123I-mIBG has been shown to be effective in assessing the areas of the heart being activated involuntarily by the sympathetic nervous system (SNS). 123I-mIBG is an iodine based radioisotope that is chemically similar to norepinephrine (NE) in the heart. NE is responsible for the way the SNS regulates heart functions such as heart rate and the force of heart contractions. NE acts automatically to maintain a homeostasis or balance within the SNS. The amount of 123I-mIBG, mimicking NE, that appears on the nuclear image using the heart-to-mediastinum ratio (H/M ratio), was predictive of the progression of heart failure, arrhythmias (irregular heartbeats) and cardiac death.