Density-dependent reduction of nitric oxide diffusing capacity after pneumonectomy

Airway lengthening after pneumonectomy (PNX) may increase diffusive resistance to gas mixing (1/DG); the effect is accentuated by increasing acinar gas density but is difficult to detect from lung CO-diffusing capacity (Dl CO). Because lung NO-diffusing capacity (Dl NO) is three- to fivefold that of Dl CO, whereas 1/DG for NO and CO are similar, we hypothesized that a density-dependent fractional reduction would be greater for Dl NO than for Dl CO. We measured Dl NOand Dl CO at two tidal volumes (Vt) and with three background gases [helium (He), nitrogen (N2), and sulfur hexafluoride (SF6)] in immature dogs 3 and 9 mo after right PNX (5 and 11 mo of age). At maturity (11 mo), background gas density had no effect on Dl NO, Dl CO, or Dl NO-to-Dl CO ratio in sham controls. In PNX animals, Dl NO declined 25–50% in SF6 relative to He and N2, and Dl NO/Dl CO declined ∼50% in SF6 relative to He at a Vt of 15 ml/kg, consistent with a significant 1/DG. At 5 mo of age, Dl NO/Dl CO declined 25–45% in SF6 relative to He and N2 in both groups, but Dl CO increased paradoxically in SF6 relative to N2 or He by 20–60%. Findings suggest that SF6, besides increasing 1/DG, may redistribute ventilation and/or enhance acinar penetration of the convective front.