Diffusing capacity for nitric oxide: Reference values and dependence on alveolar volume

Summary

Nitric oxide (NO) has a much stronger affinity for hemoglobin than carbon monoxide (CO); therefore, the DL_NO (diffusing capacity for NO) is less influenced by changes in capillary blood volume than the DL_CO (diffusing capacity for CO), and represents the true membrane diffusing capacity.

We measured the combined single breath DL_NO/DL_CO in 124 healthy subjects, and generated reference equations for the DL_NO and K_NO. In a subset of 21 subjects the measurements were performed on different inspiratory levels.

The reference equation for DL_NO in females is $53.47*H(\mathrm{height})0.077*A(\mathrm{age})-48.28(\mathrm{RSD}5.22)$ and for males $59.84*H-0.25*A-44.20(\mathrm{RSD}6.39$). Reference equations for K_NO in females is $-2.03*H-0.025*A+11.52(\mathrm{RSD}0.48)$ and for males $-0.15*H-0.045*A+9.47(\mathrm{RSD}0.65)$. The K_CO (DL_CO/V_A) increases when V_A (alveolar volume) decreases, probably due to an increase of blood volume per unit lung volume.

The DL_NO was much stronger related to the V_A, the K_NO was almost independent of V_A. Because of the relative independence of the K_NO on V_A, the K_NO appears to be a much better index for the diffusion capacity per unit lung volume (transfer coefficient) than the K_CO.