Published online before print April 14, 2008, doi: 10.1161/HYPERTENSIONAHA.108.112003
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
(Hypertension. 2008;51:1545.)
© 2008 American Heart Association, Inc.
Original Articles |
Orthostatic Hypercoagulability
A Novel Physiological Mechanism to Activate the Coagulation System
From the J. Recanati Autonomic Dysfunction Center and the Thrombosis and Hemostasis Unit, Medicine, Rambam Health Care Campus, and Bruce Rappaport Faculty of Medicine, Technion-IIT, Haifa, Israel.
Correspondence to Giris Jacob, J. Recanati Autonomic Dysfunction Center, Medicine, Rambam Medical Center, PO Box 9602, Haifa 31096, Israel. E-mail G_Jacob{at}rambam.health.gov.il
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
Orthostatic stress causes significant plasma shift and raises transmural pressure in lower extremities, resulting in an increase in endothelial activation and plasma proteins concentrations, possibly including coagulation factors. This may lead to activation of the coagulation system during standing. To test this hypothesis, we recruited 18 healthy volunteers (9 females and 9 males; mean age: 25±1.2 years; body mass index: 21.7±0.5 kg/m2). Hemodynamics, plasma shift (extrapolated from sequential hematocrit concentration), plasma proteins, and coagulation tests, including procoagulants; fibrinogen, factor V, and factor VIII activity; prothrombin fragments 1 and 2; and endothelial activation–related factors (tissue factor and von Willebrand factor), as well as protein C global pathway, were determined at rest supine and at 15 minutes, 30 minutes, and 60 minutes of still standing. Thirty minutes of standing caused a decrease in plasma volume by 12.0±0.5% and an increase in plasma protein by 13.0±0.7%. Fibrinogen, factor V, and factor VIII activity rose by 12.0±1.2%, 13.0±1.0%, and 40.0±6.0% (P<0.002 for all), respectively. Prothrombin fragments 1 and 2 were elevated by 150.0±30.0%. Tissue factor and von Willebrand factor increased by 30.0±9.0% and 17.4±51.0% (P<0.02 for both), respectively. However, protein C assay results decreased from 0.95±0.20 to 0.83±0.16 (P<0.001). We hereby introduce a novel physiological mechanism, "orthostatic procoagulation," that should be considered during coagulation tests. Furthermore, it could be extrapolated to the pathophysiology of stasis and venous thromboembolism.
Key Words: prolonged standing • coagulation • protein C global • endothelial activity
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Hypercoagulable states predispose to the development of venous thromboembolism (VTE).1 This could result from either pathological or physiological situations, eg, cancer,2,3 major surgery,4,5 pregnancy,6 and prolonged stasis.4 Pathophysiologic mechanisms underlying these predisposing conditions are summarized in the triad of Virchow: alteration in coagulation factors, vascular endothelial injury, and blood stasis.7
Standing is known to cause pooling of a large quantity of blood with a subsequent increase of orthostatic pressure in the lower part of the body.8,9 As a result, 
12% of intravascular fluid transfers from the intravascular space to surrounding tissues.10,11 This plasma shift can be estimated indirectly by measuring sequential changes in hematocrit (Hct) or total protein (TP) concentration.10,12 Although much has been reported about the hemoconcentration effect on total plasma protein level, no data are available regarding this effect on a variety of specific circulating proteins, including coagulation factors.
During prolonged standing, orthostatic pressure is increased in both venous and arterial vasculature of the lower limbs.13 This equally involves pulsatile pressure and shear stress, acting on the vessel wall, including the endothelial monolayer. Consequently, several endothelial homeostatic mechanisms are physiologically activated, which, along with vasodilatatory function, play a pivotal role in the hemostatic processes. Under physiological conditions, endothelial cells have balanced anticoagulant and procoagulant properties.14,15 Once exposed to injuries, such as, high blood pressure or increased shear stress, their procoagulant activity prevails.16,17
Although intensive exercise was shown to be associated with increased coagulability through a mechanism involving endothelial activation,18 virtually no data are available regarding the effect of still standing on the coagulation cascade.
