Expiratory flow limitation and orthopnea in massively obese subjects

Background: Morbidly obese subjects, who often complain about breathlessnes s when lying down, breathe at low lung volume with a reduced expiratory res erve volume (ERV). Therefore, during tidal breathing the expiratory flow re serve is decreased, promoting expiratory now limitation (EFL), which is mor e Likely to occur in the supine position, when the relaxation volume of the respiratory system, and hence the functional residual capacity (FRC), decr ease because of the gravitational effect of the abdominal contents. Purpose: The aim of the study was to assess EFL and orthopnea in massively obese subjects and to evaluate whether orthopnea was associated with the de velopment of supine EFL. Methods: In 46 healthy obese subjects (18 men) with a mean (+/- SD) age of 44 +/- 11 years and a mean body mass index (BMI) of 51 +/- 9 kg/m(2), we as sessed EFL in both the seated and the supine positions by the negative expi ratory pressure method and assessed postural changes in FRC by measuring th e variations in the inspiratory capacity (IC) with recumbency. Simultaneous ly, dyspnea was evaluated in either position using the Borg scale dyspnea i ndex (BSDI) to determine the presence of orthopnea, which was defined as an y increase of the BSDI in the supine position. Results: Partial EFL was detected in 22% and 59%, respectively, of the over all population in seated and supine position. The mean increase in the supi ne IC amounted to 120 +/- 200 mt (4.1 +/- 6.4%), indicating a limited decre ase in FRC with recumbency in these subjects. Orthopnea, although mild (mea n BSDI, 1.7 +/- 1.3), was claimed by 20 subjects, and in 15 of them EFL occ urred or worsened in the supine position. Orthopnea was associated with low er values of seated ERV (p < 0.05) and was marginally related to supine EFL values (p = 0.07). No significant effect of age, BMI, obstructive sleep ap nea-hypopnea syndrome, FEV1, and forced expiratory flow at 75% of vital cap acity was found on either orthopnea or EFL. Conclusion: In morbidly obese subjects, EFL and dyspnea frequently occur wi th the subject in the supine position, and both supine EFL and low-seated E RV values are related to orthopnea, suggesting that dynamic pulmonary hyper inflation and intrinsic positive end-expiratory pressure may be partly resp onsible for orthopnea in massively obese subjects.