Patent foramen ovale (PFO) is a common finding in the healthy population, with a prevalence of 27% in one necropsy study of 965 normal hearts from patients with no history of cardioembolic events.¹ It is also the most common cardiac finding in young patients (< 55 years of age) with an unexplained cerebrovascular event, presumably caused by paradoxical emboli.² The presumed mechanism is the migration of a thrombus (or less commonly air or fat) from the venous system to the left atrium via a PFO, with subsequent systemic embolisation. Determining whether paradoxical embolism has occurred through a PFO ideally requires the presence of the PFO “triad”, which combines raised right atrial pressure, venous source of thrombosis, and the presence of PFO. The larger size of a PFO and greater number of microbubbles passing through a shunt during echocardiography has also been associated with an increased incidence of cerebrovascular events^3,4

WHEN TO DIAGNOSE A PFO

Although PFO is becoming increasingly recognised as a cause for cryptogenic cerebrovascular events, there are other situations in which documenting a right to left shunt is important. The presence of a large PFO has been associated with severe unexplained decompression sickness caused by paradoxical gas embolism.⁵ Torti and colleagues showed in 250 scuba divers that the presence of a PFO was related to a low absolute risk of suffering five major decompression illness (DCI) events per 10 000 dives, the odds of which were five times as high as in divers without PFO.⁶ In addition they showed that the risk of suffering …