Although patients may experience side effects, such as chest discomfort, shortness of breath, and flushing, serious adverse effects are rare because of the drug’s very short half-life. Adenosine may also be useful diagnostically, to unmask atrial flutter or AT, but it is uncommon for adenosine to terminate these atrial arrhythmias. It should be administered via proximal IV as a rapid bolus infusion followed by a saline flush. Continuous ECG recording during adenosine administration may help diagnostically and can also distinguish drug failure due to failure to terminate the arrhythmias versus successful termination with immediate arrhythmia reinitiation.
I| B-NR| 3\. Synchronized cardioversion is recommended for acute treatment in patients with hemodynamically unstable SVT when vagal maneuvers or adenosine are ineffective or not feasible.
See Online Data Supplement 3.| Sinus rhythm must be promptly restored in patients with SVT who are hemodynamically unstable. The safety and effectiveness of cardioversion in the prehospital setting was analyzed in a cohort of patients with hemodynamically unstable SVT who had failed to convert with vagal maneuvers and intravenous pharmacological therapy, and cardioversion successfully restored sinus rhythm in all patients. The 2010 adult ACLS guideline advises synchronized cardioversion for any persistent SVT resulting in hypotension, acutely altered mental status, signs of shock, chest pain, or acute heart failure symptoms but recommends that adenosine be considered first if the tachycardia is regular and has a narrow QRS complex.
I| B-NR| 4\. Synchronized cardioversion is recommended for acute treatment in patients with hemodynamically stable SVT when pharmacological therapy is ineffective or contraindicated.,

Background: Symptomatic tachycardia is a common admission diagnosis in the emergency department (ED). This can be a life-threatening condition and requires immediate attention. Supraventricular tachycardia (SVT) is commonly treated with adenosine, and successful treatment is limited to atrioventricular (AV) node-dependent SVTs as adenosine causes a transient heart block. However, there are limited data available for instances when the recommended dosing regimen (6 mg, 12 mg, 12 mg) fails to terminate SVT.

Case Report: A 33-year old man was evaluated in the ED with an electrocardiogram revealing a regular narrow complex tachycardia with a heart rate of 180 beats/min and a rhythm consistent with SVT. He reported experiencing 3 days of fatigue, myalgias, palpitations, and dyspnea on exertion, but was otherwise hemodynamically stable. Attempts at chemical cardioversion with standard doses of adenosine (6 mg, 12 mg, and 12 mg) were given without success. After consultation with the cardiology service, additional doses of 24 mg and then 36 mg of adenosine were administered. The last dose of 36 mg produced sustained conversion and return to a normal sinus rhythm. The patient later underwent radiofrequency ablation of a left-sided orthodromic reciprocating accessory pathway. After 3 months of medical management, the patient had an implantable cardiac defibrillator placed for prevention of sudden cardiac death. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: Each case of SVT demands immediate attention from an emergency physician. It is imperative that providers be aware of the limitations of adenosine and when it may be appropriate to deviate from standard dosing recommendations. This is in addition to collaborating with an expert in cardiac electrophysiology when initial management tactics are not successful.