EFFECT OF MEASUREMENT CONDITIONS ON MEASURED LEVELS OF PEAK EXHALED NITRIC-OXIDE

Background - It is possible to measure nitric oxide (NO) levels in exh aled air. The absolute concentrations of exhaled NO obtained by separa te workers in similar patient groups and normal subjects with apparent ly similar techniques have been very different. A study was undertaken to determine whether changes in measurement conditions alter the conc entration of exhaled NO. Method - NO concentrations measured by a chem iluminescence analyser (Dasibi Environmental Corporation) and carbon d ioxide (CO2) measured by a Morgan capnograph were analysed in single e xhalations from total lung capacity in healthy volunteers (mean age 35 .9 years). Ten subjects performed five exhalations at four different e xpiratory flow rates, at four different expiratory mouth pressures, an d before and after drinking hot (n = 5) or cold (n = 5) water. Three s ubjects performed five exhalations on a day of high background NO (mea n NO level 134 ppb) before and after a set of five exhalations made wh ile both the subject and analysers were sampling from a low NO/NO-free reservoir system. Results - The mean peak concentration of NO decreas ed by 35 ppb (95% CI 25.7 to 43.4) from a mean (SE) of 79.0 (15.5) ppb at an expiratory flow rate of 250 ml/min to 54.1 (10.7) ppb at 1100 m l/min. The mean peak concentration of NO did not change significantly with change in mouth pressure. The mean (SE) peak NO concentration dec reased from 94.4 (20.8) ppb to 70.8 (16.5) ppb (p = 0.002, 95% CI 12.9 to 33.1) with water consumption. The mean NO concentration with machi ne and subject sampling from the low NO reservoir was 123.1 (19.4) ppb , an increase from results obtained before (81.9 (10.2) ppb, p = 0.001 , 95% CI -19.9 to -62.7) and after (94.2 (18.3) ppb, p = 0.017, 95% CI 6.0 to 51.8) sampling with high ambient NO. Conclusions - The measure ment of exhaled NO must be performed in a carefully standardised manne r to enable different teams of investigators to compare results.