You have been referred a 12-day-old neonate who was brought into her local A&E by her parents an hour ago. The parents have reported a 2-day history of tachypnoea and poor feeding. They feel she is increasingly lethargic and not herself. On admission, the referring registrar reported that she was tachypnoeic (respiration rate 70 breaths per minute) with mild subcostal recession and had been started on wafting oxygen as her initial saturations were 88% in air. She was tachycardic (heart rate 190 bpm) and mottled and felt cool peripherally with mildly prolonged capillary refill time. She was not pyrexial and was neurologically appropriate with a flat fontanelle. Her abdomen was soft with a 2-cm liver palpable.