Impact of Altered Alveolar Volume on the Diffusing Capacity of the Lung for Carbon Monoxide in Obesity

Abstract

Background: Studies on the diffusing capacity of the lung for carbon monoxide (DL_CO) in obese patients are conflicting, some studies showing increased DL_CO and others unaltered or reduced values in these subjects. Objectives: To compare obese patients to controls, examine the contribution of alveolar volume (VA) and CO transfer coefficient (K_CO) to DL_CO, and calculate DL_CO values adjusted for VA. Methods: We measured body mass index (BMI), waist circumference (WC), spirometry and DL_CO in 98 adult obese patients without cardiopulmonary or smoking history and 48 healthy subjects. All tests were performed in the same laboratory. Results: Using conventional reference values, mean DL_CO and VA were lower (–6%, p < 0.05, and –13%, p < 0.001, respectively), and K_CO was higher (+9%, p < 0.05) in obese patients than in controls. VA decreased whereas K_CO increased with increasing BMI and WC in the obese group. Patients with lower DL_CO had low K_CO in addition to decreased VA. In contrast, some obese patients maintained normal VA, which, coupled with high K_CO, resulted in higher DL_CO. The main result is that diffusion capacity differences between obese patients and controls disappeared using reference equations adjusting DL_CO for VA. Conclusions: Using conventional reference equations, our obese patients show slightly lower mean DL_CO, lower mean VA and higher mean K_CO than controls, but with a large range of DL_CO values and patterns. Adjusting DL_CO for VA suggests that low lung volumes are the main cause of low DL_CO and high K_CO values in obese patients.