years were taken. Blood samples of all subjects were analyzed for all biochemical, hematological, and oxidative stress parameters.

Results:

The Erythrocyte malondialdehyde level was non-significantly changed (P = NS) in group–II patients but significantly increased (P < 0.001) in group-III patients, and no significant changes were found (P = NS) in Blood viscosity of both the groups (group-II and group-III), as compared to healthy control subjects (group-I).

Conclusions:

Our findings suggest that the monitoring of Oxidative stress and Blood Viscosity in poorly controlled Type-1 diabetic patients may be very useful marker of diabetic complications.

Sex differences and hemoglobin levels in relation to stroke outcomes

OBJECTIVE

Women have worse outcomes after stroke compared to men. Since women have lower hemoglobin values, we examined whether hemoglobin levels may associate with worse stroke outcomes in women.

METHODS

We retrospectively studied 274 patients enrolled in a prospective multicenter study. We explored the relationship of hemoglobin with clinical outcome at 6 months, as measured by the modified Rankin Scale (mRS). Ordinal logistic regression was used to evaluate the independent effect of hemoglobin on clinical outcome, and to explore the influence of sex on that association.

RESULTS

Women had a lower mean hemoglobin level (11.7 ± 1.8 g/dL) compared to men (13.3 ± 1.7 g/dL). Low hemoglobin was associated with worse 6-month mRS outcomes in univariate analysis (p < 0.001). Lower hemoglobin remained independently associated with poor outcome after adjustment for comorbid disease, stroke severity, age, and sex. The inclusion of hemoglobin in the model attenuated the independent effect of sex on outcome.

CONCLUSIONS

Sex differences in stroke outcome are linked to lower hemoglobin level, which is more prevalent in women. Further examination of this potentially modifiable predictor is warranted.

Hemorheology and microvascular disorders

The present review presents basic concepts of blood rheology related to vascular diseases. Blood flow in large arteries is dominated by inertial forces exhibited at high flow velocities, while viscous forces (i.e., blood rheology) play an almost negligible role. When high flow velocity is compromised by sudden deceleration as at a bifurcation, endothelial cell dysfunction can occur along the outer wall of the bifurcation, initiating inflammatory gene expression and, through mechanotransduction, the cascade of events associated with atherosclerosis. In sharp contrast, the flow of blood in microvessels is dominated by viscous shear forces since the inertial forces are negligible due to low flow velocities. Shear stress is a critical parameter in microvascular flow, and a force-balance approach is proposed for determining microvascular shear stress, accounting for the low Reynolds numbers and the dominance of viscous forces over inertial forces. Accordingly, when the attractive forces between erythrocytes (represented by the yield stress of blood) are greater than the shear force produced by microvascular flow, tissue perfusion itself cannot be sustained, leading to capillary loss. The yield stress parameter is presented as a diagnostic candidate for future clinical research, specifically, as a fluid dynamic biomarker for microvascular disorders. The relation between the yield stress and diastolic blood viscosity (DBV) is described using the Casson model for viscosity, from which one may be able determine thresholds of DBV where the risk of microvascular disorders is high.

Blood viscosity in horses with colic

Whole blood viscosity (WBV) was measured on six healthy horses and ten horses presented for colic surgery. A Wells-Brookfield cone-and-plate microviscometer at 6 rpm was used to determine WBV and WBV was adjusted for packed cell volume (PCV). The ten colic horses were divided into two groups: five horses that died or were euthanatized (group 1) and five horses that were discharged after surgery and medical therapy (group 2). The mean WBV for each group was compared using analysis of variance followed by Duncan multiple-range tests. Mean WBV of group 2 (5.81 +/- 1.48 centipoise [cp]) and normal horses (7.1 +/- 2.3 cp) was significantly (P less than 0.01) lower than group 1 (17.15 +/- 5.71 cp). This preliminary trial shows that horses that died or were euthanatized had a significantly higher WBV than horses surviving. Since viscosity was adjusted for PCV, the differences may represent changes in other factors such as fibrinogen, rouleaux formation, red blood cell deformability, PaO2, or intracellular calcium ion concentration. Thus, increases in WBV in horses with colic may indicate severely compromised regional perfusion that may lead to significant tissue damage. In this preliminary study, it appears that WBV may be a prognostic aid in horses with colic.

