COPD patients with peripheral airway obstruction reversibility identified by exhaled nitric oxide

Besides its role as an inflammatory marker in asthma, fractional exhaled nitric oxide (F_ENO) provides information on the extent of the airway obstruction process through evaluating its change after bronchodilation.

To investigate whether F_ENO change after bronchodilation can identify different sites of airway obstruction in COPD patients.

F_ENO, FEV₁ and the slopes (S) of the alveolar plateau of the single breath washout test (SBWT) were measured in 61 stable COPD patients (FEV₁ 34.5% predicted) before and after the inhalation of 400 μg salbutamol. SBWT used Helium (He), and sulfur-hexafluoride (SF₆). Obstruction relief occurring in pre-acinar and intra-acinar small airways is expected to decrease S_He and S_SF6, respectively. Indices changes (Δ) after bronchodilation were expressed as a percentage of pre-bronchodilation values.

F_ENO stability (∣ΔF_ENO∣ ≤ 11%) was observed in 19 patients [-2.7(6.7)%] [mean (SD)] (NO = group); ΔF_ENO > 11% [+37.4(27.7)%] in 20 patients (NO+ group) and ΔF_ENO < -11% in 22 patients [-31.2(9.8)%] (NO- group). A similar ΔFEV₁ (p = 0.583; [+9.4(9.6)%]) was found in the three groups. In NO = and NO+ groups, neither S_He nor S_SF6 changed; in NO- both S_He [-12.4(27.5)%, p = 0.007] and S_SF6 [-20.2(20.4)%, p < 0.001] significantly decreased.

Different patterns of F_ENO response to β ₂-agonists were observed in COPD most likely depending on the extent of the dilation process. A profile of airway obstruction with an extensive β ₂-agonist response down to lung periphery is identified by F_ENO reduction after acute bronchodilation in 30% of COPD patients. The clinical relevance of this profile requires further investigation.