Effect of Weight Loss on Postural Changes in Pulmonary Function in Obese Subjects: A Longitudinal Study.

BACKGROUND Postural changes are known to affect normal lung volumes. A reduction in sitting to supine functional residual capacity (FRC) is well-described in non-obese subjects adopting a supine position. However, postural changes in lung volumes in the obese require further exploration. We aimed to longitudinally address the effects of weight loss on postural changes in lung volumes and pulmonary function in obese subjects. We tested the hypothesis that supine reduction in FRC would be absent in morbid obesity and recovered upon weight loss. METHODS This was a prospective, observational, longitudinal study. Consecutive morbidly obese adults (N = 12, age: 44 ± 14 y, body mass index: 45 ± 5 kg/m(2)) enrolled in a bariatric surgery program were included. Standard pulmonary function tests and blood gas analysis were performed both before and 1 y after surgery. Pulmonary function was assessed in both the sitting and supine position using spirometry and multi-breath helium dilution. Parameters recorded before and after weight loss were compared. The main outcome measure was FRC. RESULTS Ten subjects were retested 1 y after surgery (body mass index: 31 ± 5 kg/m(2)). FRC was not affected by change in posture before surgery. Supine reduction in FRC was observed after weight loss (ΔFRC: -0.6 ± 0.4 L, sitting vs supine, P = .002). Pulmonary gas exchange improved (alveolar-to-arterial oxygen partial pressure difference: -8 ± 11 mm Hg, P = .035). CONCLUSIONS Although postural change in FRC is absent when the morbidly obese adopt a supine position, supine reduction in FRC can be recovered following gastroplasty-induced weight loss, despite residual mild to moderate obesity. This also shows that mild to moderate obesity may affect supine FRC more than morbid obesity. (ClinicalTrials.gov registration NCT02207192.).