Variable

Criterion

Oxygenation defect

Partial pressure of oxygen <80 mm Hg or alveolar–arterial oxygen gradient ?15 mm Hg while breathing ambient air

Pulmonary vascular dilatation

Positive findings on contrast-enhanced echocardiography or abnormal uptake in the brain (>6%) with radioactive lung-perfusion scanning

Liver disease

Portal hypertension (most common) with or without cirrhosis

Degree of severity

Mild

Moderate

Severe

Very severe

* Alveolar–arterial oxygen gradient ?15 mm Hg, partial pressure of oxygen ?80 mm Hg

* Alveolar–arterial oxygen gradient ?15 mm Hg, partial pressure of oxygen ?60 to <80 mm Hg

* Alveolar–arterial oxygen gradient ?15 mm Hg, partial pressure of oxygen ?50 to <60 mm Hg

* Alveolar–arterial oxygen gradient ?15 mm Hg, partial pressure of oxygen <50 mm Hg (<300 mm Hg while the patient is breathing 100% oxygen)