Alexander Evans, Karli Sutton, Selvin Hernandez and Melikhan Tanyeri*
Department of Engineering, Rangos School of Health Sciences, Duquesne University, Pittsburgh, PA, USA
Received: 31 May 2019
Accepted: 10 July 2019
Version of Record Online: 22 July 2019
Evans A, Sutton K, Hernandez S, Tanyeri M (2019) Viscoelastic Hemostatic Assays - A Quest for Holy Grail of Coagulation Monitoring in Trauma Care. J Ann Bioeng 2019 (1): 61-64.
Copyright © 2019 Melikhan Tanyeri et al. This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and work is properly cited.
Viscoelastic Hemostatic Assays (VHAs) are whole blood tests which monitor all phases of coagulation by measuring viscoelastic properties of blood during clot formation and degradation to help determine the root cause of bleeding [1-3]. VHAs help evaluate how fibrinogen forms fibrin, the protein responsible for forming clots, and initiates clot formation along with platelets and red blood cells. These tests help diagnose both inherited and acquired bleeding disorders and are used as a diagnostic tool to guide patient-specific transfusion therapy. VHAs can also identify post-traumatic coagulopathies and mechanisms underlying traumatic hemorrhage. Despite their wide potential applications in the medical field, the use of VHAs has been limited to patients with major trauma or undergoing surgical procedures such as cardiovascular surgery and liver transplantation due to their bulky instrumentation and high price tag [3,4]. Here, we provide a brief introduction to VHAs, highlight some of their current biomedical applications, and suggest a potential direction to broaden their impact in trauma care. For an extended review of VHAs and their applications, we refer the reader to excellent reviews [1-6].
The most common versions of VHA are Thromboelastography (TEG) and Thromboelastometry (TEM). Both methods rely on a transducer which measures clot firmness as a function of time. For these measurements, 300μL whole blood sample is placed into a cylindrical cup incubated typically at 37°C (Figure 1a). A suspended pin is inserted into the cup. The pin is not in contact with the cup, and the blood sample provides a physical link between the pin and the cup.
Figure 1: Overview of viscoelastic hemostatic assays. (a) Existing viscoelastic hemostatic assay systems (TEG/TEM). System and assay components consist of (1) a cup containing blood sample (2) coagulation activator (3) suspended rotating pin inserted into the cup (4) electromechanical or optomechanical transducer, and (5) data processing; (b) A typical coagulation trace obtained by viscoelastic hemostatic assays showing various stages of hemostatic process from the initiation of coagulation to fibrinolysis.
Following the addition of coagulation activators, the coagulation process is triggered by oscillating either the cup (TEG) or the pin (TEM) back and forth around the vertical axis. At the beginning of the coagulation process, the torque acting on the pin due to the rotation of the cup is minimal. As the coagulation proceeds, the blood sample starts forming a clot between the cup and the pin, thereby generating a torque proportional to the clot firmness which is transmitted to the pin. The pin is connected to a detector system (a torsion wire in TEG and an optical detector in TEM) which measures the torque applied to the pin. Therefore, during the blood coagulation process, the strength of the formed clot is directly measured via an electromechanical (TEG) or an optomechanical (TEM) transducer [3,6]. As a result, TEG/TEM curves displaying clot firmness as a function of time are obtained.
Using the TEG/TEM curves, a group of parameters representing the characteristics of the coagulation and lysis process are derived [4,5]. Typically, TEG/TEM curves reveal all stages of hemostatic process from the initiation of coagulation to fibrinolysis (Figure 1b). Reaction time (or clotting time), kinetics (or clot formation time), maximum amplitude (or maximal clot firmness), and lysis time are some of the key parameters used in characterizing a TEG/TEM curve. Briefly, reaction time (r-time, typically 15-23 minutes) represents the time of latency, the time from the start of the test to the initial fibrin formation. Kinetics (or K-time, typically 5-10 minutes) represents the time to achieve a certain level of clot strength. The α-angle measures the fibrin build-up and crosslinking speed, thereby assessing the clot formation speed. The maximum amplitude is a function of the dynamic properties of fibrinogen and represents the ultimate strength of the clot before it starts to degrade . For a healthy patient, the TEG/TEM system generates a coagulation trace in the shape of a horizontal champagne flute. Anomalies in the TEG/TEM curves are used to diagnose inherited or acquired bleeding disorders.
