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)

All criteria were determined by means of positive contrast-enhanced echocardiography (i.e., microbubble opacification of the left heart chambers three to six cycles after right atrial passage). The abbreviated formula for the alveolar–arterial gradient is as follows:

PAO₂−PaO₂ = (FIO₂ [P_atm–PH₂O] – [PaCO₂/0.8]) – PaO₂,

where PAO₂ denotes partial pressure of alveolar oxygen, PaO₂ partial pressure of arterial oxygen, FIO₂ fraction of inspired oxygen, P_atm atmospheric pressure, PH₂O partial pressure of water vapor at body temperature, and PaCO₂ partial pressure of arterial carbon dioxide (0.8 corresponds to the standard gas-exchange respiratory ratio at rest); the normal range is 4 to 8 mm Hg (0.5 to 1.1 kPa). The normal range for the partial pressure of oxygen is 80 to 100 mm Hg (10.7 to 13.3 kPa) at sea level, while the patient is at rest and breathing ambient air. For patients older than 64 years of age, a value of ≤70 mm Hg (9.3 kPa) for PaO₂ or ≥20 mm Hg for the alveolar-arterial gradient is often used. Ambient air is the respired gas unless otherwise indicated. To convert millimeters of mercury to kilopascals, multiply by 0.133.

References:

1. Rodríguez-Roisin R, Krowka MJ, Hervé P, Fallon MB; ERS Task Force Pulmonary-Hepatic Vascular Disorders (PHD) Scientific Committee. Pulmonary-Hepatic vascular Disorders (PHD). Eur Respir J. 2004 Nov;24(5):861-80. [Medline]

2. Rodriguez-Roisin R, Krowka MJ. Hepatopulmonary Syndrome - A Liver-Induced Lung Vascular Disorder. N Engl J Med. 2008 May 29;358(22): 2378-2387 [Medline]