Impact of altered alveolar volume on the diffusing capacity of the lung for carbon monoxide in obesity

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.

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.

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.

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.

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.