© 1996 American Heart Association, Inc.
Articles |
Optimal Antagonism of GPIIb/IIIa Favors Platelet Adhesion by Inhibiting Thrombus Growth
An Ex Vivo Capillary Perfusion Chamber Study in the Guinea Pig
From the Institut des Vaisseaux et du Sang (P.A., B.A., V.D., P.H., C.B. dit S., J.P.C., L.O.D.); INSERM U353 (P.A., L.O.D.), Paris; and the Faculté de Médecine (B.A.), Tours, France.
Correspondence to Ludovic O. Drouet, MD, PhD, Institut des Vaisseaux et du Sang, Hôpital Lariboisière, 8 rue Guy Patin, 75010 Paris, France.
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Abstract |
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Abstract To evaluate the involvement of the glycoprotein (GP) IIb/IIIa–dependent process in platelet deposition and thrombus growth on capillaries coated with human type III collagen, the effects of incremental doses of Lamifiban, a potent specific synthetic GPIIb/IIIa antagonist, were studied in ex vivo capillary perfusion chambers using guinea pig blood. In this model, nonanticoagulated blood was perfused for 4.5 minutes at three shear rates: 100, 650, and 1600 s-1. Platelet deposition was quantified by computer-assisted morphometry and expressed as platelet adhesion (percentage of capillary surface covered with spread and contact platelets and platelets implicated in thrombus), mean thrombus height, and total thrombus cross-sectional area. In control untreated guinea pigs, platelet adhesion and thrombus height were 63% and 2.5 µm at 100 s-1, 60.5% and 13.8 µm at 650 s-1, and 45% and 28.1 µm at 1600 s-1, respectively. At 100 s-1, Lamifiban had no effect on platelet deposition at any of the three doses administered to the guinea pigs (0.3, 1, and 3 mg/kg). At 0.3 mg/kg and shear rates of 650 and 1600 s-1, Lamifiban had no effect on platelet adhesion or thrombus size, but at 1 and 3 mg/kg and shear rates of 650 and 1600 s-1, it significantly reduced thrombus size. At 1600 s-1, 1 mg/kg Lamifiban significantly increased platelet adhesion from 45% to 62.5%, whereas at 3 mg/kg it induced a significant overall decrease from 45% to 25% and qualitatively increased the ratio of contact to spread platelets. These data suggest that at high shear rates, GPIIb/IIIa participates in platelet spreading and that there is a balance between platelet involvement in adhesion to the thrombogenic surface and the growth of the already formed thrombus. This indicates that important clinical implications of an optimal therapeutic degree of GPIIb/IIIa antagonism could be expected.
Key Words: thrombus • adhesion • guinea pig • GPIIb/IIIa antagonist • capillary
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Introduction |
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TopAbstractIntroductionMethods Results Discussion Appendix References |
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One of the main aims in the prevention and treatment of cardiovascular and cerebrovascular diseases is to inhibit the platelet aggregation involved in thrombus growth with minimal inhibition of primary hemostasis. Several groups have studied the relationships between shear rate and platelet deposition and between surface composition and platelet deposition, but very few have studied the possible relationship between platelet adhesion and thrombus formation within platelet deposition.
It is generally recognized that the platelet membrane glycoprotein (GP) IIb/IIIa is involved in platelet-to-platelet interaction (ie, thrombus growth), and this glycoprotein is also thought to be involved in the interaction of platelets with the damaged vessel wall (ie, adhesion).
Several studies have been performed on everted artery subendothelium in an annular perfusion chamber with anticoagulated blood1 2 3 and on purified subendothelial components in various flat chambers4 5 6 with anticoagulated and nonanticoagulated blood. These chambers, which require large amounts of blood, are particularly well adapted to the study of platelet deposition in large animal species and humans. To take into account the circular geometry of vessel flow channels, several laboratories have developed cylindrical perfusion chambers to study thrombogenesis.7 8 9 10 We have developed a new model of the capillary perfusion chamber, in which human type III collagen is exposed to limited amounts of nonanticoagulated blood. Such a model can be applied to small laboratory animals, thus allowing the injection of a limited amount of molecules and avoiding the artifacts of anticoagulation.