Evaluation of the yield stress of normal blood as a function of fibrinogen concentration and hematocrit

The yield stress is a sensitive index of blood fluidity at low shear. Seven healthy adults were studied at hematocrits varying between 40 and 80% and fibrinogen concentrations from 0.0 to 0.935 g/dl. Multivariable analysis was used to determine the functional dependence of yield stress on hematocrit and fibrinogen level. The major findings from this analysis include a decreasing effect of fibrinogen at high concentrations (saturation effect), a relative insensitivity of yield stress to fibrinogen at low concentration (threshold effect), and a strong interaction between the effects of hematocrit and fibrinogen concentration on yield stress. Our results give the normal range of yield stress for a given value of fibrinogen and hematocrit and can be used to predict the effect of reductions in hematocrit or fibrinogen on the yield stress of normal blood.

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.

A new method for measuring the yield stress in thin layers of sedimenting blood

A new method is presented to describe the low shear rate behavior of blood. We observed the response of a thin layer of sedimenting blood to a graded shear stress in a wedge-shaped chamber. The method allows quantitation of the degree of phase separation between red cells and plasma, and extracts the yield stress of the cell phase as a function of hematocrit. Our studies showed that the behavior of normal human blood underwent a transition from a solid-like gel to a Casson fluid. This transition began at the Casson predicted yield stress. The viscoelastic properties of blood were examined at shear stresses below the yield stress. The measured Young's elastic moduli were in good agreement with published data. The yield stress of blood showed a linear dependence on hematocrit up to 60%, and increased more rapidly at higher hematocrit.

The role of plasma hyperviscosity in subcortical arteriosclerotic encephalopathy (Binswanger's disease)

Five haemorheological variables (haematocrit, plasma viscosity, red cell aggregation, red cell deformability and yield shear stress) and the plasma fibrinogen concentration were determined in blood samples from 21 untreated patients with subcortical arteriosclerotic encephalopathy (SAE, Binswanger's disease). The results were compared with those obtained in blood samples from 40 untreated patients with lacunar infarcts and from 275 healthy control subjects without vascular risk factors. Pathological ("solid body") flow behaviour was detected in the blood of both groups of patients (SAE and lacunar infarcts). However, highly elevated plasma viscosity was a consistent feature only of SAE. The authors present the hypothesis that the high plasma viscosity in patients with SAE may account for the progressive degeneration of cerebral white matter. It is postulated that microcirculatory abnormalities due to microrheological changes may be one of the many missing links in the pathophysiology of SAE.

The haemorheological features of lacunar strokes

Clinical and haemorheological data were recorded in 40 patients with lacunar strokes confirmed clinically and by computed tomography. The following haemorheological variables were monitored: haematocrit, erythrocyte aggregation, erythrocyte deformability, plasma viscosity, fibrinogen concentration and yield shear stress. Clinically, most patients had case histories and features according to the description of Fisher. All haemorheological parameters with the exception of the haematocrit were pathological when compared with values obtained from a normal control group. In descending order of frequency the pathological changes were in erythrocyte aggregation, plasma viscosity and erythrocyte deformability.

Fibrinogen, blood viscosity, and cerebral ischemia

This study examines the effect of fibrinogen and consequent blood viscosity reduction on cerebral blood flow and cellular injury following severe cerebral ischemia for 30 minutes in 78 Wistar rats. In half of these rats 10 to 15 cc's of blood was removed and replaced with a mixture of 5% albumin and autologous red blood cells maintaining a constant hematocrit but resulting in a 30% decrease in fibrinogen and corresponding reduction in viscosity. Fibrinogen reduction in a slight increase in baseline CBF and the elimination of post-ischemic hyperemia at 24 hours. Both study and control animals showed a similar decrease in CBF at 30 minutes and 2 hours. There was no significant difference in the severity of ischemic cellular change between the fibrinogen reduction group and controls, although there was a significant inverse relationship between the amount of viscosity change and severity of cellular injury within the treatment group. Fibrinogen reduction alone cannot significantly ameliorate ischemic injury in this model. Viscosity reduction therapy should include reduction of hematocrit and alteration of red cell deformability.

