<qa>  What is the purpose of this study?The purpose of this study is to study the role of closing volume as a determinant of orthopnea in stable obese subjects. The investigators hypothesized that: 1 increase in closing volume in supine position would be greater in orthopneic than in non-orthopneic subjects, and 2 the relationship of change in closing volume to change in dyspnea with position would be dependent on expiratory flow limitation in the sitting position. 