Evaluation of the thrombolytic effect of tissue-type plasminogen activator with ultrasonic irradiation: in vitro experiment involving assay of the fibrin degradation products from the clot.

Previous studies show that the combined method using thrombolytic drugs and ultrasonic irradiation enhances the effect of thrombolysis in in vitro and in vivo experiments. However, the mechanism of the clot lysis was unclear. In previous studies the evaluation of the clot lysis effect was determined by the rate of decrease in clot weight or the decrease in radioisotope-labeled fibrinogen in the clot. However, it was unclear whether or not the fibrinogen in the clot was degraded. We measured quantitatively the fibrin degradation product, D-dimer (FDP-DD), produced from clots subjected to recombinant tissue-type plasminogen activator (rt-PA, 2.0 micrograms/ml), with and without ultrasonic irradiation. Two levels of continuous ultrasound were used, 300 kHz (100 V, 0.07 W/cm2) and 1 MHz (20 V, 0.4 W/cm2). The quantity of FDP-DD produced was 1.74 times at 300 kHz and 1.80 times at 1 MHz, greater than that measured in rt-PA solution without ultrasonic irradiation; clot lysis was not observed in normal saline with ultrasonic irradiation. Our experiment clarifies that the mechanism of clot lysis when subjected to a combination of drug and ultrasound leads to degradation of fibrin, allowing a quantitative measurement of the enhancement of clot lysis. A high correlation was observed between the FDP-DD produced from the clots and the rate of decrease in clot weight.