• Arterial thromboembolic events, defined as an ischemic stroke, transient ischemic attack, or systemic embolis [ Time Frame: Measured throughout the study ] [ Designated as safety issue: Yes ]
  
• Major bleeding, symptomatic bleeding associated with transfusion of more than two units of packed red blood cells or whole blood, or death [ Time Frame: Measured throughout the study ] [ Designated as safety issue: Yes ]
  

• Acute myocardial infarction (with ST- and non-ST-elevation), deep vein thrombosis, or pulmonary embolism [ Time Frame: Measured throughout the study ] [ Designated as safety issue: Yes ]
  
• Minor bleeding [ Time Frame: Measured throughout the study ] [ Designated as safety issue: Yes ]
  

Approximately 2 million people in North America take the anticoagulant warfarin to prevent stroke, heart attack, and other events related to blood clots. Warfarin needs to be stopped before a person undergoes surgery or certain procedures because it can cause dangerous amounts of bleeding during and after surgery. Some doctors give a low molecular weight heparin (LMWH) to patients during the 2-week period when participants are without the effects of warfarin. The LMWH has the same effect as warfarin, but it acts and then leaves the system more quickly than warfarin. However, the LMWH is expensive, may increase the risk of bleeding, and has not been proven effective. This study will determine the safety and efficacy of an LMWH in adults with atrial fibrillation who stop warfarin in preparation for surgery.

Participation in this study will last between 36 and 67 days. Participation will involve nine points of contact with researchers, at least two of which will be in-person visits at the research clinic. The others will be conducted by phone. All points of contact will include assessments on possible bleeding and any new symptoms. The first two of these points of contact, occurring between 3 and 30 days before the scheduled surgery or procedure, will involve a screening of medical records and random assignment of participants to receive either the LMWH dalteparin or placebo. Participants will self-administer a subcutaneous injection of their assigned treatment twice a day for 3 days before the surgery or procedure and for 6 days after. During the course of the study, when participants visit their primary physicians for regularly scheduled appointments, they will also undergo three international normalized ratio (INR) tests of blood clotting ability. The remaining seven points of contact will occur sometime between the day before surgery and 37 days after surgery. One of the in-person visits will occur within the first week after surgery and will include assessments on possible bleeding, any new symptoms, and INR results.