Synchronized cardioversion is recommended for acute treatment in ACHD patients and SVT who are hemodynamically unstable.75,460 See Online Data Supplement 20. A small observational study demonstrated safety and efficacy of synchronized cardioversion in ACHD patients, with sinus rhythm achieved in 94% of Guideline-directed medical therapy supports this treatment approach, stressing that electrical cardioversion is the safest and most effective way of treating SVT associated with hemodynamic instability and should be considered early in the management of such patients.75 Modification of electrode pad placement may be necessary, by using an anterior-posterior configuration in patients with marked atrial enlargement or positioning strategically according to the individual arrhythmia and anatomic substrate, including consideration of the possibility of dextrocardia. I C-LD Intravenous diltiazem or esmolol (with extra caution used for either agent, observing for the development of hypotension) is recommended for acute treatment in ACHD patients and SVT who are hemodynamically stable.461,462 See Online Data Supplement 21. Very limited data exist on the use of calcium channel blocking or beta-blocking medications for rate control or termination of AT or atrial flutter specifically associated with ACHD. Rate control of AT or atrial flutter with rapid AV conduction may be useful to improve hemodynamic status while planning conversion strategies. Patients with congenital heart disease, particularly single-ventricle physiology or systemic right ventricles, may not tolerate ventricular rates >120 bpm for many hours.29 Either drug may be associated with the development of hypotension in up to 20% of patients.462 Because of the high incidence of ventricular dysfunction in this population, particular attention should be given to monitoring the patient for development of hypotension, which would necessitate prompt dose adjustments or change of treatment strategy. I B-NR 4. Intravenous adenosine is recommended for acute treatment in ACHD patients and SVT.207,463 465 See Online Data Supplement 20. 