We performed a randomized , double-blind , placebo-controlled trial in which , after perioperative interruption of warfarin therapy , patients were randomly assigned to receive bridging anticoagulation therapy with low-molecular-weight heparin ( @ IU of dalteparin per kilogram of body weight ) or matching placebo administered subcutaneously twice daily , from @ days before the procedure until @ hours before the procedure and then for @ to @ days after the procedure .