Q: What is the occurance of PFO (Patent foramen Ovale) in general population, and what is it clinical significance

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Answer:  

In general population, PFO is present in 10–20% of adults but usually remain asymptomatic. With advancement of clinical care, and availability of ventilators - these PFOs may become symptomatic under positive pressure as 'shunt' with refractory hypoxia.

Any patient under positive pressure ventilation, whose hypoxia remain unexplained or whose hypoxia get worse with increase in PEEP - should be evaluated for PFO.

Reference:

Patent Foramen Ovale and Mechanical Ventilation - Rev Esp Cardiol. 2010;63:877-8 - Vol. 63 Num.07 (http://www.revespcardiol.org/en/patent-foramen-ovale-and-mechanical/articulo/13154121/)