Patent foramen ovale (PFO) is a fairly common, congenital, generally benign hole between the heart's left and right atria (see illustration). While a fetus is developing in utero, the wall separating the left and right atria of the heart develops from the septum primum, which grows up, and septum secundum, which grows down. The septa overlap, creating a sort of trap door (known as the "foramen ovale"), which allows oxygenated blood from the mother's placenta that has entered the fetus' right atrium to pass through to its left atrium. At birth, the baby's lungs expand, and the resulting pressure in the left atrium closes the foramen ovale. Typically, shortly after birth, this former opening fuses shut — but in about 27 percent of babies, it fails to fuse completely and results in a PFO. A PFO often causes no symptoms, and most people who have one are never aware of the fact. PFO is diagnosed by injecting a small amount of air into a vein and observing its passage through the heart using echocardiography. There are two methods of echocardiography. Transthoracic echocardiography (TTE) is easy and noninvasive — it involves simply placing an ultrasound probe on the outer wall of chest — but it detects a PFO in only 10 percent to 18 percent of the population — about half of those who probably have one. Transesophageal echocardiography (TEE) — which involves local anesthesia and intravenous sedation, so the probe can be passed into the esophagus — detects a PFO in 18 percent to 33 percent of the population. However, even though TEE is more sensitive than TTE, there are still many false-negative results with both techniques; a properly conducted TTE may in fact be more reliable than a TEE. One of the most common treatments for PFO is a procedure called transcatheter closure; it involves threading a catheter through the groin and up the femoral vein into the heart, where a device called an occluder is implanted across the PFO. Occluders come in various shapes and forms, but most act like a double umbrella that opens on each side of the atrial wall and seals the hole. With time, tissue grows over the occluder and completely covers its surface. The implantation is performed under local anesthesia and intravenous sedation, and the patient remains conscious. It takes less than an hour and can be performed on an outpatient or one-night-stay basis. Most patients can return to their normal activities in two days, but they must take anticoagulant and/or antiplatelet drugs for three to six months. Other postoperative restrictions include no elective dental care (such as cleanings) for three months, no contact sports for three months and no heavy lifting for one week. A diver who undergoes a transcatheter PFO closure must abstain from diving for three to six months. No data is available on the outcome of PFO closure in divers. But the following outcomes were recorded in patients who underwent PFO closure for the prevention of stroke (note, however, that these patients have underlying medical conditions that may contribute to a greater than average risk of adverse outcomes): - *Efficacy:* Complete closure of the opening was achieved in 95 percent of cases and incomplete closure in 4 to 5 percent of cases; no improvement was shown in only 1 percent of cases. - *Complications:* Overall mortality was less than 1/10th of 1 percent (0.093 percent). The need for a follow-up operation due to an adverse event associated with the device was less than 1 percent (0.83 percent). - *Serious complications:* The incidence of death, stroke, infection, bleeding or blood vessel injury was 0.2 percent; of device movement or dislodgement, 0.25 percent; of clot formation on the device, 0.3 percent; of major complications in the perioperative period, 1.2 percent; and of minor midterm complications, 2.4 percent.