Bolus thrombolysis in venous thromboembolism.
Thrombolytic therapy is rarely used in venous thromboembolism because of the fear of hemorrhagic complications. Preliminary clinical experiences with recombinant tissue-type plasminogen activator (rt-PA) in patients with deep vein thrombosis have shown that even this fibrin-specific plasminogen activator causes an unacceptable rate of hemorrhagic complications. Theoretical considerations and the available experimental and clinical data suggest that infusion of rt-PA over a short period of time would result in a more favorable risk-benefit ratio. Shortening the period of rt-PA infusion results in higher peak plasma levels, thus allowing a higher concentration of the plasminogen activator on the surface and inside the occluding thrombus. In addition, a bolus infusion can prevent or minimize the interaction between rt-PA and the hemostatic system, reducing the likelihood of a systemic lytic state, of a platelet function defect, and, possibly, of bleeding side effects. In venous thromboembolism animal models, the efficacy of bolus rt-PA can be further increased by the adjunctive administration of an effective antithrombotic treatment. This is because the accretion of new fibrin on the thrombi counteracts the lysis of preformed fibrin and influences negatively the final thrombus size. Effective adjunctive antithrombotic treatment includes either high doses of heparin, producing an unclottable activated partial thromboplastin time (aPTT), or doses of recombinant hirudin, doubling the aPTT. When used as an alternative to rt-PA, bolus doses of a hybrid plasminogen activator with prolonged half-life efficiently reduce thrombus size by lysing preformed and newly formed fibrin. Preliminary clinical experience in patients with pulmonary embolism seems to confirm that rt-PA infused as a bolus is at least as effective as, and probably more effective than, rt-PA infused over a longer period.