A 22 year old woman presented with a three week history of productive cough, worsening breathlessness, fever, and malaise. Her medical history included an episode of pneumonia, four years earlier, which had been complicated by type 2 respiratory failure requiring mechanical ventilation. She described pronounced orthopnoea, exertional breathlessness, and ankle swelling since then.

She had normal heart sounds, no murmurs, raised jugular venous pulse, and mild pedal oedema. On respiratory examination she had posterior, bibasal, coarse, inspiratory crackles.

On admission, blood pH was 7.08 (normal range 7.35-7.45), arterial carbon dioxide tension 13.8 kPa (4.7-6.0), arterial oxygen tension 15.4 kPa (11.1-14.4), and standard bicarbonate 20 mmol/l (22-27) on high flow supplemental oxygen therapy; she also had leucocytosis. Chest radiography showed small lung fields and bibasal shadowing.

The patient was intubated and treated for presumed community acquired pneumonia. Further investigations included an echocardiogram showing a dilated and impaired right ventricle with an estimated systolic pulmonary artery pressure of 80 mm Hg (normal <25 mm Hg). Computed tomography of the pulmonary arteries showed no evidence of pulmonary emboli; bibasal atelectasis was present, but with no additional mediastinal or parenchymal abnormality. Autoimmune screen, thyroid function tests, and HIV serology were negative. The most striking abnormality on overnight oximetry was precipitous desaturation on lying flat.