Nitric oxide diffusing capacity and alveolar microvascular recruitment in sarcoidosis

We measured diffusing capacities for carbon monoxide (DL_co) and nitric oxide, lung volume, and cardiac output by a rebreathing technique at two alveolar O₂ tensions (PA_o2) at rest and exercise. Membrane diffusing capacity for CO (DM_co) and Vc were estimated from DL _co, by the Roughton-Forster (RF) method and also from simultaneous lung diffusing capacity for NO and Dk_co measured at one O₂ tension (modified RF method). Estimates by these methods agreed closely in normal subjects (Tamhane et al., Chest 2001; 120:1850-1856). Using these methods, we studied patients with stages II-III pulmonary sarcoidosis to determine (1) whether the modified RF method accurately estimates DM _co and V_c in parenchymal disease and (2) whether sarcoidosis alters recruitment of diffusing capacity with respect to cardiac output. In patients, DM_co and Vc estimated by the two methods agreed closely. DM_co was disproportionately reduced relative to Vc at any given cardiac output, and the slope of the relationship between DL_co and cardiac output was moderately, though significantly, below normal. We conclude that in sarcoidosls (1) the modified RF method provides comparable estimates of DM_co and Vc as the standard RF method and (2) the limitation to diffusive gas transport resides primarily in the membrane barrier, although recruitment of microvascular reserves is also modestly impaired.