The present invention relates generally to apparatus and methods for the suturing of body lumens.  More particularly, the present invention relates to techniques for percutaneous closure of arterial and venous puncture sites, which are usually accessed through a tissue tract. 
A number of diagnostic and interventional vascular procedures are now performed translumenally.  A catheter is introduced to the vascular system at a convenient access location and guided through the vascular system to a target location using established techniques.  Such procedures require vascular access, which is usually established during the well-known Seldinger technique, as described, for example, in William Grossman's “Cardiac Catheterization and Angioplasty,” 3rd Ed.,  Lea and Febiger, Philadelphia, 1986, incorporated herein by reference.  Vascular access is generally provided through an introducer sheath, which is positioned to extend from outside the patient body into the vascular lumen. 
When vascular access is no longer required, the introducer sheath is removed and bleeding at the puncture site stopped.  One common approach for providing hemostasis (the cessation of bleeding) is to apply external force near and upstream from the puncture site, typically by manual or “digital” compression.  This approach suffers from a number of disadvantages.  It is time consuming, frequently requiring one-half hour or more of compression before hemostasis is assured.  Additionally, such compression techniques rely on clot formation, which can be delayed until anticoagulants used in vascular therapy procedures (such as for heart attacks, stent deployment, non-optical PTCA results, and the like) wear off.  This can take two to four hours, thereby increasing the time required before completion of the compression technique.  The compression procedure is further uncomfortable for the patient and frequently requires analgesics to be tolerable.  Moreover, the application of excessive pressure can at times totally occlude the underlying blood vessel, resulting in ischemia and/or thrombosis.  Following manual compression, the patient typically remains recumbent from four to as much as twelve hours or more under close observation so as to assure continued hemostasis.  During this time renewed bleeding may occur, resulting in blood loss through the tract, hematoma and/or pseudo-aneurysm formation, as well as arteriovenous fistula formation.  These complications may require blood transfusion and/or surgical intervention. 
The incidence of complications from compression-induced hemostasis increases when the size of the introducer sheath grows larger, and/or when the patient is anticoagulated.  It is clear that the compression technique for arterial closure can be risky, and is expensive and onerous to the patient.  Although the risk of complications can be reduced by using highly trained individuals, dedicating such personnel to this task is both expensive and inefficient.  Nonetheless, as the number and efficacy of translumenally performed diagnostic and interventional vascular procedures increases, the number of patients requiring effective hemostasis for a vascular puncture continues to increase. 
To overcome the problems associated with manual compression, the use of bioabsorbable fasteners or sealing bodies to stop bleeding has previously been proposed.  Generally, these approaches rely on the placement of a thrombogenic and bioabsorbable material, such as collagen, at the superficial arterial wall over the puncture site.  While potentially effective, this approach suffers from a number of problems.  It can be difficult to properly locate the interface of the overlying tissue and the adventitial surface of the blood vessel.  Locating the fastener too far from that interface can result in failure to provide hemostasis, and subsequent hematoma and/or pseudo-aneurysm formation.  Conversely, if the sealing body intrudes into the arterial lumen, intravascular clots and/or collagen pieces with thrombus attached can form and embolize downstream, causing vascular occlusion.  Also, thrombus formation on the surface of a sealing body protruding into the lumen can cause a stenosis, which can obstruct normal blood flow.  Other possible complications include infection, as well as adverse reaction to the collagen or other implant. 
A more effective approach for vascular closure has been proposed in U.S. Pat.  Nos.  5,417,699, 5,613,974; and PCT published Patent Application No.  PCT/US96/10271 filed on Jun. 12, 1996, the full disclosures of which are incorporated herein by reference.  A suture-applying device is introduced through the tissue tract with a distal end of the device extending through the vascular puncture.  One or more needles in the device are then used to draw suture through the blood vessel wall on opposite sides of the puncture, and the suture is secured directly over the adventitial surface of the blood vessel wall to provide highly reliable closure. 
While a significant improvement over the use of manual pressure, clamps, and collagen plugs, certain design criteria have been found to be important to successful suturing to achieve vascular closure.  For example, it is highly beneficial to properly direct the needles through the blood vessel wall at a significant distance from the puncture so that the suture is well anchored in the tissue and can provide tight closure.  It is also highly beneficial to insure that the needle deployment takes place when the device is properly positioned relative to the vessel wall.  The ease of deployment and efficacy of the procedure can further be enhanced by reducing the cross-section of that portion of the device which is inserted into the tissue tract and/or the vessel itself, which may also allow closure of the vessel in a relatively short amount of time without imposing excessive injury to the tissue tract or vessel. 
For the above reasons, it would be desirable to provide improved devices, systems, and methods for suturing vascular punctures.  Such devices would have the capability of delivering a pre-tied knot to an incision site.  It would be particularly beneficial if these improved devices provided some or all of the benefits while overcoming one or more of the disadvantages discussed above.