Airway contribution to alveolar nitric oxide in healthy subjects and stable asthma patients

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Abstract
Alveolar nitric oxide (NO) concentration (FaNO), increasingly considered in asthma, is currently interpreted as a reflection of NO production in the alveoli. Recent modeling studies showed that axial molecular diffusion brings NO molecules from the airways back into the alveolar compartment during exhalation (backdiffusion) and contributes to FaNO. Our objectives in this study were 1) to simulate the impact of backdiffusion on FaNOand to estimate the alveolar concentration actually due to in situ production (FaNO,prod); and 2) to determine actual alveolar production in stable asthma patients with a broad range of NO bronchial productions. A model incorporating convection and diffusion transport and NO sources was used to simulate FaNOand exhaled NO concentration at 50 ml/s expired flow (FeNO) for a range of alveolar and bronchial NO productions. FaNOand FeNOwere measured in 10 healthy subjects (8 men; age 38 ± 14 yr) and in 21 asthma patients with stable asthma [16 men; age 33 ± 13 yr; forced expiratory volume during 1 s (FEV1) = 98.0 ± 11.9%predicted]. The Asthma Control Questionnaire (Juniper EF, Buist AS, Cox FM, Ferrie PJ, King DR. Chest 115: 1265–1270, 1999) assessed asthma control. Simulations predict that, because of backdiffusion, FaNOand FeNOare linearly related. Experimental results confirm this relationship. FaNO,prodmay be derived by FaNO,prod= (FaNO− 0.08·FeNO)/0.92 ( Eq. 1 ). Based on Eq. 1 , FaNO,prodis similar in asthma patients and in healthy subjects. In conclusion, the backdiffusion mechanism is an important determinant of NO alveolar concentration. In stable and unobstructed asthma patients, even with increased bronchial NO production, alveolar production is normal when appropriately corrected for backdiffusion.