IntroductionThe objective was to assess the validity and the comparability of multiple constant (MCF) and dynamically chan-ging (DCF) flow analyses to calculate alveolar concentration (CalvNo)> maximum conducting airway flux (J’awNo) and airway diffusing capacity (DawNO) of NO.MethodsExhaled NO (FENo) and expiratory flow rate were measured in ten atopic and eight non atopic healthy subjects at multiple constant flows (100, 150, 200, 275 ml/s) and during a single breath maneuver after a 10-second apnea to calculate (CalvjNo, J’awNo)Rwhere R is the corrélation coefficient of the linear régression between NO output and expiratory flow rate (validity criterion of the MCF approach) and (CalvNo, J’awNo, DawNo)Δ100where Δ100 is the ratio ([observed - predicted FENo] / observed FENo) Δ100 ml/s (validity criterion of the DCF approach), respectively.ResultsMCF analysis demonstrated a non linear relationship (R < 0.80) in three subjects (two with atopy). DCF analysis was not valid (Δ 100 > 30%) in six atopic subjects. A good agreement between MCF and DCF analysis was evidenced in the subjects with R > 0.80 and Δ100 < 30% (see figure below).ConclusionModel and/or method do not accurately describe NO exchange in some healthy subjects. When validity criteria are satisfied, MCF and DCF similarly characterize NO exchange parameters.