Comparative evaluation of blood viscosity in diabetic retinopathy

The viscosity of the whole blood, plasma and serum, haematocrit and plasma fibrinogen were studied in diabetic patients with (DR) and without (D) retinopathy and in non-diabetic control subjects (C). Blood viscosity was significantly higher in diabetics than in controls. No significant differences in viscosity of the whole blood were found when various types of retinopathy were compared according to the severity of retinal damage. Plasma viscosity was significantly higher (P less than 0.01) than controls (C) only in diabetic patients with retinopathy (DR). Serum viscosity was significantly increased compared with controls (C) only in diabetic patients affected by proliferative retinopathy. Plasma fibrinogen was significantly higher than controls (C) both in diabetics with retinopathy (DR) and without retinopathy (D). Haematocrit did not show a significant difference in the three groups considered (C, D, DR).

Plasma fibrinogen levels and the clinical course of acute myocardial infarction

Plasma fibrinogen levels were followed from admission to the coronary care unit up to 3 months after an acute myocardial infarction (AMI) (51 patients at entry--49 patients at follow-up). Mean initial value as determined with 24 hours of onset of symptoms was 522 mg% +/- 205 S.D. Peak value 845 mg% +/- 215 S.D. was reached 3-5 days after onset of symptoms. Mean fibrinogen level on a later follow-up was similar to the initial value (521 mg% +/- 145 S.D.). Our data confirm that a rise in the plasma fibrinogen level is common after myocardial necrosis. No correlation was found between the maximum peak value and the clinical course of the disease. However, death, reinfarction and cardiogenic shock were more frequent among patients with an initial plasma fibrinogen level higher than 400 mg%. Our findings suggest that the acute increase in fibrinogen following necrosis is less significant to the clinical course than a chronically high plasma fibrinogen level.

Whole blood viscosity parameters and cerebral blood flow

This report describes the statistical relationship of several whole blood viscosity parameters and cerebral blood flow (CBF) in 53 consecutive patients and normal controls. Significant correlations were present between CBF and serum fibrinogen (P = .05), hematocrit (P less than .05), and a relationship involving both fibrinogen and hematocrit (P less than .01). We conclude that heightened whole blood viscosity does correlate with decreased cerebral blood flow in the ranges measured in our patients, that both fibrinogen and hematocrit must be taken into consideration in viscosity determinations, and that changes in viscosity may have an important effect on CBF in regions of low flow.

Kinetics of rouleau formation. I. A mass action approach with geometric features

In the presence of certain macromolecules, such as fibrinogen, immunoglobulin, dextran, and polylysine, erythrocytes tend to aggregate and form cylindrical clusters called "rouleaux" in which cells resemble coins in a stack. The aggregates may remain cylindrical or they may branch, forming tree, and networklike structures. Using the law of mass action and notions from polymer chemistry, we derive expressions describing the kinetics of the early phase of aggregation. Our models generalize work initiated by Ponder in 1927 who used the Smoluchowski equation to predict the concentration of rouleaux of different sizes. There are two novel features to our generalization. First, we allow erythrocytes that collide near the end of a stack of cells to move to the end of the cylinder and elongate it. Second, we incorporate geometric information into our models and describe the kinetics of branched rouleau formation. From our models we can predict the concentration of rouleaux with n cells and b branches, the mean number of cells per rouleau, the mean number of branches per rouleau, and the average length of a branch. Comparisons are made with the available experimental data.

Changes in left ventricular function after intracoronary streptokinase infusion in clinically evolving myocardial infarction