Taking into account all of the aforementioned observations, we hypothesized that prolonged standing might activate the coagulation system via endothelial induction mainly because of increasing the orthostatic shear stress on the endothelial monolayer, especially in the lower extremities. To prove this hypothesis, we studied the effect of prolonged still standing on concentrations and activities of procoagulant proteins, an anticoagulant system, coagulation indices, and endothelial-dependent coagulation factors in healthy subjects.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Subjects
Subjects were enrolled in the study if they met the following eligibility criteria: age between 21 and 45 years, body mass index of 18 to 25 kg/m2, healthy, no personal or family history of hypercoagulability, no history of cancer or thrombosis, no trans-Atlantic flights for
3 months before the enrollment, no history of syncope or orthostatic intolerance, and no history of smoking. Females were nonpregnant, had not used oral contraceptives for 3 months before the enrollment, and had no history of abortions. In addition, to avoid any effect of acute-phase response, subjects were considered noneligible if they had any history of recent illness or were consuming any sort of medications. Enrolled females were evaluated 1 week after their menstruation. The protocol was approved by the institutional review board of the Rambam Medical Center. All of the study participants gave written, informed consent.
Experimental Design
All of the investigational procedures were performed after overnight fasting in a human physiological laboratory under controlled environment conditions in a quiet and partially darkened room with an ambient temperature of 24°C, at the Recanati Autonomic Dysfunction Center.
During the study, each subject was asked to assume a rest supine posture, after which a large antecubital intravenous heparin-free lock (18 gauge) was inserted. Heart rate and blood pressure (BP) were monitored continuously using 3-lead ECG and the oscillometric cuff applied on the contralateral arm (DATEX). Thereafter, a 30-minute rest supine was allowed. During supine rest and at 15, 30, and 60 minutes of still standing, hemodynamics (heart rate and BP) were measured, 2 to 3 mL of waste blood were sampled, and then 12 mL were drawn for the assessment of Hct, TP, and coagulation studies. While standing, the subjects were limited to move within a 1-ft2 area on a towel placed on the floor and were not allowed to stretch their lower extremities.
EDTA-containing tubes and 3.2% sodium citrate tubes were used for Hct and TP and for coagulation studies, respectively. Of note, blood samples were drawn without using a tourniquet, and after each blood sampling, 3 mL of normal saline, heparin free, was injected.
Plasma Shift
Acute plasma volume (PV) shift during quiet standing was estimated using quadruplicate microcapillary venous Hct measurements (Microcapillary reader and Micro MB centrifuge, IEC), corrected for trapped plasma (0.96) and whole-body Hct (0.91), (0.96x0.91= 0.87).9 Hct was measured after centrifugation for 10 minutes at 11 500 rpm and read on a microcapillary tube reader. Acute dynamic percent changes (
) in PV were calculated from Hct, where Hct1 denoted the baseline value, and Hct2 indicated the tested value. Dynamic PV (%)=100x(Hct1–Hct2)/Hct2x(1–Hct1).11 Percentage of change in the total plasma protein (triplicates measured with Refractometer, Leica) was calculated to further confirm the PV% changes in Hct.
Coagulation Studies
Extensive procoagulant and anticoagulant profiles were assessed in addition to the endothelial activation-related factors. Procoagulant pathway included prothrombin time (PT), activated partial PT time (aPTT), fibrinogen levels, and factor V and factor VIII activities. The anticoagulant pathway, ie, ProC Global assay, protein C activity, and free protein S antigen, was also assessed. More specific markers for the coagulation activation, fragments F1 and 2 and D-dimer as a marker for the fibrin formation and fibrinolysis, were taken. Plasma levels of endothelial activation-related factors, including tissue factor (TF) and von Willebrand factor (vWF) antigens, were determined.
