Mechanisms of Orthopnea in Stable Obese Subjects

The present study explored the role of closing volume as a determinant of orthopnea in stable obese subjects. We 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, we measured the Borg dyspnea score, static lung volumes, expiratory flow limitation during tidal breathing, and single-breath nitrogen expiration test. From the latter, we 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.

Twenty-one subjects (13 women), median age (interquartile range) 55 (49-57) y and with body mass index of 39 (38-42) kg/m(2) were included, of whom 12 were orthopneic and 11 had expiratory flow limitation while seated. In the sitting position, orthopneic and non-orthopneic subjects were similar for age, body mass index, and pulmonary function tests, including single-breath nitrogen expiration test-derived variables. In the orthopneic subjects, there were no changes in any respiratory variable between positions. In the non-orthopneic subjects, there was a significant decrease in slope of phase III in the supine position from 1.67 (1.33-3.60) to 1.40 (1.25-1.66)%/L (P = .008). Overall, the subjects' Borg score significantly correlated with the slope of phase III (r = 0.63, P = .002) and opening capacity (r = -0.47, P = .03). In 10 subjects without expiratory flow limitation, it correlated with slope of phase III (r = 0.68, P = .03).

In stable obese subjects, magnitude of orthopnea correlated with an increase in the slope of phase III in subjects without expiratory flow limitation. Expiratory flow limitation should be taken into account in obese patients.