The biorheology subcommittee met to present (I.) three working party reports, and (II.) three new topics for our new working parties, as well as to consider (III.) future directions. Part I reports will be reviewed by our committee for submission to the SSC for publication in Thrombosis and Haemostasis in the coming three months. Part II reports will be prepared for final presentation at the next SSC meeting in Ljubljana in June 1998. Future directions are listed below. Attendance was excellent with up to 100 participants.

1. Working Party Final Reports

   Dr. Jeff Hubbel reported on a theoretical and practical analysis of flow chambers used in biorheological studies of thrombus formation. Model calculations for parallel-plate chambers were calculated for platelet surface deposition at shear rates of 100, 500, and 1500/sec, with emphasis on physical effects of red blood cells for platelet diffusion to surfaces. Theoretical and real data were compared for platelet-collagen deposition. Again, red blood cells dominated the "axial dependence" for platelet deposition on parallel-plate surfaces; the macroembolization process was also analyzed.

   Dr. Steve Hanson reported on the dependence of antithrombotic drug efficacy on flow. He provided an excellent review of antithrombotics targeted at clotting as well as anti-platelet receptors and demonstrated the critical observations showing the dependence of the inhibitory effects of these drugs on the shear rate operating at surface-cell-cell interactions. In particular, von Willebrand factor was shown to be increasingly important with increasing shear rate, with the extent of platelet activation and secretion being important determinants. Effects of different receptor partners were also considered. Controlled blood flow conditions simulating thrombogenic physiologic environments may be particularly useful for understanding drug mechanisms and for predicting drug efficacy.

   Dr. Gordon Lowe presented exhaustive statistical analyses on risk factor parameters for thrombotic vascular events, based on the Edinburgh Artery study (five-year follow-up) of 1500 patients. Plasma viscosity was shown to be an independent predictor for vascular disease, independent of plasma fibrinogen concentration, hematocrit and LP(a). The mechanism(s) underlying the predictive behavior of blood and plasma viscosity elevations remain a scientific challenge.

2. Topics for New Working Parties

   Drs. Y. Ikeda and Y. Cadroy reported on effects of flow on endothelial cell functions: coagulant and fibrinolytic activity (Y.I.), and antithrombotic activity (Y.C.). Dr. Ikeda reviewed use of HUVEC's in a modified cone-plate viscometer to show gene upregulation in thrombomodulin by shear stresses of 18 d/cm² for 20 hrs exposure. Differential effects of shear on changes in tissue factor and tPA by shear, and modulation by cytokines (IL1 and TNF) were presented. Dr. Y. Cadroy introduced the use of parallel perfusion chambers with coverslips coated with ligands like P-selectin or HUVEC, and direct ex vivo blood perfusion led to thrombi measured with distance from perfusion cell entry point. This distance determined the state of "blood activation." Thrombomodulin activity was varied in different ways and correlated with fibrin and thrombus formation evaluated at 50/sec shear rates.

   Drs. Hubbel and Hanson reported on local drug delivery and flow. Dr. Hubell reviewed the theoretical and real observations on use of implanted materials on blood vessels to provide highly permeable "depots" for drugs with a low permeability barrier (cap) on the luminal side. Dr. Hanson predicted and gave preliminary experimental results for using helical catheters (stent-like) to "stream" drugs on the surface of blood vessels. Drugs can thus be targeted at the wall re: thrombosis, with up to 100-1000 times less drug needed than conventionally used via total blood access protocols. Similar results were presented for vascular graft releasing drugs from an integrated drug delivery system.

   Issues relating to studies of blood cell adhesion and aggregation in flow were finally reviewed by five speakers, including shear and platelet testing at bedside.

   Dr. Frojmovic emphasized that the state of activation of platelets used (both preparation and activator-dependent) determined the shear rates needed to support shear-induced aggregation. The state of platelet activation also determined the actual ligands and platelet receptors dominating in these adhesive interactions driving shear-dependent platelet aggregation. Thus, von Willebrand factor secreted onto platelet surfaces mediates aggregation without any externally added ligand for ADP-activated platelets at shear rates from 300-1500/sec, in contrast to shear rates of 60000-10000/sec needed for "resting" platelet shear-mediated aggregation driven by soluble von Willebrand factor. Differential roles for the platelet receptors GPIb and GPIIb/IIIa were also described depending on the increasing shear rates. Dr. Ikeda reviewed "resting" platelets sheared in plasma which he has studied in cone-plate viscometers, identifying the roles of soluble fibrinogen and vWF, and the platelet receptors (GPIb and GPIIb/IIIa). Issues of better definition of platelet activation in shear and effects of shear exposure times were recognized.

   Dr. de Groot demonstrated the critical importance of comparing platelet adhesion to proteins immobilized on surfaces for static versus shear-dependent effects. VWF was unique in supporting platelet deposition continuously with shear rates upwards of 2000/sec. Roles of GPIb and vWF were analyzed, including adhesion of platelets onto different collagens. The use of platelet detachment experiments with increasing shear stresses for analyzing firmness of adhesion onto different protein surfaces was described.

   Dr. Gerard Nash reviewed the flow system used for following neutrophil transient and firm attachment to platelets immobilized on flat glass capillaries expressing P-selectin. Issues of cell preparation quality, shear rate windows analyzed, and nature of surfaces used were carefully addressed. Low platelet surface coverage was seen to provide efficient neutrophil deposition, with rolling and neutrophil activation as prerequisites for firm capture.

   Dr. D. Varon described a new flow device, the cone plate-let analyzer (CPA), which allows quantitation of platelet deposition onto microtiter wells coated with ECM at shear rate of typically 1500/sec. Fresh citrated whole blood (as little as 150 microliter) is added, "stirred" with a cone added to the well for 2 mins, and surface coverage and particle size are then analyzed with image-analysis computer interface (all potentially at bedside). Examples were given for monitoring von Willebrand disease patients for diagnosis and therapy, as well as for monitoring anti-thrombotic drug efficacy in patients with cardiovascular disease.

   Standardization issues were discussed for all the above presentations, including a need to compare fresh blood and different anticoagulants.

3. Future Directions

   1. It was agreed that a new working party would be formed on "rheological bedside devices for monitoring and managing vascular diseases: current status and future needs," with Dr. Frojmovic initiating this.
   2. A need for a party on "standards in rheological studies of cell adhesion" was also recognized, though this will be integrated mostly in all future reports.
   3. An interest in a working party on "studies of von Willebrand factor in cell adhesion in flow" was also strongly expressed.

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