• Hypoxia and normal CXR

Pulmonary

• Arteriovenous fistula (eg, idiopathic arteriovenous malformation (AVM), Osler-Weber-Rendu, hepatopulmonary)
• Atelectasis
• Pulmonary edema
• Pneumonia
• Acute respiratory distress syndrome (ARDS)

Cardiac

• Atrial septal defect
• Ventricular septal defect
• Patent foramen ovale
• Patent ductus arteriosus