The invention generally relates to medical devices and methods.  More specifically, the invention relates to energy based devices and methods for treatment of patent foramen ovale. 
Fetal blood circulation is much different than adult circulation.  Because fetal blood is oxygenated by the placenta, rather than the fetal lungs, blood is generally shunted away from the lungs to the peripheral tissues through a number of vessels and foramens that remain patent (i.e., open) during fetal life and typically close shortly after birth.  For example, fetal blood passes directly from the right atrium through the foramen ovale into the left atrium, and a portion of blood circulating through the pulmonary artery trunk passes through the ductus arteriosus to the aorta.  This fetal circulation is shown in attached FIG.  1. 
At birth, as a newborn begins breathing, blood pressure in the left atrium rises above the pressure in the right atrium.  In most newborns, a flap of tissue closes the foramen ovale and heals together.  In approximately 20,000 babies born each year in the US, the flap of tissue is missing, and the hole remains open as an atrial septal defect (ASD).  In a much more significant percentage of the population (estimates range from 5% to 20% of the entire population), the flap is present but does not heal together.  This condition is known as a patent foramen ovale (PFO).  Whenever the pressure in the right atrium rises above that in the left atrium, blood pressure can push this patent channel open, allowing blood to flow from the right atrium to the left atrium. 
Patent foramen ovale has long been considered a relatively benign condition, since it typically has little effect on the body's circulation.  More recently, however, it has been found that a significant number of strokes may be caused at least in part by PFO.  In some cases, stroke may occur because a PFO allows blood containing small thrombi to flow directly from the venous circulation to the arterial circulation and into the brain, rather than flowing to the lungs where the thrombi can become trapped and gradually dissolved.  In other cases, thrombi might form in the patent channel of the PFO itself and become dislodged when the pressures cause blood to flow from the right atrium to the left atrium.  It has been estimated that patients with PFOs who have already had cryptogenic strokes have a 4% risk per year of having another stroke. 
Further research is currently being conducted into the link between PFO and stroke.  At the present time, if someone with a PFO has two or more strokes, the healthcare system in the U.S. may reimburse a surgical or other interventional procedure to definitively close the PFO.  It is likely, however, that a more prophylactic approach would be warranted to close PFOs to prevent the prospective occurrence of a stroke.  The cost and potential side-effects and complications of such a procedure must be low, however, since the event rate due to PFOs is relatively low.  In younger patients, for example, PFOs sometimes close by themselves over time without any adverse health effects. 
Another highly prevalent and debilitating condition—chronic migraine headache—has also been linked with PFO.  Although the exact link has not yet been explained, PFO closure has been shown to eliminate or significantly reduce migraine headaches in many patients.  Again, prophylactic PFO closure to treat chronic migraine headaches might be warranted if a relatively non-invasive procedure were available. 
Currently available interventional therapies for PFO are generally fairly invasive and/or have potential drawbacks.  One strategy is simply to close a PFO during open heart surgery for another purpose, such as heart valve surgery.  This can typically be achieved via a simple procedure such as placing a stitch or two across the PFO with vascular suture.  Performing open heart surgery purely to close an asymptomatic PFO or even a very small ASD, however, would be very hard to justify. 
A number of interventional devices for closing PFOs percutaneously have also been proposed and developed.  Most of these devices are the same as or similar to ASD closure devices.  They are typically “clamshell” or “double umbrella” shaped devices which deploy an area of biocompatible metal mesh or fabric (ePTFE or Dacron, for example) on each side of the atrial septum, held together with a central axial element, to cover the PFO.  This umbrella then heals into the atrial septum, with the healing response forming a uniform layer of tissue or “pannus” over the device.  Such devices have been developed, for example, by companies such as Nitinol Medical Technologies, Inc. (Boston, Mass.) and AGA Medical, Inc. (White Bear Lake, Minn.).  U.S. Pat.  No.  6,401,720 describes a method and apparatus for thoracoscopic intracardiac procedures which may be used for treatment of PFO. 
Although available devices may work well in some cases, they also face a number of challenges.  Relatively frequent causes of complications include, for example, improper deployment, device embolization into the circulation and device breakage.  In some instances, a deployed device does not heal into the septal wall completely, leaving an exposed tissue which may itself be a nidus for thrombus formation.  Furthermore, currently available devices are generally complex and expensive to manufacture, making their use for prophylactic treatment of PFO impractical.  Additionally, currently available devices typically close a PFO by placing material on either side of the tunnel of the PFO, compressing and opening the tunnel acutely, until blood clots on the devices and causes flow to stop. 
Research into methods and compositions for tissue welding has been underway for many years.  Of particular interest are technologies developed by McNally et.  al., ( as shown in U.S. Pat.  No.  6,391,049) and Fusion Medical (as shown in U.S. Pat.  Nos.  5,156,613, 5,669,934, 5,824,015 and 5,931,165).  These technologies all disclose energy delivery to tissue solders and patches to join tissue and form anastamoses between arteries, bowel, nerves, etc.  Also of interest are a number of patents by inventor Sinofsky, relating to laser suturing of biological materials (e.g., U.S. Pat.  Nos.  5,725,522, 5,569,239, 5,540,677 and 5,071,417).  None of these disclosures, however, show methods or apparatus suitable for positioning the tissues of the PFO for welding or for delivering the energy to a PFO to be welded. 
Causing thermal trauma to a patent ovale has been described in two patent applications by Stambaugh et al. ( PCT Publication Nos.  WO 99/18870 and WO 99/18871).  The devices and methods described, however, cause trauma to PFO tissues to hopefully eventually cause scar tissue formation which will close the PFO.  Using such devices and methods, the PFO actually remains patent immediately after the procedure and only closes sometime later.  Therefore, a physician may not know whether the treatment has worked until long after the treatment procedure has been performed.  Frequently, scar tissue may fail to form or may form incompletely, resulting in a still patent PFO. 
Therefore, it would be advantageous to have improved methods and apparatus for treating a PFO.  Ideally, such methods and apparatus would help seal the PFO during, immediately after or soon after performing a treatment procedure.  Also ideally, such devices and methods would leave no foreign material (or very little material) in a patient's heart.  Furthermore, such methods and apparatus would preferably be relatively simple to manufacture and use, thus rendering prophylactic treatment of PFO, such as for stroke prevention, a viable option.  At least some of these objectives will be met by the present invention.