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.93 Adenosine may also be useful diagnostically, to unmask atrial flutter or AT, but it is uncommon for adenosine to terminate these atrial arrhythmias.91 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 Synchronized cardioversion is recommended for acute treatment in patients with hemodynamically unstable SVT when vagal maneuvers or adenosine are ineffective or not feasible.94 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.94 The 2010 adult ACLS guideline75 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. Figure 3. Atypical AVNRT. Arrows point to the P wave. The reentrant circuit involves anterograde conduction over a fast atrioventricular node pathway, followed by retrograde conduction in a slow atrioventricular node pathway, resulting in a retrograde P wave (negative polarity in inferior leads) with long RP interval. This ECG does not exclude PJRT or a low septal atrial tachycardia, which can appear very similar to this ECG. 