Flow dependency and off-line measurement of exhaled NO in children

Flow dependency and off-line measurement of exhaled NO in children.van Amsterdam JG, Zanen P, Somer S, van Loveren H, Opperhuizen A, Steerenberg PA.Laboratory of Health Effects Research, National Institute of Public Health and the Environment, PO Box 1, 3720 BA Bilthoven, The Netherlands. JGC.van.Amsterdam@rivm.nlLevels of exhaled nitric oxide (eNO) are flow-dependent, and the choice of an optimal flow rate for off-line and on-line eNO measurement has raised much debate. Recently, a flow rate of 50 ml/s was recommended, but children younger than 5-6 years are not capable of stabilizing their expiratory flow at low flow rates. The power of off-line eNO values to discriminate between normal and atopic children was therefore evaluated at different exhalation flow rates. At flow rates of both 8.3 ml/s and of 350 ml/s, children (8-12 years) sensitive to house dust mite have two-fold higher eNO values (p < 0.001) as compared with children lacking such a sensitivity. The power of eNO to discriminate between normal and atopic subjects was similar at the two flow rates (no difference in AUC of receiver operation curves, p = 0.89). All children from 4.5 to 5 years of age (n = 29) could perform a single off-line exhalation manoeuvre at high (>350 ml/s) but not at low (8.3 ml/s) flow rates. At high exhalation flow rate, eNO was 7.1 +/- 2.4 (mean +/- SD) median, 6.5 p.p.b. with a mean variation coefficient of 5.5%. Depending on their developmental level, about half of the younger children (35-46 months of age) could perform an off-line exhalation manoeuvre at high flow rate with good reproducibility (mean variation coefficient of 6.6%). It is concluded that an exhalation flow rate of 350 ml/s is feasible to determine off-line eNO-values in children from 3.5 years of age, and that this high flow rate does not compromise the power of eNO to detect allergic diseas