Here is an excellent discussion (via the Divers Alert Network magazine) of patent foramen ovale and the risk of air embolism from diving, with or without other decompression sickness.
From the discussion:
It is important to keep in mind several issues. First, the relationship between PFO and DCS is an association. While there is a plausible explanation for the apparent connection (i.e., DCS triggered by arterial bubbles), this is not yet proven. Indeed, PFO has not been linked to the most common symptoms such as pain-only bends, numbness, tingling or fatigue. Second, PFO exists in 25 to 30 percent of people, and studies have shown that venous bubbles are extremely common after recreational dives. Therefore, many divers must be experiencing arterial bubbles, yet DCS is extremely rare (especially the more serious variety that is associated with PFO). Third, if the bubble explanation is correct, a PFO could precipitate DCS only after a dive of sufficient depth and duration to generate venous gas embolism. PFO is therefore unlikely to be a factor for mild DCS cases or ones that occur after short exposures or shallow depths (i.e., "undeserved"). Finally, most cases of DCS occur in divers without a PFO. It is difficult or impossible in an individual diver to ascribe a particular DCS occurrence to a PFO, as PFO will exist in nearly one-third of cases even if there is no causal relationship. Therefore, testing for a PFO is useful only in instances where there have been several DCS incidents of a type known to be PFO-associated, and the person cannot modify depth-time exposures or breathing gas.
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