IS NITRIC-OXIDE IN EXHALED AIR PRODUCED AT AIRWAY OR ALVEOLAR LEVEL

The aim of this study was to determine whether the nitric oxide (NO) m easured in exhaled air is produced at airway or alveolar level. Exhale d NO was measured using a chemiluminescence analyser, and carbon dioxi de (CO2) concentration using a Morgen capnograph in single exhalations in 12 healthy subjects (mean age 32 yrs; 6 males and 6 females). For each subject, Five exhalations were made directly into the NO analyser and five were made through a T-piece system, which allowed measuremen t of expiratory flow rate. The peak NO levels measured via the T-piece system were 41.2 (SEM 10.8) parts per billion (ppb), significantly lo wer than direct levels 84.8 (14.0) ppb (p<0.001). The levels of NO ten ded to rise to an early peak and plateau, while the CO2 levels continu ed to rise to peak late in the exhalation. The mean times to reach pea k NO levels were 32.2 s (direct) and 23.1 s (T-piece), which were sign ificantly different from that of peak CO2 levels 50.5 s (direct) and 5 1.4 s (T-piece) (p<0.001). At peak NO level, the simultaneous CO2 leve l, mean 4.9% (SEM 0.14)%, was significantly lower than the peak CO2 re ached, 5.8 (0.21)% (p<0.001). We conclude that peak nitric oxide level s are dependent on measurement conditions. There are significant diffe rences between the time to peak of carbon dioxide and nitric oxide. Th erefore, most nitric oxide, unlike carbon dioxide, is produced in airw ays and not at alveolar level.