The influence of the pulsatility of the blood flow on the extent of platelet adhesion

Effect of Chandler loop shear and tubing size on thrombus architecture

Thrombosis can lead to a wide variety of life-threatening circumstances. As current thrombolytic drug screening models often poorly predict drug profiles, leading to failure of thrombolytic therapy or clinical translation, more representative clot substrates are necessary for drug evaluation. Utilizing a Chandler loop device to form clot analogs at high shear has gained popularity in stroke societies. However, shear-dependent clot microstructure has not been fully addressed and low shear conditions are often overlooked. We herein characterized the impact of wall shear rate (126 to 951 s^-1) on clot properties in the Chandler loop. Different revolutions (20-60) per minute and tubing sizes (3.2 to 7.9 mm) were employed to create different sized clots to mimic various thrombosis applications. Increased shear resulted in decreased RBC counts (76.9 ± 4.3% to 17.6 ± 0.9%) and increased fibrin (10 to 60%) based on clot histology. Increased fibrin sheet morphology and platelet aggregates were observed at higher shear under scanning electron microscope. These results show the significant impact of shear and tubing size on resulting clot properties and demonstrate the capability of forming a variety of reproducible in-vivo-like clot analogs in the Chandler loop device controlling for simple parameters to tune clot characteristics.

In vitro flow-based assay: From simple toward more sophisticated models for mimicking hemostasis and thrombosis

In vitro flow-based assays are widely used to investigate the role of platelets and coagulation in hemostasis and thrombosis. Their main advantage over other assays relies on the fact that they integrate blood flow that regulates many aspects of platelet function, including adhesion, activation, and aggregation. Blood flow is also central in the regulation of coagulation through its ability to modulate the local concentrations of coagulation factors within and around thrombi. The most broadly used assay to study thrombus formation consists in perfusing whole blood over immobilized fibrillar collagen through a single channel, which helps to reproduce thrombus formation as it occurs in vivo after vascular injury, with platelets adhering, becoming activated, and forming a mural thrombus. This process can also be studied under conditions of thrombin generation, notably by recalcifying blood collected in sodium citrate. In this manuscript, we briefly discuss the advantages and limits of this broadly used "in vitro thrombus formation model." The main emphasis is on the description of the most recent developments regarding design of new flow models and new techniques, and how these may advance the landscape of in vitro studies into the formation of physiological or pathophysiological thrombi. Challenges linked to mimicking the formation of a hemostatic plug in a healthy vessel or a thrombus in diseased arteries and the complexity of reproducing the various aspects of venous thrombosis are discussed. Future directions are proposed to improve the physiological or pathophysiological relevance of current flow-based assays.

Wall shear rates in human and mouse arteries: Standardization of hemodynamics for in vitro blood flow assays: Communication from the ISTH SSC subcommittee on biorheology

Hemodynamics play a central role in hemostasis and thrombosis by affecting all aspects linked to platelet functions and coagulation. In vitro flow devices are extensively used in basic research, pharmacological studies, antiplatelet agent screening, and development of diagnostic tools. Because hemodynamic conditions vary tremendously throughout the vascular tree and among different (patho)physiological processes, it is important to use flow conditions based on relevant biorheological reference ranges. Surprisingly, it is particularly difficult to find a concise overview of relevant hemodynamic parameters in various human and mouse vessels. To our knowledge, this is the first time an inventory of flow conditions in healthy, non-diseased, human and mouse vessels has been created. The objective of providing such a repertoire is to aid researchers in the field of hemostasis and thrombosis in choosing rheological conditions relevant in in vitro flow experiments and to promote harmonization of flow-based assays to facilitate comparative evaluations between studies. With reference to the human, we discuss relevant similarities and discrepancies in wall shear rates in the mouse, which are typically one order of magnitude greater in agreement with allometric scaling laws between species. Importantly, we bring the attention of the researchers to the fact that the relevant range of average wall shear rates in human arteries where clinically relevant arterial thrombosis occurs may fall as low as 100 to 200 s-1, thus significantly overlapping with what are considered "venous" shear rates. The same range for the murine arteries used for arterial thrombosis models may significantly exceed 1000 s-1 reaching values considered to be "pathological."

