Some drugs can have effects on the cardiovascular system (hearts and blood vessels) including the QT interval. The QT interval is the time between the contraction and relaxation of the chambers of the heart, called ventricles, which collect and pump blood. The use of some drugs can lead to serious issues such as QT interval prolongation (lengthen) and potentially serious arrhythmias (abnormal heart beat), and sudden death. Therefore, it is important to study effect of medications on the heart.