A 60 year old woman presented to the chest clinic with four years of worsening dyspnoea. On waking she felt unrefreshed and had a headache. She felt sleepy throughout the day. Her medical history included kyphoscoliosis, spina bifida, hypertension, and osteoporosis. The drugs she was taking included amiloride, furosemide, nitrazepam, and calcium-vitamin D supplements. She was born in the United Kingdom, was a lifelong non-smoker, and a retired teacher. She owned no pets and reported no exposure to asbestos.

On physical examination, her respiratory rate was 28 breaths/min and oxygen saturation was 86% on ambient air. Temperature and haemodynamics were normal. General inspection showed marked kyphosis and rapid shallow breathing. She had a global reduction in breath sounds but no crackles, wheeze, or rub. The rest of the examination was unremarkable, with no evidence of right heart failure.

Blood tests showed a raised haemoglobin of 157 g/L (reference range 115-165), but other haematological, biochemical, and inflammatory markers were normal. Arterial blood sampling on ambient air showed pH 7.36, arterial carbon dioxide tension 9.14 kPa, arterial oxygen tension 6.4 kPa, base excess 7.8, and bicarbonate 33 mmol/L. Her body mass index was within the normal range. Figure 1⇓ shows the chest radiograph.