Cardiovascularmiddle twist
A 24-year-old woman presents with abrupt pounding in her neck and mild breathlessness that began while she was sitting at work. She is alert, warm and normotensive, with a regular pulse of 192/min; the ECG shows narrow QRS complexes at equal intervals and no clearly visible atrial activity between them. There is no chest pain, heart failure or known structural heart disease. Which one of the following should be undertaken first?