Coagulation Assays
Blood samples were collected into 3.2% sodium citrate tubes and centrifuged at 2000g for 15 minutes. PT, aPTT, fibrinogen, D-dimer, and ProC Global assays were performed on fresh plasma samples, whereas all of the other coagulation assays were done using thawed frozen plasma samples. Plasma samples were frozen after a second centrifugation at 2000g for 15 minutes in aliquots at –70°C. Before testing, plasma aliquots were thawed in the 37.0±0.5°C water bath for 15 minutes. PT, aPTT, fibrinogen, D-dimer, and ProC Global assays were performed on the STA-R evolution analyzer (Diagnostica Stago) using recombinant human thromboplastin Dade Innovin (Dade Behring Marburg GmbH) for PT assay and STA-PTT, STA-FIBRINOGEN, and STA-LIATEST D-DI kits (Diagnostica Stago) for aPTT, fibrinogen, and D-dimer assays, respectively.
The ProC Global assay (Dade Behring) was performed as described previously.19 Endogenous activated protein C activity was estimated by an aPTT base-clotting assay. The clotting time was measured with and without protein C activation, and the ratio between these values was calculated and expressed as the protein C activation time-normalized ratio (PCAT-NR).20
Levels of protein C activity, free protein S antigen, and vWF antigen were determined on the STA-R analyzer using STA-STACHROM, STA-Liatest Free Protein S, and vWF:antigen kits, respectively (Diagnostica Stago). Levels of coagulation factors V and VIII activity were determined by a 1-stage assay using factor V and VIII, respectively, deficient plasma (Diagnostica Stago).
Prothrombin fragment F1+2 concentration was measured by an enzyme immunoassay (ELISA) using Enzygnost* F1+2 (monoclonal; Dade Behring). TF level was determined by IMUBIND Tissue Factor ELISA kit (American Diagnostica Inc).
Statistical Analysis
Results were expressed as means±SEMs. We used 1-way ANOVA for repeated measurements to test the effect of standing time on the evaluated parameters. Tukey’s multiple comparison test was applied for posthoc analysis. Paired 2-tailed t test was used to assess interindividuals for continuous data. One-phase exponential decay or association curve was used to illustrate eventual increments or decrements over time of the various parameters. Linear correlation was applied for testing the eventual association between 2 parameters. Data were analyzed with Excel software (version 2002, Microsoft) and GraphPad Prism (GraphPad Software Inc, version 4.03, 2005). A P value of <0.05 was considered to be statistically significant.
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
General characteristics of participants are presented in Table 1. Unintentionally, equal numbers of males and females were enrolled in the study. During standing, systolic BP remained unchanged, whereas a significant increase was observed in diastolic BP (by 6±2 mm Hg; P<0.005) and heart rate (by 13±2 bpm; P<0.005). Vital signs (BP and heart rate) were stable at different time points during standing. All of the subjects completed the study protocol except for 1, who developed dizziness at 45 minutes of standing.
|
Rest supine-corrected Hct and plasma TP were 39.4±0.7% and 7.6±0.1 g/dL, respectively. After 30 minutes of still standing, a steady state was reached; the Hct increased to 42.1±0.8% and plasma TP to 8.6±0.1 g/dL. Plasma shifts, calculated from changes in corrected Hct and plasma TP, are presented in Figure 1 (top). A powerful linear correlation was found between the changes in Hct and plasma TP at various sampling points (r=0.96; P<0.001). Plasma fibrinogen concentration (large macromolecular weight protein) increased similar to plasma TP and Hct changes; it increased from 260±10 mg/dL to 291±11 mg/dL at 30 minutes of standing (
13%). A linear correlation was observed between the percentage changes in fibrinogen and the corresponding changes in plasma (r=0.59; P<0.001; Figure 1, bottom).
