Use of different exhaled nitric oxide multiple flow rate models in COPD

Multiple flow rates of exhaled nitric oxide (eNO) fraction (F(e,NO)) data can be modelled to estimate airway wall concentration of nitric oxide (C(aw,NO)), diffusing capacity of nitric oxide (D(aw,NO)), alveolar nitric oxide concentration (C(alv,NO)) and total maximal flux of nitric oxide in the airway compartment (J'(aw,NO)). F(e,NO) at 10, 30, 50, 100 and 200 mL.s(-1) from 50 chronic obstructive pulmonary disease (COPD) patients and 35 healthy controls (smokers and nonsmokers) modelled using five different methods was compared and the effect of the number of flow rates was investigated. All methods showed that current smoking reduced C(aw,NO) in COPD patients, with some methods showing that smoking reduced J'(aw,NO). Smoking did not affect C(alv,NO) or D(aw,NO). The methods gave similar results for C(aw,NO) but there was variability between methods for J'(aw,NO), C(alv,NO) and D(aw,NO). The median error by least squares fitting between modelled and actual data was significantly lower for the nonlinear method (1.96) compared with the mixed methods (3.31 and 3.62). Parameters calculated using the nonlinear method using five and four flow rates were significantly different. The ratio (95% confidence interval) of C(aw,NO) was 2.02 (1.45-2.83). Nitric oxide models give different results, although airway conductance of nitric oxide is relatively model independent. Nonlinear modelling has the least error, suggesting it is the best method. The number of flow rates should be standardised.