Nutritional status and airflow obstruction: two independent contributors to CO diffusing capacity impairment in COPD

The association between weight loss and Chronic Obstructive Pulmonary Disease (COPD) has been recognised from many years. Based on the evidence that nutritional status reflects metabolic disturbances in COPD, the relationship between body mass index (BMI), severity of airflow obstruction and CO diffusing capacity (DL(CO)), that is the functional hallmark of emphysema, is relevant to the management of COPD phenotypes.

We reviewed 104 patients with COPD (82 males), aged 66 +/- 9 years (mean +/- SD). Height averaged 165 +/- 8 cm, weight 71 +/- 16 Kg, FEV1 50 +/- 18 (% of predicted), RV 169 +/- 49%, and DL(CO) 56 +/- 26%. Multiple linear regression was performed using BMI, FEV1 and RV, as explanatory variables for DL(CO). Patients were also classified into four groups according to BMI < or = 18.5 (low), > 18.5 and < or = 25 (ideal), > 25 and < or = 30 (overweight), > 30 (obese), and post-bronchodilator FEV1 < 50%. Using this categorisation, a two-factor analysis of variance, testing for interaction and main effects (BMI and FEV1) was performed as confirmatory analysis for the association between BMI (kg/m2), FEV1% and DL(CO)%.

FEV1 and BMI were significantly and independently associated to DL(CO) according to the equation: DL(CO) = -18.32 + 0.65 x FEV1 + 1.59 x BMI (R2 = 0.40, p < 0.0001). The contribution of RV % to DL(CO) % was largely non-significant (p = 0.16). A close relationship was found between BMI (kg/m2) and DL(CO) %, for all of the four BMI groups segregated by post-bronchodilator FEV1 %, (p < .0001). No interaction was found between these two factors (p = 0.30).

Nutritional status as assessed by BMI contributes substantially to impairment of DL(CO) independently of the severity of airflow obstruction. This data confirms the association between emphysematous process and weight loss in advanced COPD, independent of the airflow obstruction severity.