The guinea pig was chosen because of its size and homology to human hemostasis. Guinea pig platelet membrane glycoproteins have been found to exhibit a very close homology to human platelet GPIIb/IIIa and GPIb/IX,11 12 and their platelet functions resemble those of human platelets in their responsiveness to ADP and the arachidonic acid cascade.13 In the present study, we evaluated the influence of a growing thrombus on platelet adhesion to the surrounding thrombogenic surfaces. The relationship between thrombus growth and platelet adhesion was studied using Lamifiban, a specific and synthetic GPIIb/IIIa antagonist that is active in the guinea pig,14 15 at three representative shear rates: venous (100 s-1), arterial (650 s-1), and mild stenosed arterial (1600 s-1). The results were analyzed by computer-assisted morphometry.
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Methods |
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TopAbstractIntroductionMethods Results Discussion Appendix References |
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Lamifiban (Ro44-9883), kindly provided by Dr S. Roux (F. Hoffmann–La Roche), is a synthetic nonpeptidic GPIIb/IIIa inhibitor with an IC50 of 25 and 450 nmol/L in human and guinea pig platelet-rich plasma,16 respectively. Lamifiban was injected intravenously into guinea pigs as a bolus 10 minutes before blood perfusion.
Animal Selection and Anesthetic Administration
Ninety male
Hartley strain guinea pigs (Saint Antoine) weighing
700±100 g were used. Experiments were conducted according to the
legislation of the French Ministry of Agriculture, the guidelines of
the Institut National de la Santé et de la Recherche
Médicale, and the institutional policies of the Institut National
de la Recherche Agronomique. Guinea pigs were anesthetized
intraperitoneally with 50 mg/kg sodium thiopental
(Nesdonal, Specia Rhône Poulenc).
Capillary Perfusion Chamber
The capillary perfusion chambers
A, B, and C consisted of glass
capillaries that were 64, 100, and 127 mm long with radii of 0.315,
0.4, and 0.78 mm, respectively (Microcaps Drummond, Polylabo). Chamber
A capillaries were used to create a mild stenotic
arterial shear rate of 1600 s-1,
chamber B capillaries to create an arterial shear rate of
650 s-1, and chamber C capillaries to create a
venous shear rate of 100 s-1 at blood flow rates of 2.36,
1.96, and 2.06 mL/min for chambers A, B, and C, respectively. The
Reynolds numbers were always <22, indicating laminar blood flow
conditions in all tubes (Table 1
,
"Appendix").
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Blood entered and left the chambers through
Silastic tubes (Sigma
Medical) with radii of 0.317, 0.381, and 0.737 mm for the tubes
connected to chambers A, B, and C, respectively. The proximal end of
the collagen-coated capillary was connected to an identical
capillary that was not collagen coated to avoid possible flow
turbulence at the entrance to the thrombogenic capillary (Table
1
, Fig 1).
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Coating of the Internal Capillary Surface With
Collagen
Human type III collagen (Sigma Chimie) was purified from a
lyophilized human placental pepsin extraction by selective salt
precipitation.17 Collagen was allowed to polymerize and
form fibrils by dialysis for 48 hours against 20 mmol/L
Na2HPO4, pH 7.4.18 Internal
capillary surfaces were prepared for coating with three washes of 10
minutes each with chromic acid, three rinses with deionized water, and
drying at 75°C for 1 hour. Capillaries were filled by capillarity
with a 1-mg/mL solution of human type III collagen. To obtain a
homogeneous circumferential coating, they were
centrifuged for 15 minutes in their longitudinal axis at 2100
rpm and stored at 22°C for 1 hour. They were then connected, undried,
to Silastic tubing that was first filled with rinsing buffer ([mmol]
130 NaCl, 2 KCl, 12 NaHCO3, 2.5
CaCl2·2H2O, 0.9
MgCl2·6H2O, and 5 glucose, pH 7.4).
Characterization and Quantitation of Immobilized
Collagen
The amount of collagen coating inside the glass capillaries
was
measured in a spectrophotometer at 230 nm with 125I-labeled
collagen. The optical density measured in a fixed volume of buffer that
rinsed the filled collagen capillary for 1 minute was compared with the
optical density of an equal capillary volume of collagen added directly
to the same fixed volume of buffer. Type III 125I-collagen
was prepared according to the Bolton and Hunter method.19
The 125I-collagen deposited in the capillaries was pump
rinsed with the rinsing buffer for 1 minute at the appropriate shear
rate. The amount of collagen deposited in the capillaries was counted
in a Beckman model 5500 gamma counter. The degree of removal of coated
collagen was assessed by increasing the rinsing period. There were no
significant differences between the amounts of collagen deposited in
the capillaries after rinsing periods of 1 minute and 5.5 minutes as
measured by spectrophotometry with 125I-labeled collagen.
