Basal and induced NO formation in the pharyngo-oral tract influences estimates of alveolar NO levels

Abstract

The present study analyzed how models currently used to distinguish alveolar from bronchial contribution to exhaled nitric oxide (NO) are affected by manipulation of NO formation in the pharyngo-oral tract. Exhaled NO was measured at multiple flow rates in 15 healthy subjects in two experiments: 1) measurements at baseline and 5 min after chlorhexidine (CHX) mouthwash and 2) measurements at baseline, 60 min after ingestion of 10 mg NaNO(3)/kg body wt, and 5 min after CHX mouthwash. Alveolar NO concentration (Calv(NO)) and bronchial flux (J'aw(NO)) were calculated by using the slope-intercept model with or without adjustment for trumpet shape of airways and axial diffusion (TMAD). Salivary nitrate and nitrite were measured in the second experiment. Calv(NO) [median (range)] was reduced from 1.16 ppb (0.77, 1.96) at baseline to 0.84 ppb (0.57, 1.48) 5 min after CHX mouthwash (P < 0.001). The TMAD-adjusted Calv(NO) value after CHX mouthwash was 0.50 ppb (0, 0.85). The nitrate load increased J'aw(NO) from 32.2 nl/min (12.2, 60.3) to 57.1 nl/min (22.0, 119) in all subjects and Calv(NO) from 1.47 ppb (0.73, 1.95) to 1.87 ppb (10.85, 7.20) in subjects with high nitrate turnover (>10-fold increase of salivary nitrite after nitrate load). CHX mouthwash reduced Calv(NO) levels to 1.15 ppb (0.72, 2.07) in these subjects with high nitrate turnover. All these results remained consistent after TMAD adjustment. We conclude that estimated alveolar NO concentration is affected by pharyngo-oral tract production of NO in healthy subjects, with a decrease after CHX mouthwash. Moreover, unknown ingestion of dietary nitrate could significantly increase estimated alveolar NO in subjects with high nitrate turnover, and this might be falsely interpreted as a sign of peripheral inflammation. These findings were robust for TMAD.