Background: Morbidly obese subjects, who often complain
about breathlessness when lying down, breathe at low lung volume with a
reduced expiratory reserve volume (ERV). Therefore, during tidal
breathing the expiratory flow reserve is decreased, promoting
expiratory flow limitation (EFL), which is more likely to occur in the
supine position, when the relaxation volume of the respiratory system,
and hence the functional residual capacity (FRC), decrease 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 development 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/m2, we assessed EFL in both the seated and the
supine positions by the negative expiratory pressure method and
assessed postural changes in FRC by measuring the variations in the
inspiratory capacity (IC) with recumbency. Simultaneously, dyspnea was
evaluated in either position using the Borg scale dyspnea index (BSDI)
to determine the presence of orthopnea, which was defined as any
increase of the BSDI in the supine position.
Partial EFL was detected in 22% and 59%, respectively, of the overall
population in seated and supine position. The mean increase in the
supine IC amounted to 120 ± 200 mL (4.1 ± 6.4%), indicating a
limited decrease in FRC with recumbency in these subjects. Orthopnea,
although mild (mean BSDI, 1.7 ± 1.3), was claimed by 20 subjects,
and in 15 of them EFL occurred or worsened in the supine position.
Orthopnea was associated with lower 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 apnea-hypopnea
syndrome, FEV1, and forced expiratory flow at 75% of vital
capacity was found on either orthopnea or EFL.
Conclusion: In morbidly obese subjects, EFL and dyspnea
frequently occur with the subject in the supine position, and both
supine EFL and low-seated ERV values are related to orthopnea,
suggesting that dynamic pulmonary hyperinflation and intrinsic positive
end-expiratory pressure may be partly responsible for orthopnea in
massively obese subjects.