The distribution of collagen along the capillary was assessed by
dividing the capillaries into 1-cm segments that were individually
counted (n=3 for each perfusion period, Table 2). The
amount of collagen in each proximal capillary section was twice that in
the next five sections and varied from 2.52 to 2.79
µg/cm2 (Table 2). The thickness of the layer of
collagen
deposited was revealed by transmission electron microscopy (Philips CM
10), which revealed the deposition of a homogeneous layer
along the capillary wall, with a mean thickness of 0.45±0.12 µm at
the proximal end of the capillaries (mean±SEM, n=9, P.A.,
unpublished
data, 1995). Some of the glass capillaries were examined with scanning
electron microscopy (JEOL 35 C SEM). For this purpose, the internal
surface of the capillaries was fixed in 1%
glutaraldehyde and 4% paraformaldehyde
in 0.1 mol/L phosphate buffer (pH 7.4), post-fixed in 0.1% osmium
tetroxide, dehydrated in graded ethanols, critical point–dried
with carbon dioxide, and coated by gold sputtering.20
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Perfusion System
Capillary chambers were perfused with
nonanticoagulated blood
drawn directly from the abdominal aorta punctured with a 19-gauge
butterfly infusion set (Venisystems, Abbott Laboratories; Fig
1). The
infusion set was then connected to the capillary chamber inlet through
a T-piece shunt with Silastic tubing. The outlet of the capillary
chamber was connected to a roller pump (2120 VarioperpexII Pump, LKB
Bromma) to obtain selected blood flow rates. Just before the beginning
of blood perfusion, the collagen-coated inner surface of the
capillaries was rinsed with 2 mL of buffer at the respective shear
rates of chambers A, B, and C. No bubbles were allowed to form before
the blood reached the collagen-coated surface. Perfusion
apparatus, tubing, and chambers were thermostated by
immersion in a permanent circulating water bath at 37°C. The chambers
were then perfused with blood for 4.5 minutes. At the end of that time,
they were rinsed through the T piece with the rinsing buffer for 20
seconds and then fixed in 2.5% glutaraldehyde
cacodylate (0.1 mol/L, pH 7.4) buffer at 4°C for 50 seconds at the
appropriate shear rate. The reactive collagen-coated capillary was
then separated from the Silastic tubing. Fixation was prolonged by
immersing the capillary in freshly prepared fixative for 2 hours at
4°C and storing it in 7% sucrose and 0.1 mol/L cacodylate buffer (pH
7.4) at 4°C until further postfixation (see below). Thus, one
capillary chamber was perfused for each guinea pig.
Postfixation, Dehydration, Epon Embedding, Sectioning, and
Staining
Capillaries were rinsed with 0.1 mol/L cacodylate, pH 7.4,
and
then post-fixed with 0.05% KMnO4 for 1 hour. They were
dehydrated, embedded in Epon 812, and allowed to polymerize at 50°C
for 1 hour and then at 63°C for 16 hours.21 Glass
capillaries were broken in water at 4°C under magnifying binoculars.
The Epon rods that contained the platelet deposits at their
periphery were reembedded in new Epon 812. Cross sections (1 µm) of
the embedded preparation were cut perpendicular to the direction of the
blood flow at 5 mm (proximal), 30 mm (middle), and 60 mm (distal) from
the proximal end. Sections were stained and mounted on microscope
slides according to the method of Sakariassen et al.21
Computer-Assisted Morphometric Quantitation of Semithin
Sections
The microscope view of the sections was displayed on a color
video monitor (Microvitec, HL series) at a final magnification of
x1400 by a video camera (Sony, 3CCD) fitted on the photographic
pathway of the microscope (Zeiss, Axioplan). Total platelet
deposition, percent capillary surface coverage with platelets,
percent surface coverage with thrombus >5 µm, mean thrombus height,
and total thrombus surface area on the surface coated with collagen
were automatically recorded and contrasted by a color-effect
generator (NS15000). Data were managed with the Lucie program
(Microvision) and a PC 486DX33 computer (Elonex). In accordance with
the convention usually adopted by other users of in vitro or ex vivo
perfusion chambers,21 thrombi were defined as platelet
aggregates >5 µm. Total sectional thrombus surface area was
calculated on the basis of the type B capillary cross-section
area.
