The hepatopulmonary syndrome results from the formation of microscopic intrapulmonary arteriovenous dilatations in patients with both chronic, and far less commonly acute
liver failure. The mechanism is unknown but is thought to be due to increased liver production or decreased liver clearance of
vasodilators, possibly involving
nitric oxide. The dilation of these blood vessels causes overperfusion relative to ventilation, leading to
ventilation-perfusion mismatch and hypoxemia. There is an increased
gradient between the
partial pressure of oxygen in the
alveoli of the lung and adjacent arteries (alveolar-arterial [A-a] gradient) while breathing room air. Patients with HPS have platypnea-orthodeoxia syndrome (POS); that is, because intrapulmonary vascular dilations (IPVDs) predominate in the bases of the lungs, standing worsens hypoxemia (orthodeoxia)/dyspnea (platypnea) and the supine position improves oxygenation as blood is redistributed from the bases to the apices. Additionally, late in
cirrhosis, it is common to develop high output failure, which would lead to less time in capillaries per
red blood cell, exacerbating the hypoxemia. ==Diagnosis==