|
Evidence for thrombin formation was provided by a significant increase in prothrombin activation fragment F1+2. After 60 minutes of standing, it rose steeply from 150±20 pmol/L to 375±60 pmol/L (P=0.002; Figure 2). D-dimer reached significant increase at only 30 minutes (P=0.02; Figure 2, bottom). Moreover, aPTT and PT reduced significantly as shown in Figure 2 (second row), with the aPTT decrease of 2.25±0.38 seconds (P<0.001). For coagulant factors V and VIII, activity increased by 13% and 40%, respectively, after 60 minutes of still standing (Figure 2, third row).
|
Analysis of the protein C anticoagulant pathway revealed that PCAT-NR levels significantly decreased after 15 minutes of standing (from 0.95±0.2 to 0.83±0.16; P<0.001) and reached a nadir within 30 minutes (0.75±0.17; P<0.001; Figure 3, top). However, free protein S antigen and protein C activity levels significantly increased with standing and reached a plateau after 15 and 60 minutes, respectively (Figure 3, middle and bottom). Of note, only 3 of all of the participants were found to have a PCAT-NR <0.8 (cutoff normal values) in the rest supine, and 2 were heterozygote for factor V Leiden mutation.
|
Simultaneous to the decrease in PCAT-NR levels, a significant increase in the endothelial activation–related factors, ie, TF and vWF antigen, was demonstrated (Figure 2, top). TF antigen level significantly rose after 15 minutes of standing (from 59±41 to 68±36 pg/mL; P=0.007) and decreased gradually to 64±50 pg/mL after 60 minutes. However, the vWF antigen level increased significantly during the first 30 minutes of standing (from 92±32 to 110±10 mg/dL; P<0.001) and remained at a level of 113 mg/dL after standing for another 30 minutes. Percentage changes in both factors are demonstrated in Table 2 and Figure 2 (top). It is noteworthy that maximal changes in the levels of vWF and TF antigens amounted to 21.6±7.0% and 44.0±13.0%, respectively.
|
Moreover, during 60 minutes of still standing, a significant correlation was demonstrated between the levels of vWF antigen and factor VIII activity (r=0.75; P<0.002). Although factor VIII activity increased by 51.0±8.0%, that of vWF antigen rose by 21.6±7.0% only. Furthermore, throughout still standing, an inverse correlation was found between PCAT-NR and F1+2 (r=–0.52; P=0.03) and D-dimer levels (r=–0.51; P=0.03).
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Although much is known about the effect of orthostatic stress on hemodynamics and plasma shift,10,11 little, if any, information is available about the coagulation system in this setting. To the best of our knowledge, the present study has shown for the first time that prolonged still standing significantly activates the coagulation cascade.
Orthostatic stress causes an increase in transcapillary orthostatic pressure. As a result, 12% of plasma (water and micromolecules) crosses into the extravascular compartment. This hemoconcentration phenomenon leads to an increase in blood viscosity and stands behind the increment in Hct and TP.8,10,21 According to the data obtained in the current study, it also increases the concentration of fibrinogen (13%) and protein S (10%) and contributes, at least partially, to the rise in other proteins in the coagulation system (see Table 2). Interestingly, vWF antigen, factor VIII activity, and TF increase by 22%, 51%, and 45%, respectively, which is far beyond the expected hemoconcentration effect. This suggests that other mechanisms are also involved in the latter phenomenon.
Persistent increase in fibrinogen, factor VIII activity, and most likely other procoagulant factors, is associated with high incidence of thrombotic and cardiovascular events.22,23 However, no data are available regarding the effect of hyperacute physiological increment on these components. Although hemoconcentration affects blood rheology by increasing viscosity24 and coagulant proteins concentrations, it is not the sole determinant in the present remarkable coagulant stimulation. It is noteworthy that the viscosity level measured in lower limbs while standing or sitting is higher compared with that seen in the upper limbs.25 The increase in viscosity could contribute to activation of the coagulation system through various mechanisms, including augmentation of shear stress.26
Undoubtedly, orthostatic pressure and shear stress are increased in the lower extremities during standing.10 Previous studies have shown that orthostatic stress enhances endothelial activation. Moreover, shear stress promotes activation of the coagulation cascade and fibrinolytic system. It stimulates endothelial cells, enhances the expression of the TF,27 modulates the release of endothelial vWF from Weibel Palade bodies,28 and also increases the levels of tissue plasminogen activator and plasminogen activator inhibitor-1. In the current study, on 15 minutes of still standing, the TF antigen first significantly increased and subsequently decreased (see Figure 2). TF is believed to be shed into the circulation from activated cells, including platelets and endothelium,29 or from the injured surrounding tissue, consequently creating hypercoagulable blood.30 The orthostatic stress-induced increase in TF in our healthy subjects is associated with a remarkable rise in vWF. This simultaneous elevation in both biomarkers and tissue-type plasminogen activator (unpublished data) is in further support of the suggestion of orthostatic endothelial activation.
