It is effective in suppressing VA related to acquired long QT syndrome. However, 2 RCTs that investigated the use of intravenous magnesium in patients with cardiac arrest and refractory VF found no In a study of out-of-hospital car- diac arrest, administration of 2 to 4 g magnesium intravenously did not improve survival to hospital In a similar study involving inpa- tient cardiac arrest, magnesium did not improve return of spontaneous circulation, survival to 24 hours, or survival to hospital discharge.S6-28 There are exceptions such as marked hypokale- mia or medication-induced torsades de pointes in which administration of intravenous magne- sium is warranted. 14. Several studies have tested the hypothesis that prophylactic administration of antiarrhythmic medications could reduce the incidence of post- MI VA and lead to better outcomes. One meta- analysis assessed studies in which beta blockers, class I antiarrhythmic agents such as lidocaine and procainamide, and amiodarone were given in the setting of AMI. The routine use of lidocaine and procainamide was associated with increased mor- tality, whereas beta blockers were associated with a significantly lower mortality rate.S6-16 Limited data with amiodarone appeared to be promising, but a subsequent RCT involving 1073 patients found that administration of high-dose amio- darone led to a higher mortality rate, although a moderate dose of amiodarone was not superior 15. With a stable, wide QRS complex tachycardia, differentiation between SVT with aberrancy and VT is often possible by review of the patient s history and the 12-lead ECG during tachycar- dia. Patients with wide QRS complex tachycar- dia and known structural heart disease should be presumed to have VT until proven otherwise. 