Hypoxia and fractions: an example of how to uncover this uncommon connection

### Learning Point for Clinicians Anatomic shunts such as those associated with congenital heart disease and pulmonary arteriovenous malformations complete the wide differential of hypoxia. When the common causes of hypoxia are ruled unlikely by investigations these serious and treatable connections need to enter the differential by, as this case highlights, calculating a shunt fraction. Pulmonary arteriovenous malformations (PAVM) are rare abnormal communications between arterial and venous vessels. They affect 1 in 15 000–24 000 within a large tertiary centre (The Mayo Clinic, New York) recording 4.5 cases per year.1,2 Despite their rarity they can result in life-threatening complications including cerebral abscess and stroke.3 Diagnosis requires a high clinical suspicion. It is estimated that 70% are associated with hereditary haemorrhagic telangiectasia, which can present …