The early increase in TF on 15 minutes of still standing, according to the current study, leads to coagulation activation (extrinsic pathway) and to the conversion of prothrombin into active thrombin, as expressed by the remarkable increment in the prothrombin fragment F1+2. Thrombin promotes further procoagulant effects by activating coagulation factors V and VIII,31,32 as demonstrated by the present study. Such increment in both factors is inversely correlated with the orthostatic progressive shortening of aPTT and PT. The shortening in aPTT has been suggested to be a predictor of VTE.33 Taken together, these findings, along with the increase in D-dimer, confirm that prolonged standing (at least after 30 minutes) activates the coagulation cascade and causes a hypercoagulable state.
Interestingly, although the main anticoagulation system, protein C pathway, is activated, its function decreases continuously with prolonged standing. In fact, despite the increment in the levels of free protein S antigen and protein C activity, the global activity of protein C pathway (as measured by PCAT-NR) decreases remarkably (Figure 3). This reduction might be because of the ongoing increase in factor VIII activity. The increment in factor VIII activity is almost 4-fold compared with the increase in protein C and S levels. This may overwhelm the ability of the protein C pathway to inactivate factor VIIIa, as demonstrated by the continuous decrease in PCAT-NR during standing.34 Factor V activity, which is enhanced during standing, could also contribute to the decrease in PCAT-NR, because factor Va, similar to factor VIIIa, is inactivated by the protein C pathway.35
ProC Global assay has emerged as an important test in the evaluation of the risk of VTE,19,36 recurrent VTE, and gestational complications.37 In the current study, the result of the ProC Global assay was found to be associated with coagulation activation during still standing.
In summary, both endothelial-dependent procoagulant and anticoagulant systems are activated during prolonged still standing. Apparently, although significantly activated, the protein C system fails to counteract completely the ongoing thrombin generation, thereby leading to a hypercoagulable state.
Moreover, other mechanisms could be involved in this orthostatic hypercoagulability. Orthostatic stress causes activation of the sympathetic nervous system, renin-angiotensin system, and increases plasma levels of vasopressin.21 Vasopressin releases vWF via V2 endothelial receptor activation,38 and catecholamines promote the procoagulant state possibly through a β2-adrenergic mechanism,39 yielding a net procoagulant effect. However, the renin-angiotensin system activates both coagulation and fibrinolytic pathways by increasing TF expression and plasma plasminogen activator inhibitor -1 levels.16 These interesting interrelationships deserve a detailed investigation in the future.
Limitations
Our hypothesis was based on previous studies that reasonably stated that orthostatic pressure is definitely increased in lower extremities while standing. Therefore, we did not measure intravascular pressure during standing. Also, we did not perform simultaneous measurements of vWF and TF, the markers of endothelial activation, in both the forearm and leg. These measurements, in addition to the evaluation of other specific markers of endothelium, such as P-selectin, adhesion molecules (vascular cell adhesion molecule and intercellular adhesion molecule), tissue-type plasminogen activator, and plasminogen activator inhibitor-1, could strengthen our findings. In addition, detailed exploration of the fibrinolytic system and the cellular component of the coagulation are warranted in this regard.
