What is the initial treatment of choice for a patient presenting with narrow-complex supraventricular tachycardia that does not respond to vagal maneuvers?

The initial treatment of choice for a patient presenting with narrow-complex supraventricular tachycardia (SVT) who does not respond to vagal maneuvers is adenosine 6 mg IV. Adenosine acts as a rapid cardiodepressant and can effectively interrupt reentrant pathways in the atrioventricular (AV) node, which is often the underlying mechanism in SVT. When administered intravenously, adenosine causes a brief, nearly complete block of conduction through the AV node, which can terminate the tachycardia in most cases.

The 6 mg dose is recommended as the initial bolus because it is sufficient to elicit the desired response without significant side effects in a majority of patients. Should the initial dose fail, subsequent doses can be given, typically in a double increment (e.g., 12 mg), but the first step in management remains this low-dose adenosine.

In patients with stable narrow-complex SVT, this rapid-action drug is preferred due to its efficacy and safety profile when compared with other options, making it a cornerstone treatment in the acute management of the condition.