The aim of this study was to determine whether the nitric oxide (NO) measured in exhaled air is produced at airway or alveolar level. Exhaled NO was measured using a chemiluminescence analyser, and carbon dioxide (CO2 concentration using a Morgan 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 measurement 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 lower than direct levels 84.8 (14.0) ppb (p<0.001). The levels of NO tended to rise to an early peak and plateau, while the CO2 levels continued to rise to peak late in the exhalation. The mean times to reach peak NO levels were 32.2 s (direct) and 23.1 s (T-piece), which were significantly different from that of peak CO2 levels 50.5 s (direct) and 51.4 s (T-piece) (p<0.001). At peak NO level, the simultaneous CO2 level mean 4.9% (SEM 0.14)%, was significantly lower than the peak CO2 reached, 5.8 (0.21)% (p<0.001). We conclude that peak nitric oxide levels are dependent on measurement conditions. There are significant differences between the time to peak of carbon dioxide and nitric oxide. Therefore, most nitric oxide, unlike carbon dioxide, is produced in airways and not at alveolar level.

Nitric oxide (NO) can be measured directly in expired air in adults. The purpose of our study was to measure NO levels in children and to compare these values with adults. Exhaled NO was measured in 39 normal prepubertal children (23 girls), aged 9–11 years (mean, 9.9 years). Exhaled NO was measured by the chemiluminescence method that is sensitive in a range of 2 to 4,000 ppb of NO on an adapted analyzer (Dasibi Environmental). Wearing a nose clip, 5 measurements were recorded in each child with exhalation 1) directly into the NO analyzer (flow rate 240 mL/min) with measurements of NO, carbon dioxide, and mouth pressure; and 2) using a T‐piece to allow measurements at a different flow rate. For all measurements, background NO levels were less than 10 ppb. The mean direct level was 49.6 ppb, SD 37.8 (range, 11.5–197.2 ppb) compared with T‐piece levels of 29.2 ppb, SD 27.1 (range, 5.1–141.2 ppb). There was no significant difference between boys and girls for direct or T‐piece recordings. Mean direct NO in boys was 43.1 ppb, SD 40.5 and in girls 55.2 ppb, SD 35.4, mean T‐piece in boys was 25.6 ppb, SD 29.2, and in girls 33.8 ppb, SD 25.1. Mean NO levels in prepubertal children are lower than in adults and show no difference between males and females. Pediatr Pulmonol. 1996; 22:396–401. © 1996 Wiley‐Liss, Inc