Blood-borne TF is believed to be shed into circulation from activated cells, including monocytes, platelets, and endothelium,29 or from the injured surrounding tissue, consequently creating a hypercoagulable state.30,40 The present results demonstrate that orthostatic stress induces an increment in the TF antigen level but do not promote identification of the type or source of the TF antigen. This question might be elucidated in future studies.
Perspectives and Clinical Implications
Prolonged orthostatic stress, ie, still standing, significantly activates the coagulation cascade through endothelial activation, possibly because of an increased shear stress in the lower extremities. In parallel, the main counterregulatory system, protein C pathway, fails to neutralize this ongoing orthostatic thrombogenesis.
The present study suggests that posture should be considered a significant factor affecting coagulation studies, including such tests as PT and aPTT. A simple in vivo method (prolonged still standing) to activate the procoagulant and anticoagulant pathways is proposed. Possible pathophysiological projections of the obtained findings could be further considered. Prolonged recumbency and sitting were shown to activate the coagulation cascade significantly.41 According to the present findings, prolonged standing, but of much shorter duration, produces a similar effect. Therefore, occupations requiring prolonged standing, such as cashiers, military services, and guards, could predispose to increased hypercoagulability. The economy-class syndrome (trans-Atlantic flights), is considered a risk factor for VTE,42 and its pathophysiology remains to be explored. The results of the current study suggest that endothelial activation or injury may contribute to this setting.
|
Acknowledgments |
|---|
Source of Funding
This study was supported by the Yahel Foundation (Israel).
Disclosures
None.
Received February 14, 2008; first decision March 8, 2008; accepted March 21, 2008.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
1. Rosenberg RD, Aird WC. Vascular-bed-specific hemostasis and hypercoagulable states. N Engl J Med. 1999; 340: 1555–1564.
2. Chong BH, Lee SH. Management of thromboembolism in hematologic malignancies. Semin Thromb Hemost. 2007; 33: 435–448.[CrossRef][Medline] [Order article via Infotrieve]
3. Lee AY. Epidemiology and management of venous thromboembolism in patients with cancer. Thromb Res. 2003; 110: 167–172.[CrossRef][Medline] [Order article via Infotrieve]
4. Edelsberg J, Hagiwara M, Taneja C, Oster G. Risk of venous thromboembolism among hospitalized medically ill patients. Am J Health Syst Pharm. 2006; 63: S16–S22.
5. Keenan CR, White RH. Age as a risk factor for venous thromboembolism after major surgery. Curr Opin Pulm Med. 2005; 11: 398–402.[CrossRef][Medline] [Order article via Infotrieve]
6. Brenner B. Thrombophilia and pregnancy complications. Pathophysiol Haemost Thromb. 2006; 35: 28–35.[CrossRef][Medline] [Order article via Infotrieve]
7. Chung I, Lip GY. Virchow’s triad revisited: blood constituents. Pathophysiol Haemost Thromb. 2003; 33: 449–454.[CrossRef][Medline] [Order article via Infotrieve]
8. Hinghofer-Szalkay H, Moser M. Fluid and protein shifts after postural changes in humans. Am J Physiol. 1986; 250: H68–H75.[Medline] [Order article via Infotrieve]
9. Persson PB. Modulation of cardiovascular control mechanisms and their interaction. Physiol Rev. 1996; 76: 193–244.
10. Hagan RD, Diaz FJ, Horvath SM. Plasma volume changes with movement to supine and standing positions. J Appl Physiol. 1978; 45: 414–417.
11. Jacob G, Raj SR, Ketch T, Pavlin B, Biaggioni I, Ertl AC, Robertson D. Postural pseudoanemia: posture-dependent change in hematocrit. Mayo Clin Proc. 2005; 80: 611–614.
12. Johansen LB, Gharib C, Allevard AM, Sigaudo D, Christensen NJ, Videbaek R, Hammerum M, Drummer C, Norsk P. Adaptation of plasma volume in humans to prolonged head-down bed rest. J Gravit Physiol. 1996; 3: 37–41.[Medline] [Order article via Infotrieve]
13. Rowell LB. Neuro-Humoral Adjustment to Orthostasis and Long Term Control. London, United Kingdom: Oxford University Press; 1993.
