An uncommon cause of acute severe respiratory failure: a case report

In the literature there are some case reports of hypoxemia due to right to left blood shunting [2–4]. Most causes of right to left shunting through a patent foramen ovale are attributed to right atrial pressure exceeding that of the left, forcing open a potentially patent foramen ovale. A patent foramen ovale is present in 25–30% of healthy subjects [5], and is responsible for a right to left shunt when pulmonary hypertension is present. In the absence of pulmonary hypertension there are three main pathophysiologic mechanisms [6]: extrinsic compression of the right atrium (right hydrothorax or a localized pericardial effusion) causing an increase in the right heart pressure; a decrease in the compliance of the right ventricle (due to right ventricle ischemia or after pneumonectomy); and an abnormal anatomic relationship between the vena cava and the atrial septum or an anatomic distortion of the heart with downward displacement of the right atrium due to the enlargement of the thoracic aorta with distortion of the position of the atrial septum relative to caval inflow and favouring interatrial flow.