Postoperative period: Dalteparin or placebo will be administered daily (starting the morning after the procedure), provided surgical hemostasis is achieved, and will be continued for at least four days and until the INR is>2.0. Patients considered at high risk for a postoperative major bleed will be given dalteparin or placebo at a dose of 5,000 IU sc daily. Patients who undergo procedures that are considered low risk for bleeding complications will resume dalteparin or placebo at 200 IU/Kg s.c. daily.