Mechanisms of Orthopnea in Stable Obese Patients

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. In stable obese subjects, in sitting and supine positions, the investigators measured Borg dyspnea score, static lung volumes, expiratory flow limitation, and single-breath nitrogen expiration test, from which the investigators determined closing volume and closing capacity, slope of phase III, and opening capacity. Orthopnea was defined as any increase in the Borg score in the supine position from its value in the sitting position