A twenty-eight-year-old arrives in the emergency department saying her heart "switched on" while she was sitting at her desk. Not built up — switched on, from one second to the next. She is anxious, feels her chest fluttering, and is otherwise well. The monitor shows a regular narrow-complex rhythm at 188 with no P waves visible anywhere. The abrupt on-off quality is the history worth eliciting, because it separates a re-entry rhythm from the gradual acceleration of a heart responding to fever, pain, blood loss, or anxiety. A sinus tachycardia ramps; this rhythm flips. Supraventricular tachycardia refers to a fast regular rhythm originating at or above the atrioventricular node, in which an impulse circles a self-sustaining electrical loop. It is common in otherwise healthy young adults and is usually not dangerous in a structurally normal heart — but it is uncomfortable, frightening, and in a client with coronary disease or heart failure it can precipitate genuine compromise. The reason this rhythm is a favourite teaching topic is that the treatment is a physiological lever the nurse participates in directly: interrupt...