The extent of fibrinolysis was calculated 3 h after the injection of rt-PA bolus as the ratio between the radioactivity remaining in the vein segment and that originally incorporated in the thrombus. Efforts were made to avoid overestimation of the fibrinolysis, a common pitfall with this animal model. The cotton swabs, syringes, and needles were kept for radioisotope counting. The amount of radioactivity delivered to the clot was calculated by subtracting the radioactivity remaining in the cotton swabs, syringes, and needles from the original amount of radioactivity in the syringe. To avoid overestimation of the fibrinolysis due to liberation into the blood of radioactive fibrinogen not incorporated in the thrombus at the moment of clamp removal, a 1-ml sample was drawn 5 min before and immediately after the removal of the clamps to measure the radioactivity. The amount of fibrinogen not incorporated in the thrombus was evaluated as total blood radioactivity, assuming a theoretical blood volume of 60 ml/kg of body weight. The total blood radioactivity was also subtracted from the original amount of radioactivity in the syringe.
Treatments: Single-chain rt-PA (Actilyse, Boehringer Ingelheim Italia, Florence, Italy) was injected as a bolus (2 min) at doses of 0.2, 0.4, 0.6, and 0.8 mg/kg. Each treatment group consisted of 8 rabbits in both the thrombolysis and the thrombus-growth experiments.