Ex Vivo Platelet Aggregation
Blood samples were collected in
0.129 mol/L trisodium citrate
(9:1 vol/vol). Platelet-rich plasma was obtained by
centrifugation at 200g for 10 minutes and
platelet-poor plasma by centrifugation at
1000g for 20 minutes. Platelet aggregation was monitored
by the turbidimetric method on a Chrono-log aggregometer (Coulter).
Aggregation was induced by 1.5 µmol/L ADP (Diagnostica
Stago), 10 µg/mL human type III collagen (Sigma Chimie), and 1.5
mg/mL ristocetin (Diagnostica Stago). Platelet
aggregation was reported as the variation in light transmittance.
Statistical Analysis
Student's t test was used
to test the effect of
collagen deposition on platelet coverage and thrombus formation.
Two-way ANOVA was used to test the effects of Lamifiban on
platelet adhesion (percent capillary surface coverage with
platelets), thrombus formation (mean thrombus height), and fibrin
deposition (percent fibrin). Multiple comparisons were performed with
the Tukey test. Values of P<.05 were considered
significant.
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Results |
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TopAbstractIntroductionMethods Results Discussion Appendix References |
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Platelet and Fibrin Deposition With Blood From Untreated Animals
Quantitative Analysis
Platelet adhesion (capillary surface coverage with platelets). Maximal adhesion was observed at the proximal part of the type III collagen–coated capillaries. Platelet adhesion at the proximal end of the capillary decreased with increasing shear rates from 63% at 100 s-1 to 60.5% at 650 s-1 and 45% at 1600 s-1. Platelet adhesion decreased with length along the internal capillary surface (Table 3
).
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Thrombus size. At the proximal part
of the capillaries, the
percentage of thrombus was shear rate–dependent (0.75% at 100
s-1, 31% at 650 s-1, and
39.8% at 1600 s-1) and so was mean thrombus height (2.5,
13.8, and 28.1 µm at 100, 650, and 1600 s-1,
respectively). Both the percentage of thrombus and mean thrombus height
decreased along the capillaries from the proximal to distal parts
(Table 3).
Fibrin deposition. Fibrin deposition was minimal at the proximal part of the three capillaries (value <5%). A significant axial increase in fibrin deposition (P<.001) was observed from the proximal to distal parts of the capillaries for the three shear rates as follows: from 5.1% (proximal) to 80% (distal) at 100 s-1, 3.5% (proximal) to 13% (distal) at 650 s-1, and 2.7% (proximal) to 11.5% (distal) at 1600 s-1.
Qualitative Analysis: Relation
Among Platelet Adhesion
and Thrombus and Fibrin Formations
Increasing the shear rate increased
mean thrombus height at the
proximal part of the capillary and reduced platelet adhesion (Table
3). The decrease in adhesion was related to a reduction of the
number
of thrombi, whose size increased (Fig 2A through 2D). At
the venous shear rate, platelet adhesion dropped along the
capillary from 63% (proximal) to 4.5% (distal), which is related to
an increase in fibrin formation from 5.1% (proximal) to 80%
(distal).
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Effects of Increasing In Vivo Administration of the GPIIb/IIIa
Antagonist (Lamifiban) on Ex Vivo Platelet Deposition
at the Proximal Part of the Collagen-Coated
Capillary
Quantitative Analysis
Platelet
adhesion. At shear rates of 100 s-1
and 650 s-1, none of the three doses of Lamifiban
(0.3, 1, and 3 mg/kg) significantly affected platelet adhesion. At
1600 s-1, the lowest and highest doses
significantly reduced adhesion (30.5% and 25%, respectively;
P<.05), but the intermediate dose increased platelet
adhesion (62.5%, P<.05) (Fig 3).
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Thrombus size. At the shear rate of 100
s-1, none of the three Lamifiban doses
significantly affected thrombus formation. At shear rates of 650
s-1 and 1600 s-1, Lamifiban induced a
dose-dependent decrease in thrombus formation (Figs 3B and
3C and
4A, 4B, and 4C).
At 1600 s-1 and 1 mg/kg, mean thrombus height decreased from 28.1 (control) to 6.5 µm, and this decrease was correlated with a significant increase in platelet adhesion.
Fibrin deposition. Lamifiban did not significantly affect axial fibrin formation along the capillary at any shear rate.
Qualitative Analysis
At 100 s-1,
none of the doses of Lamifiban
significantly affected the relative distribution of the reactive
surfaces covered by contact or spread platelets, or of those
involved in thrombus (Fig 5A). At 650
s-1, Lamifiban increased the relative percentage of
contact platelets. The percentage of spread platelets remained
stable, but the thrombus percentage decreased (Fig 5B). The
greatest
effect of Lamifiban was observed at 1600 s-1. At this
shear rate, it induced a dose-dependent decrease in the percentage
of thrombus, which correlated with a linear increase in the relative
surface covered by contact platelets (Fig 5C).