Bleeding disorders are routinely diagnosed by a panel of Conventional Coagulation Tests (CCTs) including Hemoglobin (Hb) concentration, hematocrit, platelet count, fibrinogen level, Prothrombin Time (PT), activated Partial Thromboplastin Time (aPTT), International Normalized Ratio (INR), fibrinogen level, and D-dimer . In these tests, blood samples are treated in vitro with an array of activation factors to measure the coagulation time. For instance, Prothrombin Time (PT) is a plasma-based assay which evaluates the extrinsic and common pathways (specifically factors VII, X, V, II and fibrinogen) where calcium and thromboplastin (tissue factor and platelet phospholipids) are added to blood plasma to initiate the fibrin clot formation. Similarly, activated Partial Thromboplastin Time (aPTT) evaluates the intrinsic and common pathways where calcium, platelet phospholipids, and an activator (silica, celite, kaolin or ellagic acid) are added to blood plasma to determine the clotting time in the absence of tissue factors. In both tests, total time to fibrin gel formation is reported; typically, 10-14s for PT and 20-50s for aPTT. Prolongation of the PT indicates defective extrinsic and/or common pathways, whereas prolongation of the aPTT indicates defective intrinsic and/or common pathways .
While CCTs are widely used in clinic to assess blood clotting function, and are extremely useful for identifying and characterizing bleeding disorders of secondary hemostasis, they do not measure the balance of the hemostatic components through all phases, from clot initiation through clot lysis. PT/aPTT do not assess the overall strength and stability of clots as they are measured at the initiation of fibrin polymerization. CCTs in general, and in vitro plasma-based tests such as PT and aPTT in particular, are based on the cascade model of coagulation, and do not take into account the important interactions between molecular and cellular components (e.g., platelets, fibroblasts and clotting factors) in platelet activation and thrombin generation. Furthermore, CCTs provide only endpoint measurements, do not account for the balance between coagulation and fibrinolysis, and therefore, are not sufficient to explain the pathways leading to hemostasis in vivo. For instance, PT/aPTT fail to detect hyperfibrinolysis or platelet dysfunction and is not prolonged until fibrinogen falls to exceedingly low levels [9,10]. In addition, CCTs cannot identify the root cause for certain bleeding disorders, especially induced coagulopathies, such as trauma-induced coagulopathy . CCTs are also limited in detecting hypercoagulability as the shortening of PT/aPTT time is not a consistent predictor of hypercoagulability . Finally, CCTs are not point-of-care assays and the long processing time may lead to treatment delay with associated morbidity and mortality, specifically under time-constrained circumstances such as trauma and surgery [13,14].
Alleviating some of the shortcomings of CCTs in the diagnosis of bleeding disorders, VHAs provide an overview of global hemostatic function by providing information on all phases of hemostasis including initial fibrin formation, fibrin-platelet plug construction, and clot lysis. VHAs are capable of diagnosing not only hypocoagulable state but also hypercoagulable conditions that are not evident with CCTs. As a result, VHAs have proved superior in predicting and diagnosing coagulopathies, guiding transfusion therapy, and found clinical applications in cardiac surgery, liver transplantation and obstetric hemorrhage. In cardiac surgery, where the risk of surgical bleeding or induced coagulopathy is higher, VHAs help identify patients needing a perioperative (mid-operation) or postoperative transfusion therapy . VHAs are more effective than CCTs and lead to cost-effective practices in both trauma  and cardiac surgery patients . For instance, transfusion therapy guided by VHAs reduces the frequency of blood product transfusions (red blood cell and platelet), and major bleeding following cardiac surgery [18,19]. VHAs allow for point-of-care testing which yields results quickly, enabling adaptation of the method in emergency and operating rooms.
While VHAs allow for rapid diagnosis of coagulopathies, thereby reducing overall transfusion requirements, its use in medical procedures is so far limited to a number of applications (transfusion therapy, liver transplant, trauma, and cardiac surgery) due to its high instrumentation cost and benchtop-size equipment. Portable and low-cost devices performing viscoelastic hemostatic assays would significantly facilitate and spread the use of point-of-care hemostatic tests. Miniaturization of VHAs using microfluidics would help reduce the footprint and the overall cost of the analytical system, thereby broadening the potential applications of the VHAs in medical practice beyond emergency and surgical procedures. Furthermore, shrinking device size would enable field-deployable devices and extensively widen the applications of VHAs. Currently, commercial VHA instruments are mainly used in operating rooms and emergency care units. A miniaturized VHA could potentially be utilized by military medics in trauma scenarios and used immediately after an injury to determine the best course of action. This reduces the diagnosis time and increases the effectiveness of treatment for trauma patients. Improvements in blood sample handling, full automation of the assay protocols, simultaneous testing with multiple activators, integrated analysis software, and enhancing the robustness of the device would increase the versatility of VHAs in the biomedical fields.
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