A 32-year-old woman presents to the emergency department with complaints of palpitations, dizziness, and shortness of breath that began suddenly two hours ago. She describes the palpitations as rapid and regular, with no identifiable triggers. She reports having experienced similar episodes in the past but notes that this episode lasted longer and was more intense.

Her past medical history is unremarkable, with no history of hypertension, diabetes, or heart disease. She is not taking any regular medications and denies any drug use. There is no family history of cardiac conditions.

On examination, her heart rate is 170 beats per minute, blood pressure is 110/70 mmHg, and she is mildly diaphoretic but otherwise hemodynamically stable. Cardiovascular examination reveals a regular, rapid heart rate without any murmurs or added sounds. Her lung fields are clear on auscultation.

An ECG performed in the emergency department shows a narrow complex tachycardia with a heart rate of 175 beats per minute and no visible P waves, consistent with a diagnosis of supraventricular tachycardia (SVT). After administration of adenosine, the heart rate suddenly decreases to 80 beats per minute, and normal sinus rhythm is restored. The patient experiences immediate relief of her symptoms.

A detailed history and review of the ECG pattern led to a diagnosis of atrioventricular nodal re-entrant tachycardia (AVNRT). The patient is counselled on the benign nature of the condition and is advised on lifestyle modifications and potential triggers. She is referred to cardiology for further evaluation and consideration of long-term management options, including catheter ablation if episodes become frequent or debilitating.

The patient is discharged with follow-up and instructions on how to perform vagal manoeuvres if similar episodes occur in the future.