The current role of thrombolytic therapy in the treatment of acute myocardial infarction

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.

Acceleration of fibrinolysis by high-frequency ultrasound: the contribution of acoustic streaming and temperature rise

High-frequency ultrasound has been shown to accelerate enzymatic fibrinolysis. One of the supposed mechanisms of this effect is the enhancement of mass transport by acoustic streaming, i.e., ultrasound-induced macroscopic flow around the clot. In this study, which is aimed at further elucidating the mechanisms of the acceleration of fibrinolysis by ultrasound, we investigated whether ultrasound would accelerate fibrinolysis if the flow around the thrombus is already present, as may occur in vivo. The effect of the ultrasound-induced temperature rise was also studied. In a model of a plasma clot submerged in plasma, containing tissue-type plasminogen activator, mild stirring of the outer plasma producing a shear rate of 40 seconds(-1) at the surface of the clot resulted in a two-fold acceleration of lysis. A similar effect was obtained with ultrasound (1 MHz, 2 W/cm(2)). Furthermore, if ultrasound was applied together with stirring, only 30% acceleration by ultrasound was documented, fully attributable to the concomitant temperature rise. In a model with tissue-type plasminogen activator incorporated throughout a plasma clot, the effect of ultrasound (two-fold shortening of lysis time) was fully attributable to the concomitant temperature rise of a few degrees. We concluded that the acceleration of enzymatic plasma clot lysis by high-frequency ultrasound in the models we used can be largely explained by a combination of the effects of heating and acoustic streaming, equivalent to mild stirring. The thermal effects can hardly be utilized in vivo due to the danger of tissue overheat. The therapeutic advantage of transcutaneous high-frequency ultrasound as an adjunct to thrombolytic therapy may appear limited to the situations where there is no flow in the direct environment of the thrombus.