Ultrasound-mediated catheter delivery of tissue plasminogen activator promotes thrombolysis by altering fibrin fibre thickness and clot permeability.

It has been proposed that low power, high frequency ultrasound can augment the ability of thrombolytic agents to dissolve clot in patients with venous thromboembolism. We created a bench model to examine what role and mechanism ultrasound may have in this process. Fibrin polymerisation was analysed through modified light-scattering experiments with the inclusion of catheter-mediated ultrasound application. We studied fibrin fibre diameters through scanning electron microscopy of ultrasound treated fibrin clots. Clot porosity was investigated using permeation tests, while fibrinolysis was analysed through lightscattering experiments, and by changes in porosity of lysing clots under flow. Application of ultrasound did not change initial fibrin polymerisation but did induce a reversible change in maximal turbidity of already formed fibrin clots. This change in turbidity was caused by a reduction in fibrin fibre diameter and was associated with an increase in clot porosity. These reversible structural changes were associated with a linear increase in fibrinolysis rates under static conditions, while an exponential increase in rates was observed under flow. The use of ultrasound augmentation of thrombolysis enhances clot dissolution through greater and more rapid fibrin degradation. This is due to conformational change created by the ultrasound in clot structure, a reversible phenomenon that may increase binding sites for lytic agent and could potentially allow the use of lower doses and shorter infusion times of ultrasound-assisted thrombolytic to treat venous thromboembolism in-vivo.