14. Bassenge E. Endothelial function in different organs. Prog Cardiovasc Dis. 1996; 39: 209–228.[CrossRef][Medline] [Order article via Infotrieve]
15. Felmeden DC, Lip GY. Endothelial function and its assessment. Expert Opin Investig Drugs. 2005; 14: 1319–1336.[CrossRef][Medline] [Order article via Infotrieve]
16. Dielis AW, Smid M, Spronk HM, Hamulyak K, Kroon AA, ten CH, de Leeuw PW. The prothrombotic paradox of hypertension: role of the renin-angiotensin and kallikrein-kinin systems. Hypertension. 2005; 46: 1236–1242.
17. Lin MC, mus-Jacobs F, Chen HH, Parry GC, Mackman N, Shyy JY, Chien S. Shear stress induction of the tissue factor gene. J Clin Invest. 1997; 99: 737–744.[Medline] [Order article via Infotrieve]
18. Feng DL, Murillo J, Jadhav P, McKenna C, Gebara OC, Lipinska I, Muller JE, Tofler GH. Upright posture and maximal exercise increase platelet aggregability and prostacyclin production in healthy male subjects. Br J Sports Med. 1999; 33: 401–404.
19. Sarig G, Aberbach I, Schliamser L, Blumenfeld Z, Brenner B. Evaluation of ProC Global assay in women with a history of venous thromboembolism on hormonal therapy. Thromb Haemost. 2006; 96: 578–583.[Medline] [Order article via Infotrieve]
20. Dati F, Hafner G, Erbes H, Prellwitz W, Kraus M, Niemann F, Noah M, Wagner C. ProC Global: the first functional screening assay for the complete protein C pathway. Clin Chem. 1997; 43: 1719–1723.
21. Jacob G, Ertl AC, Shannon JR, Furlan R, Robertson RM, Robertson D. Effect of standing on neurohumoral responses and plasma volume in healthy subjects. J Appl Physiol. 1998; 84: 914–921.
22. Anderson FA Jr, Spencer FA. Risk factors for venous thromboembolism. Circulation. 2003; 107: I9–I16.[CrossRef][Medline] [Order article via Infotrieve]
23. Wang TJ, Gona P, Larson MG, Tofler GH, Levy D, Newton-Cheh C, Jacques PF, Rifai N, Selhub J, Robins SJ, Benjamin EJ, D'Agostino RB, Vasan RS. Multiple biomarkers for the prediction of first major cardiovascular events and death. N Engl J Med. 2006; 355: 2631–2639.
24. Dormandy JA. Clinical significance of blood viscosity. Ann R Coll Surg Engl. 1970; 47: 211–228.[Medline] [Order article via Infotrieve]
25. Hitosugi M, Niwa M, Takatsu A. Rheologic changes in venous blood during prolonged sitting. Thromb Res. 2000; 100: 409–412.[CrossRef][Medline] [Order article via Infotrieve]
26. Jen CJ, McIntire LV. Characteristics of shear-induced aggregation in whole blood. J Lab Clin Med. 1984; 103: 115–124.[Medline] [Order article via Infotrieve]
27. Mazzolai L, Silacci P, Bouzourene K, Daniel F, Brunner H, Hayoz D. Tissue factor activity is upregulated in human endothelial cells exposed to oscillatory shear stress. Thromb Haemost. 2002; 87: 1062–1068.[Medline] [Order article via Infotrieve]
28. Galbusera M, Zoja C, Donadelli R, Paris S, Morigi M, Benigni A, Figliuzzi M, Remuzzi G, Remuzzi A. Fluid shear stress modulates von Willebrand factor release from human vascular endothelium. Blood. 1997; 90: 1558–1564.
