A 2-month-old baby girl was brought into the A&E of her local hospital by her parents. Their main concern was a change in her breathing and a day’s history of feeding less well. She had been born at term and been well until 2 days prior to admission, when she had developed mild coryzal symptoms with a cough. She had not really fed properly on the day of admission and her parents were very concerned. During triage her heart rate was noted to be 270bpm. She was moved to the resuscitation area in A&E for further management. A single-lead ECG showed a narrow-complex tachycardia. The observations in Resus are noted in Table 22.1 with the first blood gas in Table 22.2.