Abstracts: ISEE 20th Annual Conference, Pasadena, California, October 12–16, 2008: Contributed Abstracts
Field measurement of exhaled nitric oxide (eNO) may be useful in epidemiologic assessment of respiratory health, and also facilitates short-term air pollution exposure assessment, in that ambient nitric oxide (aNO) can be measured concurrently on-site with the same apparatus. “On-line” eNO measurement, with real-time direct sampling of exhaled breath, maximizes precision and quality control. “Off-line” measurement, with breath collection in Mylar bags for delayed analysis, is sometimes more practical. Cross-comparisons of these methods are needed to maximize data value. We obtained concurrent on-line and off-line measurements of eNO and aNO in 386 schoolchildren at 15 schools in 8 Southern California communities–a subsample from the longitudinal Asthma Incidence Risk Study (AIRS), in which both methods have been applied at different times in a larger population.
Following common practice, off-line breath samples were collected at 100 ml/sec expiratory flow with dead-space discard, while on-line eNO measurements were made at 50 ml/sec expiratory flow. Scrubbing of NO from inhaled air was nearly 100% effective with the on-line apparatus, but less effective with the off-line apparatus. Off-line samples were stored on “blue ice” and analyzed 2–36 hr later at a central laboratory.
Off-line and on-line eNO showed a highly but not completely linear relationship (r2 = 0.91); their means (ranges) were 10 (3–104) and 16 (3–181) ppb respectively. The aNO concentration mean (range) was 31 (0–212) ppb. Off-line eNO showed a significant (P < .001) positive relationship to aNO; while the corresponding relationship for real-time on-line measurements was less significant, with a smaller effect size. This suggests that off-line eNO might have been influenced by artifacts related to incomplete removal of inhaled NO. Separate studies using the same off-line bag-collection technique documented artifacts related to lag time between collection and analysis (positive at low and negative at high initial NO concentrations, likely attributable to a combination of gas-phase reactions and bag surface phenomena), and to bag aging (a positive effect of the number of prior uses). Even with adjustment for artifacts and for host factors known to influence eNO, off-line eNO differed significantly (P < .001) between schools, while on-line eNO did not. This suggests that off-line measurements were influenced by differences in storage and transport conditions between schools. Variance introduced by artifacts was small in comparison with between-subject variance of eNO.
Both on-line and off-line NO measurements in the field can be useful for health and exposure assessment. It is important to be aware of and account for possible artifacts in off-line measurement.