The reason why the patients with PAHassociated with
small defects have an intermediate survival between patients with
idiopathic PAH and patients with PAH and larger uncorrected
defects is unclear. Possible explanations include the favourable prognostic
influence of the small defects, which may allow a
pulmonary-to-systemic shunt in the advanced stages, limiting the
progressive reduction of the systemic output. This mechanism,
which may explain the better survival compared with idiopathic
PAH patients, may not be sufficient to reach the more favourable
prognosis of patients with larger defects and larger shunts.