Gregor Tratar

Turbulent axially directed flow of plasma containing rt-PA promotes thrombolysis of non-occlusive whole blood clots in vitro

The rate of thrombolysis markedly decreases after a thrombosed vessel is partly recanalized and the remaining clot poses serious risk for rethrombosis. We studied in vitro how thrombolysis depends on penetration of plasma containing thrombolytic agents - 0.2 micro g/ml rt-PA or 250 IU/ml streptokinase (SK) - and the magnetic resonance contrast agent Gd-DTPA (at 1 mmol/l) into non-occlusive clots under conditions of fast (turbulent) or slow (laminar) axially directed flow. Cylindrical non-retracted (fresh) or retracted (aged) whole blood clots were pierced lengthways and connected to a perfusion system. Dynamical spin-echo MRI was used for measuring the penetration of labeled plasma into clots and for assessing the remaining clot size. In both types of clots fast flow enhanced the penetration of Gd-DTPA-labeled plasma into clots in comparison to slow flow. In non-retracted clots, lysis with rt-PA and to a lesser extent also lysis with SK followed the path of plasma penetration into clots. After 40 minutes of fast axially directed flow rt-PA resulted in almost complete lysis and SK left only about a third of the clot undissolved, whereas with slow flow lysis was much slower (undissolved clot: 86 +/- 5 % with rt-PA and 95 +/- 1 % with SK). In retracted clots, substantial lysis was possible only with rt-PA and rapid flow (53 +/- 28% of the clot undissolved after 60 min), whereas the use of SK or slow flow precluded meaningful lysis. We conclude that rapid (turbulent) axially directed flow of plasma along non-occlusive blood clots causes forceful exchange of serum inside the clot with outer plasma which enhances both fibrin-specific and non-fibrin-specific lysis of fresh clots. Dissolution of non-occlusive retracted (aged) clots occurs only under fibrin-specific conditions combined with adequate transport of rt-PA into clots.

Modelling the effect of laminar axially directed blood flow on the dissolution of non-occlusive blood clots

Axially directed blood plasma flow can significantly accelerate thrombolysis of non-occlusive blood clots. Viscous forces caused by shearing of blood play an essential role in this process, in addition to biochemical fibrinolytic reactions. An analytical mathematical model based on the hypothesis that clot dissolution dynamics is proportional to the power of the flowing blood plasma dissipated along the clot is presented. The model assumes cylindrical non-occlusive blood clots with the flow channel in the centre, in which the flow is assumed to be laminar and flow rate constant at all times during dissolution. Effects of sudden constriction on the flow and its impact on the dissolution rate are also considered. The model was verified experimentally by dynamic magnetic resonance (MR) microscopy of artificial blood clots dissolving in an in vitro circulation system, containing plasma with a magnetic resonance imaging contrast agent and recombinant tissue-type plasminogen activator (rt-PA). Sequences of dynamically acquired 3D low resolution MR images of entire clots and 2D high resolution MR images of clots in the axial cross-section were used to evaluate the dissolution model by fitting it to the experimental data. The experimental data fitted well to the model and confirmed our hypothesis.

Comparison of Plasmin With Recombinant Tissue-Type Plasminogen Activator in Lysis of Cerebral Thromboemboli Retrieved From Patients With Acute Ischemic Stroke

Background and Purpose— Plasmin is a direct-acting thrombolytic with a better safety profile than recombinant tissue-type plasminogen activator (rtPA) in animal models. With the application of retrieval devices for managing acute ischemic stroke, extracted thromboemboli are available for ex vivo examination. We ask whether such thrombi are amenable to plasmin thrombolysis and whether such activity is different with rtPA. Methods— Thromboembolic fragments (total 29) were retrieved from the intracranial carotid artery system of 15 patients with acute ischemic stroke and randomly assigned to ex vivo thrombolysis with plasmin or rtPA. After an initial 2-hour exposure, residual material was exposed to the other agent for an additional 2 hours. Thrombolysis was quantified by change in thrombus area and released D-dimer. Results— Plasmin induced significant ex vivo thrombolysis of cerebral arterial thromboemboli, decreasing area by 45.9%±29.4% and 69.2%±52.5% and inducing median D-dimer release of 108 180 &mgr;g/L (range, 16 780 to 668 050 &mgr;g/L) and 16 905 &mgr;g/L (range, 240 to 403 085 &mgr;g/L) during the first and second 2-hour incubation periods, respectively. These changes were not different from those obtained with rtPA, which decreased area by 34.7%±57.8% (P=0.63) and by 68.4%±26.9% (P=0.97) and induced median D-dimer release of 151 990 &mgr;g/L (range, 9870 to 338 350 &mgr;g/L; P=0.51) and 34 520 &mgr;g/L (range 3794 to 325 400 &mgr;g/L; P=0.19) during the first and second 2-hour incubations. Conclusions— Retrieved human cerebral thromboemboli were amenable to ex vivo lysis by plasmin, the rate and degree of which was not different than that achieved with rtPA.

