Ultrasound enforced thrombolysis

The goal of the proposed study is to improve a thrombolytic drug therapy for patients suffering from peripheral artery occlusions, by means of externally applied high frequency ultrasound. As we have shown in our previous study (FWF project Pi 3288-MED), ultrasound of therapeutic quality significantly enhances in vitro thrombolysis induced by recombinant tissue plasminogen activator (rt-PA). Therefore, when applied simultaneously with rt-PA therapy, ultrasound has the potential to re-establish perfusion in the occluded vessel much faster than rt-PA alone. The benefit of ultrasound-accelerated thrombolysis ("sonothrombolysis") could be gained from reducing the time until reperfusion and from decreasing the amount of thrombolytic agent, which could be advantageous to patients with higher bleeding risks. In this study, 50 patients with peripheral vascular occlusions will be enrolled for receiving an external non-invasive ultrasound treatment in addition to common standardized thrombolytic drug therapy. Sonication will be performed with commonly used ultrasound therapy equipment which will be adapted for sonothrombolysis. Before being applied on human patients, the entire treatment procedure will be tested and optimized on an anthropomorphal ultrasound phantom. Patients will be randomised in the first 6 months and will be examined clinically as well as with duplex-sonography four times in a 12 months follow-up in order to evaluate the effects of ultrasound on clot lysis. The mechanisms by which high frequency ultrasound accelerates thrombolysis are complex and include oscillatory and radiation forces, acoustic streaming, shear stress, bubble associated phenomena and heating. At present, it is not clear which of these mechanisms are essential for acceleration of clot lysis. Therefore, another important goal of this project is to investigate the effects of ultrasound mechanisms underlying the thrombolysis enhancement in order to detect the dominant one(s) and to support it (them) most efficiently by sonication. Thereby, ELISA (Enzyme Linked Immuno Sorbent Assay) technique will be applied for measuring the concentration of fibrin degradation products and fluorescence microscopy for visualizing the spatial distribution of fibrinolytic components in blood clots during thrombolysis. In this study, ultrasound enhanced thrombolysis on human patients will be established. This method is easier to handle than surgical interventions and its application causes lower risks and costs than other treatment strategies.

During the project several studies were performed to elucidate the positive role of ultrasound in pharmacological thrombolysis (Sonothrombolysis): Ultrasound has shown to enhance thrombolysis in addition to fibrinolytic enzymes (t-PA). Referring to a possible in vivo application our in vitro data suggest that an intermittent application of a 2MHz high frequency ultrasound using a traveling wave field would be the most potent application for lysing blood clots. Sonothrombolysis in a stroke model: Recently, three clinical trials revealed encouraging results in recanalization and clinical outcome in acute stroke patients when 2 MHz transcranial Doppler monitoring was applied. By employing an in vitro stroke model, we investigated the potential of a commercial diagnostic ultrasound device to facilitate thrombolysis. Because of the high attenuation of ultrasound by temporal bone, no thrombolytic effect was observed in our in vitro model. The success of Sonothrombolysis is dependent on the sonication mode applied. Clot dissolution, as evaluated by measuring the clot weight loss and the concentration of the fibrin degradation product Ddimer in supernatant, was most successful when ultrasound was delivered in intermittent mode with 50% duty cycle. Ultrasound promotes the penetration of rt-PA into the thrombus. Therefor, we speculate that ultrasound-induced changes in the spatial distribution of fibrinolytic encymes in whole blood clots contribute to the augmenting effect of ultrasound on enzymatic thrombolysis. A review article giving an overview on ultrasound thrombolysis is in preparation. An ultrasound phantom has been developed that mimicks the human thigh in terms of propagation of ultrasound and thermal tissue parameters. Work in progress: The recordings of acoustic streaming on video tapes are examined and the sequences with streaming patterns of interest are digitised. A vector map of the velocities (i.e. a map of flow) will be constructed by means of PIV software. The vibrations of the silicone tubing as investigated by laser vibrometry are analysed. Influence of temperature and ultrasound on t-PA activity.