Increased erythrocyte adhesion to VCAM-1 during pulsatile flow: Application of a microfluidic flow adhesion bioassay

Sickle cell disease (SCD) is characterized by microvascular occlusion mediated by adhesive interactions of sickle erythrocytes (SSRBCs) to the endothelium. Most in vitro flow adhesion assays measure SSRBC adhesion during continuous flow, although in vivo SSRBC adhesive interactions occur during pulsatile flow. Using a well-plate microfluidic flow adhesion system, we demonstrate that isolated SSRBCs adhere to vascular cell adhesion molecule (VCAM-1) at greater levels during pulsatile versus continuous flow. A significant increase in adhesive interactions was observed between all pulse frequencies 1 Hz to 2 Hz (60-120 beats/min) when compared to non-pulsatile flow. Adhesion of isolated SSRBCs and whole blood during pulsatile flow was unaffected by protein kinase A (PKA) inhibition, and exposure of SSRBCs to pulsatile flow did not affect the intrinsic adhesive properties of SSRBCs. The cell type responsible for increased adhesion of whole blood varied from patient to patient. We conclude that low flow periods of the pulse cycle allow more adhesive interactions between sickle erythrocytes and VCAM-1, and sickle erythrocyte adhesion in the context of whole blood may better reflect physiologic cellular interactions. The microfluidic flow adhesion bioassay used in this study may have applications for clinical assessment of sickle erythrocyte adhesion during pulsatile flow.

Obstacles in haemocompatibility testing

ISO 10993-4 is an international standard describing the methods of testing of medical devices for interactions with blood for regulatory purpose. The complexity of blood responses to biomaterial surfaces and the variability of blood functions in different individuals and species pose difficulties in standardisation. Moreover, in vivo or in vitro testing, as well as the clinical relevance of certain findings, is still matter of debate. This review deals with the major remaining problems, including a brief explanation of surface interactions with blood, the current ISO 10993 requirements for testing, and the role of in vitro test models. The literature is reviewed on anticoagulation, shear rate, blood-air interfaces, incubation time, and the importance of evaluation of the surface area after blood contact. Two test categories deserve further attention: complement and platelet function, including the effects on platelets from adhesion proteins, venipuncture, and animal derived- blood. The material properties, hydrophilicity, and roughness, as well as reference materials, are discussed. Finally this review calls for completing the acceptance criteria in the ISO standard based on a panel of test results.

Brodde M. State of the art in platelet function testing

Platelets perform many functions in hemostasis but also in other areas of physiology and pathology. Therefore, it is obvious that many different function tests have been developed, each one conceived and standardized for a special purpose. This review will summarize the different fields in which platelet function testing is currently in use; diagnostics of patients with bleeding disorders, monitoring patients' response to anti-platelet therapy, monitoring in transfusion medicine (blood donors, platelet concentrates, and after transfusion), and monitoring in perioperative medicine to predict bleeding tendency. The second part of the review outlines different methods for platelet function testing, spanning bleeding time, and platelet counting as well as determining platelet adhesion, platelet secretion, platelet aggregation, platelet morphology, platelet signal transduction, platelet procoagulant activity, platelet apoptosis, platelet proteomics, and molecular biology.

A low-volume, single pass in-vitro system of high shear thrombosis in a stenosis

INTRODUCTION

Arterial thrombosis leading to heart attack and stroke requires the rapid accumulation of millions of platelets under pathologically high shear. Previous in vitro systems studying platelets typically use endpoints that emphasize platelet-surface effects rather than large-scale platelet-platelet accumulation that precedes occlusion. Further, most platelet tests do not recreate shear rates present during arterial occlusion. We present an alternative flow system to study large thrombus formation under pathologic shear conditions in an anatomic stenosis with reasonable volumes of human blood.

