Adenosine can be considered as both a therapeutic and diagnostic agent in narrow-complex tachyarrhythmias. It will acutely terminate AVNRT in approximately 95% of patients and will unmask atrial activity in arrhythmias, such as atrial flutter or AT.91,236 I B-NR Synchronized cardioversion should be performed for acute treatment in hemodynamically unstable patients with AVNRT when adenosine and vagal maneuvers do not terminate the tachycardia or are not feasible.94,208 See Online Data Supplement 10. Sinus rhythm must be promptly restored in patients with AVNRT who are hemodynamically unstable. The safety and effectiveness of cardioversion has been proven in patients with hemodynamically unstable SVT who had failed to convert with vagal maneuvers and intravenous pharmacological therapy.94 I B-NR Synchronized cardioversion is recommended for acute treatment in hemodynamically stable patients with AVNRT when pharmacological therapy does not terminate the tachycardia or is contraindicated.87,95 See Online Data and 9. Synchronized cardioversion is highly effective in terminating SVT (including AVRT and AVNRT).94 Most stable patients with SVT respond to pharmacological therapy, with success rates of 80% to 98% for agents such as verapamil, diltiazem, or adenosine. In some resistant cases, a second drug bolus or higher dose of initial drug agent is often effective.87,96 Nevertheless, in rare instances, drugs may fail to successfully restore sinus rhythm, necessitating synchronized cardioversion. IIa B-R Intravenous beta blockers, diltiazem, or verapamil are reasonable for acute treatment in hemodynamically stable patients with See Online Data Supplement 10. Intravenous diltiazem and verapamil are particularly effective in converting AVNRT to sinus rhythm. These drugs should be used only in hemodynamically stable patients. It is important to ensure the absence of VT or pre-excited AF, because patients with these rhythms may become hemodynamically unstable and develop ventricular fibrillation if administered diltiazem or verapamil. 