Risk of Thromboembolic Events in Patients with Non-Valvular Atrial Fibrillation After Dabigatran or Rivaroxaban Discontinuation – Data from the Ljubljana Registry

Background and Aim Interruption of anticoagulant treatment with warfarin or non-vitamin K antagonist oral anticoagulants (NOAC) represents a vulnerable period with an increased risk of thromboembolic events. What is the incidence of thromboembolic events in real-life patients with non-valvular atrial fibrillation treated with NOAC who had a discontinuation or cessation of treatment in comparison to patients on continuous treatment? Patients and Methods Registry data from 866 patients with non-valvular atrial fibrillation, aged 74.3 (SD 9.8) years, with an average CHADS2 score of 2.1 (SD 1.2), who were started on dabigatran or rivaroxaban, were analysed for thromboembolic events and survival. Patients who had temporary or permanent discontinuation of NOAC were compared to patients on continuous NOAC treatment. Results Among 866 patients started on NOAC, 705 were treated without interruption, 84 patients had temporary interruption (69 because of planned invasive procedures, 10 due to bleeding, 5 for other causes) and 77 had permanent cessation of NOAC treatment. In patients without interruptions, the incidence of thromboembolic events was 1.0 (95% CI 0.4–2.1) per 100 patient-years, while in patients with interruption/cessation the rate of thromboembolic events was 21.6 (95% CI 10.3–45.2) per 100 patient-years, p < 0.001. There was a distinct clustering of thromboembolic events in the first weeks of NOAC discontinuation with the median occurring on day 14 (range 1–37 days) after discontinuation. Conclusion Dabigatran and rivaroxaban offered good protection against thromboembolic events during treatment, but interruption of NOAC treatment increased the short-term thromboembolic risk more than 20-fold.

Blood Clot Dissolution Dynamics Simulation during Thrombolytic Therapy

Nonocclusive blood clots only partially fill blood vessels and together with the adjacent vessel wall form a channel through which blood flows at usually much higher velocities than in normal vessels. Our aim was to find a theoretical explanation for the experimentally observed fact that fast flowing blood through the channel has a large effect on the increase of the clot dissolution rate compared to the dissolution rate in the absence of flow. Blood flow through the channel increases transport of dissolution agents to the clot and also exerts large forces to the surface of the clot along the channel. Proposed is a model for clot dissolution which assumes that the clot dissolution rate is proportional to the forces of flowing blood to the surface of the clot multiplied by the average blood velocity. The model has been verified by fitting to experimental magnetic resonance imaging data obtained by dynamical magnetic resonance microscopy of clots dissolved by recombinant tissue plasminogen activator in an artificial blood flow system.

Flow-induced permeation of non-occlusive blood clots: an MRI study and modelling.

The success of clot thrombolysis very much depends on efficient clot permeation with blood plasma carrying the thrombolytic agent. In this paper clot permeation was studied by dynamic magnetic resonance imaging (MRI) on artificial non-occlusive blood clots inserted in an artificial circulation system filled with blood plasma to which an MRI contrast agent was added. The MRI results revealed that clot permeation is much faster and more efficient at the entrance of the flow channel across the clot. Clot permeation with fluid was simulated numerically as well. The simulation was based on numerical solution of Navier–Stokes equations for the flow in the channel and within the clot. The clot was considered as a porous material with known permeability and porosity. Based on the calculated velocity profiles, concentration profiles of fluid in the clot were modelled. These agreed well with the MRI results. The presented model of clot permeation with fluid may also serve as a useful extension to numerical modelling of dissolution of non-occlusive blood clots during thrombolytic therapy.

Laminar Axially Directed Blood Flow Promotes Blood Clot Dissolution: Mathematical Modeling Verified by MR Microscopy

Understandig process of thrombolysis is a key for a corresponding medical treatment. Thrombolysis of nonocclusive blood clots is significantly accelerated by axially directed blood plasma flow. When fast blood flow occurs, the increase of the dissolution rate is too big that it could be explained just by better permeation of the thrombolytic agent into the clot and more efficient biochemical degradation. Viscous forces caused by shearing of blood play an essential role in addition to the known biochemical fibrinolytic reactions. We developed an analytical mathematical model based on a hypothesis that clot dissolution dynamics is proportional to the power of the blood plasma flow dissipating along the clot. The model assumes cylindrical non-occlusive blood clots with centrally placed flow channel and the flow is assumed laminar at a constant rate all times during dissolution. Effects of sudden constriction on the flow and its impact on the dissolution rate are considered as well. The model of clot dissolution was verified experimentally by dynamic magnetic resonance (MR) microscopy in in-vitro circulation system containing plasma with a magnetic resonance imaging contrast agent and recombinant tissue-type plasminogen activator (rt-PA).