Obesity-related Changes in Diffusing Capacity and Transfer Coefficient of the Lung for Carbon Monoxide and Resulting Patterns of Abnormality across Reference Equations

Rationale In 2017, an American Thoracic Society/European Respiratory Society Task Force report recommended further research on the effects that body mass index (BMI) has on diffusing capacity of the lung for carbon monoxide (Dl CO), the transfer coefficient (Kco), and the alveolar volume (VA). Objectives Our goals were to 1) quantify the magnitude and direction of change to measured and predicted Dl CO values as BMI increases in patients free of cardiopulmonary disease and 2) identify how BMI and obesity-related changes differ by reference set. Methods Using data from a prospective cohort study of service members free of cardiopulmonary disease, we modeled the effect that BMI has on measured values of Dl CO, Kco, and VA, after adjusting for age, sex, hemoglobin (Hgb), and height. We then referenced Dl CO, Kco, and VA to normal values using four different reference equations. Results There were 380 patients with data available for analysis, and 130 had a BMI ⩾ 30 kg/m2 (87.7% class I obesity). After controlling for age, sex, Hgb, and height, increased BMI was significantly associated with Kco (β = 0.09, P < 0.01) and VA (β = −0.15, P < 0.01) but not Dl CO. After adjustment for Hgb, for every 5-kg/m2 increase in BMI, the mean increase in percent predicted (PPD) values ranged from 4.2% to 6.5% and from 5.0% to 7.5% for Dl CO and Kco, respectively; and the mean decrease in VA PPD was 3.2–4.0%. In the presence of obesity (BMI ⩾ 30 kg/m2), the prevalence of Dl CO and Kco abnormalities dropped by 4.1–12.1% and 0.4–16.3%, respectively, across equations, whereas VA abnormalities increased from 7.7% to 9.9%. Eliminating 163 patients with abnormal trans-thoracic echocardiogram (TEE), high-resolution computed tomographic (HRCT) scan, or Hgb altered the magnitude of relationships, but significance was preserved. Conclusions In an otherwise healthy population with predominantly class I obesity and normal TTE, HRCT scan, and Hgb, we found that Kco and VA were more affected by BMI than Dl CO. Increases in PPD values varied across equations and were modest but significant and could change clinical decision making by reducing sensitivity for detecting gas-exchange abnormalities. BMI and obesity had the smallest effect on Global Lung Function Initiative PPD values.