The PFO is back.  A recent research study published in the Journal of the American College of Cardiology Intervention edition suggests that technical diving exposures produce enough venous bubbles that a large PFO might under those circumstances contribute to decompression sickness. Usual recreational diving exposures (No-D, not deeper than 130 feet) are less likely to produce significant venous bubbles.  Small PFOs do not appear to produce an increased risk.