29. Kagawa H, Komiyama Y, Nakamura S, Miyake T, Miyazaki Y, Hamamoto K, Masuda M, Takahashi H, Nomura S, Fukuhara S. Expression of functional tissue factor on small vesicles of lipopolysaccharide-stimulated human vascular endothelial cells. Thromb Res. 1998; 91: 297–304.[CrossRef][Medline] [Order article via Infotrieve]
30. Osterud B, Bjorklid E. Sources of tissue factor. Semin Thromb Hemost. 2006; 32: 11–23.[CrossRef][Medline] [Order article via Infotrieve]
31. Ovanesov MV, Ananyeva NM, Panteleev MA, Ataullakhanov FI, Saenko EL. Initiation and propagation of coagulation from tissue factor-bearing cell monolayers to plasma: initiator cells do not regulate spatial growth rate. J Thromb Haemost. 2005; 3: 321–331.[CrossRef][Medline] [Order article via Infotrieve]
32. Panteleev MA, Ovanesov MV, Kireev DA, Shibeko AM, Sinauridze EI, Ananyeva NM, Butylin AA, Saenko EL, Ataullakhanov FI. Spatial propagation and localization of blood coagulation are regulated by intrinsic and protein C pathways, respectively. Biophys J. 2006; 90: 1489–1500.[CrossRef][Medline] [Order article via Infotrieve]
33. Reddy NM, Hall SW, MacKintosh FR. Partial thromboplastin time: prediction of adverse events and poor prognosis by low abnormal values. Arch Intern Med. 1999; 159: 2706–2710.
34. Toulon P, Adda R, Perez P. Sensitivity of the ProC global assay for protein C pathway abnormalities. Clinical experience in 899 unselected patients with venous thromboembolism. Thromb Res. 2001; 104: 93–103.[CrossRef][Medline] [Order article via Infotrieve]
35. Hoffman M, Monroe DM III. A cell-based model of hemostasis. Thromb Haemost. 2001; 85: 958–965.[Medline] [Order article via Infotrieve]
36. Toulon P, Perez P, Rapp J, Adda R. An abnormal ProC Global test result is associated with an increased risk of venous thromboembolism independent of test sensitivity for protein C pathway abnormalities. Thromb Haemost. 2007; 98: 228–233.[Medline] [Order article via Infotrieve]
37. Sarig G, Lanir N, Hoffman R, Brenner B. Protein C global assay in the evaluation of women with idiopathic pregnancy loss. Thromb Haemost. 2002; 88: 32–36.[Medline] [Order article via Infotrieve]
38. Kaufmann JE, Oksche A, Wollheim CB, Gunther G, Rosenthal W, Vischer UM. Vasopressin-induced von Willebrand factor secretion from endothelial cells involves V2 receptors and cAMP. J Clin Invest. 2000; 106: 107–116.[Medline] [Order article via Infotrieve]
39. Wirtz PH, Ehlert U, Emini L, Rudisuli K, Groessbauer S, Mausbach BT, von KR. The role of stress hormones in the relationship between resting blood pressure and coagulation activity. J Hypertens. 2006; 24: 2409–2416.[Medline] [Order article via Infotrieve]
40. Hathcock JJ, Hall CL, Turitto VT. Active tissue factor shed from human arterial smooth muscle cells adheres to artificial surfaces. J Biomater Sci Polym Ed. 2000; 11: 1211–1225.[CrossRef][Medline] [Order article via Infotrieve]
41. Schobersberger W, Mittermayr M, Innerhofer P, Sumann G, Schobersberger B, Klingler A, Simmer M, Streif W, Fischbach U, Fries D. Coagulation changes and edema formation during long-distance bus travel. Blood Coagul Fibrinolysis. 2004; 15: 419–425.[CrossRef][Medline] [Order article via Infotrieve]
42. Lapostolle F, Surget V, Borron SW, Desmaizieres M, Sordelet D, Lapandry C, Cupa M, Adnet F. Severe pulmonary embolism associated with air travel. N Engl J Med. 2001; 345: 779–783.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||