BACKGROUND Expiratory flow rate, soft palate closure, and dead space air may influence exhaled levels of nitric oxide (NO). These factors have not been evaluated in the reservoir collection of NO.

RESULTS NO collected in a reservoir containing silica gel was stable for 24 hours. Nasally delivered 4.8% argon measured by mass spectrometry did not contaminate exhaled argon levels (0.1 (0.02)%) in five volunteers during exhalation against a resistance (10 (0.5) cmH₂O), hence proving an effective soft palate closure. Exhaled NO in the reservoir was 11 (0.2) ppb, 8.6 (0.1) ppb, 7.1 (0.6) ppb, and 6.6 (0.4) ppb in five normal subjects and 48.3 (18) ppb, 20.3 (12) ppb, 16.9 (0.3) ppb and 10.1 (0.4) ppb in 10 asthmatic subjects at four studied expiratory flows (5–6, 7–8, 10–11, and 12–13 l/min, respectively), with NO levels equal to direct measurement (7.3 (0.5) ppb and 17.4 (0.5) ppb for normal and asthmatic subjects respectively, p<0.05) at the flow rate 10–11 l/min. Elimination of dead space proved necessary to provide NO levels comparable to the direct measurement. Exhaled NO collected into the reservoir without dead space during flow controlled exhalation against mild resistance provided close agreement (mean (SD) difference –0.21 (0.68), coefficient of variation 4.58%) with direct measurement in 74 patients (NO range 1–69 ppb).

CONCLUSIONS Flow and pressure controlled collection of exhaled NO into a reservoir with silica gel provides values identical to the direct measurement and may be used to monitor asthma at home and where analysers are not on site.