The prevalence of patent foramen ovale (PFO) is high. As identified at autopsy it is found in approximately 25% of the general population. Anatomically a PFO represents a channel through which unidirectional blood flow from the right to the left atrium may occur. This potential interatrial shunt of unoxygenated venous blood into the oxygenated arterial system may lead to hypoxaemia. Usually right to left shunting across a PFO is transient and without clinical significance. Increased pulmonary arterial pressure may give rise to left-right pressure gradient reversal and right to left shunting across a PFO. High pressure in the right heart chambers, even without pulmonary arterial hypertension, can potentially lead to the reopening of a foramen ovale. In other cases inferior vena cava flow deviation might lead to right to left shunting across a PFO. Right to left shunting without pressure increase inside the right heart chambers is usually transient and even positional and its diagnosis is more difficult.