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Effects of Lamifiban on Ex Vivo Platelet
Aggregation
Increasing doses of Lamifiban had no effect on
ristocetin-induced ex vivo platelet agglutination (Table 4).
The highest dose of Lamifiban (3 mg/kg) totally
inhibited both ADP- and collagen-induced aggregation. The
intermediate dose (1 mg/kg) inhibited ADP-induced aggregation by 84%
and collagen-induced aggregation by 78%. The lowest dose (0.3
mg/kg) inhibited ADP-induced aggregation by 55% and
collagen-induced aggregation by 49%.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion Appendix References |
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Thrombus composition varies according to shear stress. At high shear stress it is mainly composed of platelets,1 22 23 thus explaining their predominant role in cardiovascular and cerebrovascular diseases. The molecular interactions involved in platelet adhesion and aggregation have been extensively studied from a biochemical point of view, but fewer experiments have been designed to study the hemorheological circulation conditions that might preferentially induce platelet aggregation or adhesion and their interrelationship. Platelet adhesion to subendothelial components24 is mediated by at least one plasma component, von Willebrand factor (vWF),4 and by at least three distinct platelet receptors, ie, GPIb/IX, GPIIb/IIIa,25 26 27 28 and GPIa/IIa.29 30 31 32 33 34 35 36 At high shear rates, GPIb/IX is the specific platelet receptor for subendothelial-bound vWF in the initial contact phase.37 38 39 This phase induces platelet activation and spreading and then thrombus formation, which involves the binding of adhesive molecules including fibrinogen and vWF to activated GPIIb/IIIa.40 41 The predominant involvement of GPIb/IX-vWF in platelet adhesion and GPIIb/IIIa-fibrinogen in thrombus formation has been established in anticoagulated blood,27 42 43 but the in vivo mechanisms governing these processes remain partially unknown. To find out how circulation conditions could favor either thrombus growth or platelet adhesion, the present experiments were performed in an original perfusion chamber with pharmacological modulation of GPIIb/IIIa reactivity by a molecular antagonist, Lamifiban.
For this study, we developed a new type of capillary perfusion chamber that allowed us to study ex vivo thrombogenesis in small animal species. Capillaries of various diameters were chosen to mimic the range of physiological blood shear rates present in the vasculature, and these capillaries were coated with human type III collagen. The guinea pig was chosen because of its small size and the homology exhibited by its platelet glycoprotein receptors to the human receptors involved in thrombogenic mechanisms.11 12 Lamifiban was chosen because it has been shown to be active in the guinea pig by abolishing cyclic flow variations in an in vivo model of arterial thrombosis.14
In the present system, human type III collagen induced ex vivo platelet adhesion and thrombus formation in flowing, native, nonanticoagulated guinea pig blood. Platelet deposition was dependent on reactive surface length. Nevertheless, the axial dependence of platelet-collagen interaction previously reported by Sakariassen and Baumgartner44 was present at all the shear rates tested. This dependence was apparently enormously increased at the venous blood shear rate in our setting. At this shear rate, the drop in platelet adhesion from the proximal to distal end of the capillary did not depend on the upstream consumption of platelets because there were no formed thrombi in the capillary; it was due to the marked increase in fibrin formation. Fibrin reactivity is perhaps species dependent, since it was much higher than what we had observed in humans (P.A., unpublished data, 1995).
At the proximal part of the capillary, raising the shear rate induced a significant increase in thrombus growth, which correlated with a moderate decrease in platelet adhesion. One possible explanation for this observation is that the local microturbulences created by the growing thrombus prevented the platelets from coming into contact with the surrounding collagen. An alternative possibility is that the thrombus surface is more thrombogenic than the collagen surface, which is in agreement with the findings of Badimon et al.45 Our data differ from those of Weiss et al37 and Sakariassen et al,6 who, using anticoagulated and nonanticoagulated blood, found shear rate–dependent increases in both platelet adhesion and thrombus formation. This might be because in our experiment (1) the capillaries were coated with 10 times less collagen than in previous rectangular perfusion chamber studies,6 (2) the collagen never dried, and (3) the species reactivity of platelets may affect the quality of platelet deposition. The shear rate–dependent increase in platelet adhesion at the middle part of the capillary might be related to the decrease in mean thrombus height observed at this level, in accordance with our hypothesis that a growing thrombus slightly reduces platelet adhesion in the guinea pig.
