Origins of breath nitric oxide in humans

Study objectives: Nitric oxide (NO) exists in the human breath, but little is known about its site of origin or enzyme source. The aims of this study were to locate the main site of NO release into human breath and to decide whether the inducible isoform of NO synthase (iNOS) and nasal bacteria contribute to breath NO. Design: Using a chemiluminescence assay, NO levels were measured in air exhaled from the nose, mouth, trachea, and distal airway. The susceptibility of breath NO to treatment with a topical corticosteroid (to inhibit iNOS; intranasal beclomethasone dipropionate for 2 weeks) and with antibiotics (systemic amoxicillin plus clavulanic acid and intranasal bacitracin zinc, 5 to 10 days) was also tested. Participants: Twenty-one healthy subjects, 9 intubated patients, and 7 patients undergoing bronchoscopy. All subjects were nonsmokers free of pneumonia, rhinitis, and bronchitis. Measurements and results: Breath NO levels, collected in the gas sampling bags, were greater (p<0.05) in the nose (25 ± 2 parts per billion [ppb]) than in the mouth (6 ± 1 ppb), trachea (3 ± 1 ppb), or distal airway (1 ± 2 ppb). Similar results were obtained when NO was sampled directly by cannula from nose or mouth during resting breathing. Nasal breath NO signal increased sharply during 30 s of breath-holding. Beclomethasone, but not antibiotics, decreased nasal NO levels without changing oral breath NO. Conclusions: Most NO in normal human breath derives locally from the nose where it can reach high levels during breath-holding. NO is synthesized, at least in part, by a steroid-inhibitable, nonbacterial, NO synthase, presumably iNOS.