Alveolar-Membrane Diffusing Capacity Improves in the Morbidly Obese after Bariatric Surgery

Abstract

Morbidly obese individuals may have impaired alveolar-membrane diffusing capacity (DmCO). The purpose of this study was to measure pulmonary diffusing capacity for NO (DLNO) as an index of DmCO pre- and postbariatric surgery in the morbidly obese. Twenty-one patients [age = 40 +/- 9 years, body mass index (BMI) = 48.5 +/- 7.2 kg/m2] with an excess weight of 72 +/- 17 kg scheduled for bariatric surgery were recruited. Pulmonary function and arterial blood-gases were measured pre- and postsurgery. DmCO was 88 +/- 23% of predicted before surgery (p < 0.05). There was loss in BMI and excess weight of 7.7 +/- 2.0 kg/m2 and 31 +/- 8%, respectively. Because DmCO = DLNO/2.42, the increase in DLNO postsurgery resulted in a normalization of the predicted DmCO to 97 +/- 29% predicted, or an improvement of DLNO by 11 +/- 18 (95% CI = 3.5, 19.1; p = 0.01) milliliters per minute per millimeter of mercury without any improvement in DLCO. The DLNO/DLCO ratio and alveolar volume both increased, respectively (p < 0.05), and pulmonary capillary blood volume to DmCO ratio decreased postsurgery (p < 0.01). Multiple linear regression revealed that the change in DLNO was most strongly associated with changes in alveolar volume and the waist-to-hip ratio (adjusted r2 = 0.76; p < 0.001) and was not related to the reduction in the alveolar-to-arterial PO2 difference. Alveolar-membrane diffusion normalizes within 10 weeks after bariatric surgery. This is likely due to the increase in alveolar volume from the reduction in the waist-to-hip ratio.