Patent foramen ovale in adult life.

Description and prevalence In the fetus venous blood enters the right atrium and is directed into either the right ventricle (predominantly from the superior vena cava) or the left atrium (predominantly from the inferior vena cava, being guided by the eustachian valve across the foramen ovale). At birth the decrease in pulmonary vascular resistance and increase in systemic vascular resistance cause left atrial pressure to exceed right atrial pressure. This draws the valve of the foramen ovale against the surrounding tissue of the secundum atrial septum. The components of the septum usually adhere to seal the foramen within the first year of life but sometimes fusion is incomplete and the foramen ovale persists.' 2 Even if a potential channel between the atria remains, the pressure in the left atrium keeps the flap-like valve of the foramen opposed to the septum. If right atrial pressure exceeds left atrial pressure the valve can be pushed open to create a right-to-left shunt. The shunt volume depends on the pressure gradient and the size of the patent foramen ovale (PFO); both vary from moment to moment. The prevalence of PFOs in necropsy studies was 20-35%.'1 4 Size is measured by finding the largest probe that fits the PFO without stretching the tissue. Formalin fixation of the samples examined may result in underestimation of the true size in vivo. In 1100 consecutive necropsies from all age groups a small probe-patent PFO was found in 29% (mean diameter 2 mm) and a larger pencil-patent PFO was found in 6%.3 Half of the subjects with pencil-patent PFOs were aged less than 6 months. This suggests that a PFO of moderate size is uncommon in adults. Hagen and colleagues studied subjects from each of the first nine decades of life but excluded those aged less than one year.4 The size of the PFO ranged from 1 mm to 19 mm (mean 4 9 mm) and three quarters were 1-6 mm in diameter. Despite the variation within each age group, the mean size of the channel was larger in older subjects. In addition, the prevalence decreased with age (34% in those aged 1-29 years, 25% in those 30-79 years, and 20% in those aged 80 years or more). One explanation is that the foramen ovale may seal completely after the first year of life and that the smaller the channel the more likely it is to seal.