You're a part of the advanced life support team, arriving at the sleep lab in the small hours of the morning. Worried staff narrate a story of a morbidly obese sleep apnoea patient who presented for a sleep study and has been observed all night with continuous pulse oximetery.

Prolonged apnoeic pauses were witnessed, with desaturation into the 60s. However, each time the patient snored back into life after a few seconds.

However, recently the saturation probe had become dislodged, and in spite of fiddling with it the sleep lab staff could not get it to read a "realistic" result. A resident had collected an arterial blood gas, "to make sure". The pulse oximeter continued to offer very depressing numbers. Concerned that they may be real, the staff attempted to rouse the patient - and could not.

The resident returned with this blood gas.