The Specific Aim #2 of this study is to determine the relationship between 123I-MIBG labeling of sympathetic activity and physiological measures of cardiopulmonary and autonomic function. This aim is to test the hypothesis that impaired cardiac sympathetic activity, determined by 123I-MIBG imaging will be associated with poorer submaximal exercise gas exchange (higher ventilation - carbon dioxide (CO2) slopes, low end tidal CO2, reduced oxygen pulse and a more rapid frequency response) as well as reduced heart rate power spectral frequencies, a blunted response to positional changes and a delayed heart rate recovery.