A significant proportion of exhaled nitric oxide arises in large airways in normal subjects.

Nitric oxide (NO) of endogenous origin is present in exhaled breath. An increase in exhaled NO concentration (ENO) has been described in bronchial asthma and ENO falls after inhaled steroid therapy. The sources of ENO may include pulmonary blood, the gas exchange region, conducting airways and the nasal cavity. In four healthy volunteers, a catheter was placed in a main bronchus after topical anesthesia in order to sample airway NO (CNO). Exhaled nitric oxide of bronchopulmonary and oropharyngeal origin (ENO(b/o)) was measured while excluding nasal NO and was controlled for expiratory flow. During the same exhalation, ENO(b/o) was compared to CNO at multiple sites in the airway as the catheter was progressively withdrawn. Mean CNO concentration in a position corresponding to a main bronchus was 51.4 +/- 10.8% of ENO(b/o). As the catheter was withdrawn, mean CNO concentration progressively increased both in absolute values and as a proportion of ENO(b/o), until in the oropharynx, it was 96.1 +/- 5.2% ENO(b/o). We conclude that a significant proportion of ENO(b/o) arises in the large airways and trachea in normal subjects and contains a minor oropharyngeal component.