They inject you with saline that has been jiggled about a bit to introduce bubbles. Then they measure the amount of bubbles that get shunted from your right ventricle through to your left ventricle. AFAIK
I was also worried about the possibility of having a PFO- well was more than worried as my son has had a slight murmur and I wasnt sure if it was a hereditary thing. I went for an HSE medical this week and was told that the only time you will find one is if you get a bend and they do an ultra sound or some other type of scan. Was also told that an ECG wont pick it up. Bit scary aint it when the stats say one if five of us have one.
And then they watch the bubbles go through the flap that is your PFO with an ultrasound.
ECG won't pick it up, otherwise you wouldn't have to drive halfway up the country to see an expert, your GP could do it.
And the stats i was given were 1 in 3.