A 66 year old female presented with a three week history of lethargy and malaise, reduced oral intake, and nausea but no vomiting. Medical history included hypertension, hypercholesterolaemia, and cervical spondylosis, and she was a lifelong smoker. Medications included simvastatin, aspirin, bendroflumethiazide, amlodipine, co-codamol, atenolol, and furosemide. Pulse oximetry showed reduced oxygen saturation (90% on air), but the patient was not tachypnoeic. She had a mild reduction in skin turgor; otherwise, the rest of her examination was unremarkable.

The patient’s electrocardiogram is shown in the figure⇓. In view of the reduced oxygen saturations, an arterial blood gas was performed. This test showed a pH of 7.631 (normal range 7.35-7.45), a pCO₂ of 6.49 kPa (4-6 kPa), a pO₂ of 7.79 kPa (10-13 kPa), a HCO₃ of 59.9 mmol/l (22-26 mmol/l), and a base excess of +35.2 (−2 to +2). …