Left ventricular (LV) contrast cineventriculograms were obrained in 174 patients with evolving acute myocardial infarction (AMI) treated by intracoronary streptokinase (SK) infusion. Ejection fraction (EF) increased slightly, from 52% +/- 13% before admission SK intervention to 58% +/- 13% immediately after early recanalization of the completely obstructed coronary vessel (p less than 0.0005, n = 68), while local wall motion usually improved. Immediately improved LV function appears to be the result of early reperfusion and/or decreased LV afterload. In 55 chronic (long-term follow-up) post-AMI (2 to 4 weeks after successful early SK reperfusion) patients, repeat angiography revealed late reocclusion of the infarct vessel in nine patients. Chronic EF was not significantly different from pre-SK intervention EF in these nine patients with late reocclusion, whereas in the remaining 46 patients with persistent patency, EF increased mildly from 52% +/- 13% (before SK intervention) to 56% +/- 16% (in long-term recanalization) (p less than 0.025). Ventricular loading conditions were not different at pre-SK intervention angiography and chronic angiography. EF rose modestly from 51% +/- 14% to 57% +/- 18% in the 23 patients recanalized on follow-up, in whom pre-SK intervention angiography revealed collaterals to the infarcting area (p less than 0.025), whereas there was no EF change in the 23 chronically recanalized patients without pre-SK collaterals. In the eight patients recanalized at late follow-up, who had preadmission chest pain for less than 3 hours and successful SK reperfusion within 4 hours of symptom onset (group A), EF increased significantly from 51% +/- 12% to 65% +/- 6% (p less than 0.025). In the 25 chronically recanalized patients admitted 3 to 6 hours after onset of chest pain, in whom SK reperfusion was successful (group B), EF was unchanged at late study. In the 13 chronically recanalized patients with preadmission chest pain lasting longer than 6 hours, in whom SK reperfusion was successful (group C), EF increased from 52% +/- 13% before SK intervention to 56% +/- 16% at late study (p less than 0.025). Pre-SK intervention angiography revealed collateral vessels to the infarct area in 69% of group C patients but in only 42% of patients in groups A and B. These findings suggest that the critical interval for myocardial salvage, as well as for early and long-term improvement of LV function by SK reperfusion at admission, is 4 hours after onset of chest pain in the majority of AMI patients. This time interval may be longer in patients with well-developed collateral supply to the region of the acute infarct.

[Coagulation studies and rheological measurements during streptokinase therapy of myocardial infarction (author's transl)]

In a multicenter, randomized trial of streptokinase in acute myocardial infarction one group of patients was given streptokinase for 24 h; the remainder served as controls and received a placebo infusion instead. Coagulation assays and rheological measurements were serially performed on patients entered into the trial at one of the participating centers. Streptokinase was found to improve considerably the flow properties of blood for a period of time exceeding the duration of its administration. These results may well explain the positive clinical effect of streptokinase therapy observed in this trial.

Viscosity, haematocrit, fibrinogen and plasma proteins in maternal and cord blood

The viscosity at low shear rate of whole blood, plasma and serum, the haematocrit, plasma fibrinogen, plasma proteins, and IgM were measured in maternal and cord blood immediately following delivery. The whole blood viscosity in maternal blood (mean 19.96 +/- 3.74 centipoise or c/p) was similar to cord blood viscosity (mean 19.44 +/- 4.28 c/p). The mean haematocrit in maternal blood (0.385 +/- 0.026) was significantly lower than the cord haematocrit (0.488 +/- 0.39). Mean plasma viscosity (1.86 +/- 0.47 c/p) and plasma fibrinogen (4.25 +/- 0.75 g/litre) in maternal blood were significantly higher than cord plasma viscosity (1.12 +/- 0.19 c/p) and cord plasma fibrinogen (1.81 +/- 0.61 g/l). Mean serum viscosity (1.22 +/- 0.28 c/p) and mean IgM levels (2.38 +/- 0.36 g/l) in maternal blood were significantly higher than cord serum viscosity (0.90 +/- 0.15 c/p) and cord IgM (0.48 +/- 0.06 g/l). The low low levels of IgM and fibrinogen may protect the fetus from the increased viscosity that could be expected in association with a high haematocrit.

Mathematical and experimental methods for design and evaluation of membrane oxygenators

Three categories of membrane oxygenators are considered: passive flow, secondary flow induced by the mainstream, secondary flow induced by an external application of energy. The current status of mathematical methods for analysis of fluid mechanics, O2 and CO2 exchange for these categories are briefly reviewed. Emphasis is given to approximate methods for calculation of gas exchange. Practical methods for experimental design optimization studies are outlined; these methods are extended to evaluation of O2 and CO2 exchange in clinical operation. A new method for estimation of internal ventilation and perfusion maldistribution and diffusion resistance is described. A brief assessment of blood damage in clinical application of the oxygenator is presented from the point of view of deterioration of gas exchange performance.

Effect of low-dose subcutaneous heparin on whole-blood viscosity

The change in blood-viscosity at low shear-rates (0.77 s-1 and 2-62 s-1 was measured in eighteen normal subjects and postoperatively in sixteen patients after administration of 5000 I.U. of subcutaneous heparin. In both groups there was a significant decrease in the mean blood-viscosity 4 to 6 hours after the injection of heparin. This fall in blood-viscosity may be involved in the prophylactic effect of low-dose subcutaneous heparin in preventing venous thrombosis.

