[Fiona]

<font color='#0000FF'>OK, size isn't everything or so I am told.  Just found this:

How common are PFO's?   A post mortem study from the Mayo Clinic found a prevalence of around 20-25% depending on age. This included very small holes which were described as probe patent, measuring only 1-2 mm in diameter. Studies in normal populations have been hampered by detection methods, and conventional echocardiography detected only 20-50% of cases (as compared with transoesophageal echocardiography, the gold standard). Holes big enough to cause significant paradoxical gas embolism are probably only present in 1-2% of the normal population.