Aim 1: Primary Prevention of Post-operative Cardiac Arrhythmias

1. To evaluate the efficacy of dexmedetomidine vs. the combination of Magnesium Sulfate and dexmedetomidine for the prevention of post-operative cardiac arrhythmias in children and young adults undergoing open heart surgical repair for congenital or acquired heart disease using cardiopulmonary bypass. Dexmedetomidine is currently being administered to almost all patients after coming off cardiopulmonary bypass and in the CICU. For this aim, the investigator will be comparing dexmedetomidine administered alone or in combination with Magnesium Sulfate.
2. To evaluate medication safety based on the frequency of Serious Adverse Events (SAEs) and Adverse Events (AEs)

Aim 2: Secondary treatment of those Patients that develop a clinically significant arrhythmia despite having received either Dexmedetomidine alone or Magnesium Sulfate with Dexmedetomidine at the time of cardiac surgery

1. To evaluate the efficacy of intravenous (IV) Amiodarone vs. IV Procainamide for the control of postoperative cardiac arrhythmias developing after the failure of Stage 1, Preventative Trial. IV amiodarone and IV procainamide are standardly used treatment agents for the treatment of postoperative cardiac arrhythmias in this setting. The investigator will be assessing the comparative effectiveness of these agents in controlling post-operative cardiac arrhythmias.
2. To evaluate antiarrhythmic medication safety based on the frequency of SAEs and AEs

Objective: Postoperative cardiac arrhythmias (POCA) are common in children undergoing congenital heart surgery using the heart-lung machine (cardiopulmonary bypass) and have been reported in 5-38% of patients. The occurrence of arrhythmias frequently delays the patient's postoperative recovery, prolongs Cardiac Intensive Care Unit (CICU) and hospital stay, increases costs for hospital care and increases postoperative morbidity and mortality.

Magnesium Sulfate has been one agent used to reduce the occurrence of postoperative arrhythmias. The investigators recently published a propensity-score matched study of the intra-operative administration of Magnesium Sulfate revealing that as many as one-third of pediatric patients at Children's National Hospital undergoing open-heart surgery using cardiopulmonary bypass have postoperative arrhythmias. The use of intraoperative Magnesium Sulfate was associated with a reduction in the occurrence of postoperative arrhythmias. However, despite the intraoperative use of Magnesium Sulfate there continued to be a fairly high residual occurrence rate of post-operative arrhythmias (total arrhythmia frequency -18%).

Dexmedetomidine has also been showed to reduce the occurrence of POCA and has become the preferred agent used for post-operative sedation and pain control. In several studies reported in the medical literature, dexmedetomidine has been found to decrease the occurrence of POCA, like Magnesium Sulfate.

In this IRB study, the investigator is proposing a clinical study to test if the investigator can further decrease the occurrence of postoperative arrhythmias. The investigator will be studying if Magnesium Sulfate administrated in conjunction with an intravenous anesthetic medication, known as dexmedetomidine, can further decrease the primary occurrence of post-operative arrhythmias. The investigator hypothesizes that the effect achieved with the simultaneous administration of Magnesium Sulfate and dexmedetomidine will be much greater than either medication used alone.

For those patients who fail preventative medical strategies and develop clinical significant post-operative arrhythmias in need of treatment, rescue arrhythmia treatment options include the use of potent anti-arrhythmic medications that can have very dramatic effects on the patient's heart rate and blood pressure (amiodarone or procainamide). Scant published data exists providing us with guidance on the comparative efficacy and safety of these agents in the postoperative cardiac surgical patient. The investigator will compare the efficacy and adverse effects of the two agents in treating clinically significant arrhythmias that occur despite trying to prevent their onset.

Inclusion Criteria:

• 1) Any infant, child, adolescent, or young adult with congenital or acquired heart disease undergoing open heart surgical repair using cardiopulmonary bypass is eligible to be enrolled in this trial.
• 2) No age, gender or ethnic group restrictions.

Exclusion Criteria:

• 1) Non-cardiopulmonary bypass repair of a congenital or acquired cardiac defect.
• 2) Significant pre-operative arrhythmia history
• 3) Past history of adverse effect to any of the study medication agents

Patients will be randomized within each participating center, not by center, into study groups:

Stage 1: Group 1 - Dexmedetomidine vs. Group 2- Magnesium Sulfate + Dexmedetomidine

Stage 2: If clinically significant arrhythmias in need of treatment develop, patients will be randomized to Group 1 - amiodarone vs. procainamide.

Randomization will occur through a module in Redcap.