Impaired red cell deformability in peripheral vascular disease

By means of a recently developed technique, red-cell deformability was measured in 44 patients with peripheral vascular disease and in 44 age and sex matched normal control subjects. 28 patients had intermittent claudication and 16 rest pain or gangrene. The ability of the red cells to deform was significantly reduced in patients and significantly less in patients with rest pain or gangrene than in those who only had intermittent claudication. A reduction in red-cell deformability by retarding blood-flow through the microcirculation may be an important factor in states of peripheral vascular insufficiency.

Rheology of leukocytes, leukocyte suspensions, and blood in leukemia. Possible relationship to clinical manifestations

Suspensions of leukemic lymphocytes and myeloblasts and blood of leukemic patients were studied to examine (a) the effect of leukemic cells on blood viscosity and (b) the ability of leukemic cells to traverse channels of capillary diameter. The viscosity of suspensions of leukemic cells was dependent logarithmically on (a) shear strain rate and (b) cytocrit, although, suspensions of small lymphocytes and of myeloblasts had a similar viscosity at equivalent shear rates and cytocrit. The minimum apparent viscosity (MAV) of leukemic cells and red blood cells, measured over shear rates of 2.3-230 s(-1) was dependent logarithmically on cytocrit. However, MAV was slightly greater for leukemic cells than for red cells at cytocrits up to 20%. At cytocrits above 20%. MAV of leukemic cells increased more rapidly than that of erythrocytes. For example, at a 15% cytocrit MAV(WBC) (1.85 centipoise) was only slightly greater than MAV(RBC) (1.59); whereas, at 45% cytocrit MAV(WBC) (14.9) was markedly greater than MAV(RBC) (3.81). The blood of subjects with leukemia with marked elevation of leukocyte concentration (leukocrits of 6-32%) had 24% higher mean MAV (3.72) than blood with a similar total cytocrit composed of red cells (3.00). A negative correlation was present between leukocrit and erythrocrit in chronic lymphocytic (r = - 0.82) and chronic granulocytic (r = - 0.81) leukemia. Therefore, the modest increase in whole blood MAV in leukemia can be explained by (a) the negative association of leukocrit and erythrocrit and (b) the rarity of leukocrits over 20% and total cytocrits over 45%. However, the MAV of blood of leukemic patients was 71% greater than expected on the basis of their packed red cell volume. Hence, the ratio of hemoglobin concentration (O(2) carrying capacity) to MAV was abnormally low in the subjects with leukemia studied. Individual leukemic leukocytes were nearly rigid. The mean deformability index (DI) of leukemic myeloblasts (1.22; 1.18) and lymphocytes (1.22; 1.40) as measured by filtration and elastometry, respectively, at 50 mm H(2)O negative pressure, approached that of a rigid body (1.0) as compared to red cells studied by filtration (3.09) or elastometry (4.23). The ability of leukemic cells to traverse nucleopore filter or micropipette channels was related to cell diameter. The relevance of the rheology of leukemic cells to the interruption of blood flow and of tissue oxygen delivery and thereby to clinical manifestations of leukemia is considered.

Viscoelasticity of human blood

Measurements made for oscillatory flow of blood in circular tubes show that blood possesses elastic properties which make consideration of its viscous properties alone inadequate. Results are for a frequency of 10 Hz while varying the amplitude of the velocity gradient for red blood cells in plasma at concentrations ranging from 0 to 100% apparent hematocrit. For velocity gradients less than 1-2 sec(-1) both the viscous and elastic components of the shearing stress are linearly related to the gradient. For hematocrits above 20% the elastic component of the complex coefficient of viscosity increases with hematocrit approximately to the third power while the viscous component increases exponentially. Oscillatory flow measurements at very low hematocrits, when extrapolated to zero cell concentration, give the intrinsic viscosity of the average individual isolated red cell. The viscous part of this is found to be 1.7 which is compared with theoretical values from the rigid ellipsoid model for which the minimum possible value is 2.5. This difference is attributed to cell deformability. With increasing velocity gradient nonlinear properties develop. The viscous component of the complex viscosity becomes of the order of the steady flow viscosity at high gradients while the elastic component tends to decrease in inverse proportion to the gradient. Thus, the elastic component of the oscillatory stress tends to saturate, this tendency appearing at the approximate level of the yield stress.