Using incremental doses of Lamifiban, we studied the effects of inhibiting GPIIb/IIIa on the platelet adhesion and thrombus formation induced in type III collagen–coated capillaries at three shear rates in nonanticoagulated blood.
At the venous blood shear rate, Lamifiban had no effect on platelet adhesion in the proximal part of the capillary, where fibrin deposition did not influence platelet deposition. This seems to indicate that at the venous blood shear rate, the involvement of GPIIb/IIIa in platelet contact and spread is minimal. At the higher shear rates, raising the Lamifiban dose significantly reduced thrombus height, but platelet adhesion did not correspond to the degree of inhibition of GPIIb/IIIa. The low dose had a minimal effect on both platelet adhesion and thrombus growth, whereas it induced a 50% inhibition of ADP and collagen-induced ex vivo platelet aggregation. One explanation is that in vivo, numerous mechanisms (such as the vWF-GPIb/IX axis) are implicated in thrombus formation in the guinea pig,46 but such mechanisms did not occur in the aggregometer model. The medium dose significantly inhibited thrombus growth and, due to the balance between adhesion and thrombus formation, apparently increased platelet adhesion; the high dose abolished thrombus formation and impaired adhesion. Our observation of an apparent balance between platelet adhesion and thrombus formation is in agreement with other in vitro and ex vivo observations.47 48 The involvement of the GPIIb/IIIa axis in platelet spreading deduced from our results with Lamifiban is in accordance with the results of experiments with monoclonal antibodies and thrombasthenic platelets.26 49 50
If experimental thrombus growth is indeed related to clinical thrombogenesis and experimental platelet adhesion and spreading are related to clinical hemostasis, the latter observations are of great clinical interest. The GPIIb/IIIa antagonists currently tested in numerous clinical studies have had promising effects on high-risk coronary angioplasty, but these effects were offset by a severe risk of bleeding.51 In our experiments, 1 mg/kg Lamifiban induced a 1.5-fold increase in bleeding time versus the 12-fold increase induced by 3 mg/kg (unpublished data). From the present data, we may justifiably postulate that a specific intermediate dose of GPIIb/IIIa antagonist could partially block the GPIIb/IIIa receptor and induce antithrombotic but not antiadhesive effects, with minor effects on the hemorrhagic risk.
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Acknowledgments |
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This study was supported by a scholarship from the Sanofi Foundation, Paris, France (P. André). We thank Dr Françoise F. Lafève for 125I-labeling and Khalid Azzam for expert assistance.
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Appendix |
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TopAbstractIntroductionMethods Results Discussion Appendix References |
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The capillary chamber was used because of its well-known fluid dynamic and mass transport properties and its similarity to blood vessel geometry. With this chamber, fluid movement is well defined and is dependent on the volumetric flow rate (Q), pressure drop (P), tube length (L), radius (R), and fluid viscosity (µ), according to the well-known Hagen-Poiseuille equation52 : Q=
P
R4/8µL. The velocity gradient or shear
rate (
) reflects the local movement of fluid near the wall. At the
wall, the shear rate value (w) is given by the following
equation53 : w=4Q/R3.
Nevertheless, blood is not a homogeneous newtonian fluid.
It is a concentrated suspension of cells and proteins and may be
treated as homogeneous, with viscosity dependent primarily
on the red blood cell concentration. Plasma viscosity and red
blood cell deformability and aggregation are relatively minor
parameters compared with the red blood cell concentration.
For most of the physiological range of shear rates,
blood viscosity is relatively constant (in the range of 0.03 to 0.04
mPa/s at 37°C) and does not rise above 100
s-1.53 Guinea pig blood viscosity at 37°C
was reported to be 0.035 mPa/s at 100 s-1 and 0.032 mPa/s
at >200 s-1.54 Reynolds number (Re) reflects
the ratio of inertial to viscous terms and gives further information
about flow stability: Re=2Q 114 /µR. As blood enters a tube
from a
large reservoir, there is an entrance length (Le) of parabolic profile
establishment, in which the shear stress, which is directly
proportional to the shear rate (
=
), increases:
Le=0.07R·Ret. In this study, shear rates were
calculated according to these rheological parameters.
Received June 19, 1995; accepted October 11, 1995.
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References |
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TopAbstractIntroductionMethods Results Discussion Appendix References |
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