MATERIALS AND METHODS

An in-vitro system using a syringe pump was created to subject low volume (<30 mLs), whole blood samples to very high shear rates (>3,500 s(-1)) through a stenosis. Thrombus was quantified using an optical microscope from initial deposition to large scale accumulation. Images were taken using a high definition camera in real time.

RESULTS AND CONCLUSIONS

Occlusive thrombus blocks the collagen-coated lumen with millions of platelets using human whole, heparinized blood. Rapid Platelet Accumulation rates in human blood are 4.5±2.4 μm(3)/μm(2)/min (n=21). There is an initial lag time of 7.4±3.8 min (n=21) before the onset of large scale thrombosis. The rates of platelet accumulation in vitro are consistent with the clinical timescale of coronary or carotid artery occlusion. Porcine blood has a faster accumulation rate of 9.6±6.1 μm(3)/μm(2)/min (n=7, p<0.05) and a shorter lag time of 2.7±0.5 min (n=7, p<0.05). The long lag time for large thrombus formation suggests that some in-vitro assays will miss the main mechanism creating thrombotic occlusion.

Sources of variability in platelet accumulation on type 1 fibrillar collagen in microfluidic flow assays

Microfluidic flow assays (MFA) that measure shear dependent platelet function have potential clinical applications in the diagnosis and treatment of bleeding and thrombotic disorders. As a step towards clinical application, the objective of this study was to measure how phenotypic and genetic factors, as well as experimental conditions, affect the variability of platelet accumulation on type 1 collagen within a MFA. Whole blood was perfused over type 1 fibrillar collagen at wall shear rates of 150, 300, 750 and 1500 s⁻¹ through four independent channels with a height of 50 µm and a width of 500 µm. The accumulation of platelets was characterized by the lag time to 1% platelet surface coverage (Lag_T), the rate of platelet accumulation (V_PLT), and platelet surface coverage (SC). A cohort of normal donors was tested and the results were correlated to plasma von Willebrand factor (VWF) levels, platelet count, hematocrit, sex, and collagen receptors genotypes. VWF levels were the strongest determinant of platelet accumulation. VWF levels were positively correlated to V_PLT and SC at all wall shear rates. A longer Lag_T for platelet accumulation at arterial shear rates compared to venous shear rates was attributed to the time required for plasma proteins to adsorb to collagen. There was no association between platelet accumulation and hematocrit or platelet count. Individuals with the AG genotype of the GP6 gene had lower platelet accumulation than individuals with the AA genotype at 150 s⁻¹ and 300 s⁻¹. Recalcified blood collected into sodium citrate and corn trypsin inhibitor (CTI) resulted in diminished platelet accumulation compared to CTI alone, suggesting that citrate irreversibly diminishes platelet function. This study the largest association study of MFA in healthy donors (n = 104) and will likely set up the basis for the determination of the normal range of platelet responses in this type of assay.

Measurement of whole blood thrombus formation using parallel-plate flow chambers - a practical guide

Custom-made and commercial parallel-plate flow chambers are widely used for studies of platelet activation and thrombus formation in whole blood at defined shear rates. When used in a reproducible way, such flow chamber devices give valuable information on the thrombogenic potential of human, mouse, or rat blood. This article aims to provide a practical guide for the use of parallel-plate flow chambers in combination with routine microscopic imaging techniques. The following methodological aspects are addressed: preparation of surface coatings, calculation of blood flow and shear rate, control of pre-analytical variables, protocols for routine performing of flow chamber tests with non-coagulating or coagulating blood, and procedures for real-time and end-point analysis of thrombus formation. Frequently encountered experimental problems and artifacts are discussed, as well as possibilities for using flow chamber devices as a diagnostic tool to test antithrombotic medication.